Heterodyne vs superheterodyne

Heterodyne vs superheterodyne DEFAULT

A superheterodyne receiver contains a combination of amplification with frequency mixing, and is by far the most popular architecture for a microwave receiver. To heterodyne means to mix two signals of different frequencies together, resulting in a "beat" frequency.


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Also, how does a heterodyne receiver work?

The "heterodyne" or "beat" receiver has a local oscillator that produces a radio signal adjusted to be close in frequency to the incoming signal being received. When the two signals are mixed, a "beat" frequency equal to the difference between the two frequencies is created.

Similarly, what is meant by image frequency? The image frequency is an undesired input frequency which is demodulated by superheterodyne receivers along with the desired incoming signal. This results in two stations being received at the same time, thus producing interference.

Moreover, what is super heterodyne method?

The Superheterodyne ReceiverWe have discussed that superheterodyning is simply reducing the incoming signal is frequency by mixing. In a radio application we are reducing the AM or FM signal which is centered on the carrier frequency to some intermediate value, called the IF (intermediate frequency).

What is image frequency and how it is rejected?

The image rejection ratio, or image frequency rejection ratio, is the ratio of the intermediate-frequency (IF) signal level produced by the desired input frequency to that produced by the image frequency. The image rejection ratio is usually expressed in dB.

Sours: https://findanyanswer.com/what-is-the-difference-between-heterodyne-and-superheterodyne

RF Wireless World

Advantages of Superheterodyne Receiver | disadvantages of heterodyne Receiver

This page covers Advantages and Disadvantages of Superheterodyne receiver architecture. It mentions benefits or advantages of superheterodyne receiver and drawbacks or disadvantages of superheterodyne receiver.

What is Superheterodyne receiver?

Introduction:
• Heterodyne receiver uses single RF mixer for conversion of modulated RF signal to baseband I/Q signals.
• Superheterodyne receiver uses dual RF mixers for conversion of modulated RF signal to baseband I/Q signals.

Heterodyne and Superheterodyne receiver types use different LO (Local Oscillator) frequency than received signal frequency.

Heterodyne receiver

The figure-1 depicts Heterodyne receiver architecture. Here fIF = fRF - fLO
Refer RF Mixer basics>> and RF Mixer tutorial to understand up conversion and down conversion.

Superheterodyne receiver

The figure-2 depicts Superheterodyne receiver architecture.
Here Here fIF1 = fRF - fLO1 ...equation-1 at stage-I
Here fIF2 = fIF1 - fLO2 ...equation-2 at stage-II
The modulated fIF2 is processed to achieve baseband I/Q signals at zero frequency.
Refer Homodyne Vs Heterodyne Receiver>>.

Benefits or advantages of Superheterodyne Receiver

Following are the benefits or advantages of superheterodyne Receiver and heterodyne receiver architecture types:
➨As it converts high frequency to low frequency, all processing takes place at lower frequencies. The devices are cheaper at such lower frequencies compare to higher frequencies.
➨It is easy to filter IF signal compare to RF signal.
➨It offers better sensitivity compare to homodyne receiver architecture.
➨Heterodyne uses single conversion and Superheterodyne uses double conversion. The Superheterodyne receiver prevents image noise foldover due to use of two IF frequencies before conversion to baseband.

Drawbacks or disadvantages of Superheterodyne Receiver

Following are the disadvantages of superheterodyne Receiver and heterodyne receiver architecture types:
➨It requires additional LOs (Local Oscillators) and RF Mixers to convert signal from RF to IF before conversion to baseband. This increases cost of overall receiver.
➨Moreover filters are also needed to remove any LO leakage as well as undesired frequency components to prevent image frequencies. This also increases cost as well as complexity of the receiver.

Also refer advantages and disadvantages of Homodyne receiver >>.

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Heterodyne receiver vs Homodyne receiver-difference between Heterodyne receiver and Homodyne receiver

This page on Heterodyne receiver vs Homodyne receiver describes difference between Heterodyne receiver and Homodyne receiver. There are two main architecture prevails in radio receiver of any system i.e. Heterodyne and Homodyne. Both heterodyne and homodyne converts modulated RF signal to baseband I/Q signal at zero IF frequency.

Heterodyne receiver

In Heterodyne receiver, it requires one mixer to bring the modulated RF signal to modulated IF signal, which is applied to I/Q demodulator which brings the modulated low IF to baseband at zero IF.


Heterodyne receiver

In super heterodyne receiver, it requires two mixers to bring the modulated RF signal to modulated-IF signal. The first mixer brings RF signal to high IF signal and the later mixer brings high IF signal to low IF signal. This is applied to I/Q demodulator which brings the low IF signal to zero IF baseband signals.


super heterodyne receiver

Homodyne receiver

In homodyne receiver, it does not require any mixers at RF stage. The modulated RF signal is directly applied to I/Q demodulator which gives baseband signals out (I and Q) at zero IF.


homodyne receiver

Figure below depicts typical I/Q demodulator circuit which is used in almost all the modems which converts modulated IF/RF signal to baseband signal at zero frequency. For this appropriate frequency f0 is choosen. Here W0=2*pi*f0. Here f0 is the same as RF frequency of modulated signal.


IQ demodulator

The principle of homodyne receiver is depicted in the figure-3 and figure-4. The signal is first amplified at a low noise stage known as LNA. After the low noise amplification signal is directly converted to the baseband (i.e. direct current signal). If RF frequency signal and LO frequency signals are equal, this circuit works as phase detector. In other words, if LO is synchronized in phase with incoming carrier frequency signal, then the receiver is called homodyne receiver.

Maximum information from modulated I/Q signal is obtained using quadrature down conversion. This is illustrated in fig-4. As shown modulated signal is first split into two channels. These two signals are multiplied by A*sin(w0*t) and A*cos(w0*t). This results into complex signal (I+j*Q) consisting I and Q parts. This vector signal will have magnitude of sqrt(I2+Q2) and phase of tan-1 (Q/I).

The homodyne receiver is also known as direct conversion receiver. The main problem in this type of receiver is LO leakage. This LO leakage need to be as low as possible in order to make RF Transceiver work efficiently to deliver baseband I/Q signals.

Also refer advantages and disadvantages of Homodyne receiver >> and Heterodye receiver >> types.

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SUPERHETERODYNE Receiver - Modulation Techniques
Introduction to Naval Weapons Engineering


Objective

What Heterodyning is

What Superheterodyning is

The Superheterodyne Receiver

Advantages of Using Superheterodyning

Summary


Objectives

1. Know how a superheterodyne receiver works and what its advantages are.

What Heterodyning is

To heterodyne means to mix to frequencies together so as to produce a beat frequency, namely the difference between the two. Amplitude modulation is a heterodyne process: the information signal is mixed with the carrier to produce the side-bands. The side-bands occur at precisely the sum and difference frequencies of the carrier and information. These are beat frequencies (normally the beat frequency is associated with the lower side-band, the difference between the two).

What Superheterodyning is

When you use the lower side-band (the difference between the two frequencies), you are superheterodyning. Strictly speaking, the term superheterodyne refers to creating a beat frequency that is lower than the original signal. Although we have used the example of amplitude modulation side-bands as an example, we are not talking about encoding information for transmission. What superheterodying does is to purposely mix in another frequency in the receiver, so as to reduce the signal frequency prior to processing. Why and how this is done will be discussed below.

The Superheterodyne Receiver

We have discussed that superheterodyning is simply reducing the incoming signal is frequency by mixing. In a radio application we are reducing the AM or FM signal which is centered on the carrier frequency to some intermediate value, called the IF (intermediate frequency). For practical purposes, the superheterodyne receiver always reduces to the same value of IF. To accomplish this requires that we be able to continuously vary the frequency being mixed into the signal so as to keep the difference the same. Here's what the superheterodyne receiver looks like:

This is essentially the conventional receiver with the addition of a mixer and local oscillator. The local oscillator is linked to the tuner because they both must vary with the carrier frequency. For example, suppose you want to tune in a TV station at 235 MHz. The band-pass filter (which only permits signals in a small range about the center frequency to pass) must be centered at 235 MHz (or slightly higher in SSB). The local oscillator must be set to a frequency that will heterodyne the 235 MHz to the desired IF of 452 kHz (typical). This means the local oscillator must be set to 234.448 MHz (or alternatively to 235.452 MHz) so that the difference frequency will be exactly 452 kHz. The local oscillator must be capable of varying the frequency over the same range as the tuner; in fact, they vary the same amount. Therefore, the tuner and the local oscillator are linked so they operate together.

Advantages of Using Superheterodyning

Now, we easily see that this type of receiver can be constructed, but for what purpose? All we have accomplished is to reduce the frequency to the IF value. We still must process the signal as before. So why are so many receivers using the superheterodyne method? There are three main advantages, depending on the application used for:

  • It reduces the signal from very high frequency sources where ordinary components wouldn't work (like in a radar receiver).
  • It allows many components to operate at a fixed frequency (IF section) and therefore they can be optimized or made more inexpensively.
  • It can be used to improve signal isolation by arithmetic selectivity

Reduction in frequency

AT very extremely high frequencies, many ordinary components cease to function. Although we see many computer systems that work at previously unattainable frequencies like 166 MHz, you certainly never see any system that works at radar frequencies like 10 GHz (try that Intel!). There are many physical reasons for this, but suffice it to say, it can't be done (yet). So the designer of a radar interceptor (fuzz-buster, et al.) is faced with a daunting circumstance unless he/she can use a superheterodyne receiver to knock down the frequency to an IF value. It is in fact, the local oscillator (a operating at radar frequencies) of the superheterodyne radar receiver that makes your radar detector detectable by the police (in VA for example, where the use of radar detectors are illegal).

Optimization of Components

It is a typical engineering dilemma: how to make components that have outstanding performance, but can also cover a wide range of frequencies. Again, the details aren't important, but the problem is very real. A possible solution to this, is to make as much of the receiver as possible always work at the same frequency (the IF). This is accomplished by using the superheterodyne method. The majority of components can be optimized to work at the IF without any requirements to cover a wide range of frequencies.

Arithmetic Selectivity

The ability to isolate signals, or reject unwanted ones, is a function of the receiver bandwidth. For example, the band-pass filter in the tuner is what isolates the desired signal from the adjacent ones. In real life, there are frequently sources that can interfere with your signal. The FCC makes frequency assignments that generally prevent this. Depending on the application, you might have a need for very narrow signal isolation. If the performance of your band-pass filter isn't sufficient to accomplish this, the performance can be improve by superheterodyning.

Frequently, the receiver bandwidth is some fraction of the carrier frequency. If your receiver has a bandwidth of 2 % and you are tuned to 850 kHz, then only signals within the range from 2 % above and below are passed. In this case, that would be from 833 to 867 kHz.

Arithmetic selectivity takes that fraction and applies it to the reduced frequency (the IF). For the fixed IF of 452 kHz, that means signals which are superheterodyned to the range of 443 to 461 kHz will pass. Taking this range back up into the carrier band, only carrier frequencies in the range of 841 to 859 kHz will pass. If this is confusing, recall that the local oscillator is set to reduce the 850 kHz to 452 kHz (i.e. must be set at 398 kHz). Thus, the 850 kHz is superheterodyned to 452 kHz. Any adjacent signals are also superheterodyned but remain the same above or below the original signal. An example might clear this up:

Suppose there is an interfering signal at 863 kHz while you are tuned to 850 kHz. A conventional 2 % receiver will pass 833 to 867 kHz and so the interfering signal also passes. The superheterodyne receiver mixes both signals with 398 kHz to produce the desired signal at 452 kHz and the interference at 465 kHz. At 2 %, the IF section only passes 443 to 461 kHz, and therefore the interference is now suppressed. We say that the superheterodyne receiver is more selective. With a little thought, the reason is simple: it operates at a smaller frequency, so the 2 % actually involves a smaller range. That is why it is called arithmetic selectivity. Bandwidths that are expressed as a percentage are smaller when the center frequency is smaller (the same way that 2 % of $10 is less than 2 % of $10,000,000 ).

Whether or not, you need to take advantage of arithmetic selectivity depends on the application. If you have no problems with interference at your current bandwidth and/or it is not difficult or expensive to reduce the bandwidth of your receiver, then you don't need it. However, in cases where selectivity is important or the frequency is very high (like radar) then superheterodyning can greatly improve performance.

Summary

  • Superheterodyne receivers reduce the signal frequency be mixing in a signal from a local oscillator to produce the intermediate frequency (IF).
  • Superheterodyne receivers have better performance because the components can be optimized to work a single intermediate frequency, and can take advantage of arithmetic selectivity.
Sours: https://man.fas.org/dod-101/navy/docs/es310/superhet.htm

Superheterodyne heterodyne vs

Superheterodyne receiver

Common type of radio receiver that shifts the received signal to an easily-processed intermediate frequency

A 5-tube superheterodyne receiver made in Japan circa 1955

A superheterodyne receiver, often shortened to superhet, is a type of radio receiver that uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original carrier frequency. It was long believed to have been invented by US engineer Edwin Armstrong, but after some controversy the earliest patent for the invention is now credited to French radio engineer and radio manufacturer Lucien Lévy.[1] Virtually all modern radio receivers use the superheterodyne principle.

History[edit]

Heterodyne[edit]

Early Morse code radio broadcasts were produced using an alternator connected to a spark gap. The output signal was at a carrier frequency defined by the physical construction of the gap, modulated by the alternating current signal from the alternator. Since the output of the alternator was generally in the audible range, this produces an audible amplitude modulated (AM) signal. Simple radio detectors filtered out the high-frequency carrier, leaving the modulation, which was passed on to the user's headphones as an audible signal of dots and dashes.

In 1904, Ernst Alexanderson introduced the Alexanderson alternator, a device that directly produced radio frequency output with higher power and much higher efficiency than the older spark gap systems. In contrast to the spark gap, however, the output from the alternator was a pure carrier wave at a selected frequency. When detected on existing receivers, the dots and dashes would normally be inaudible, or "supersonic". Due to the filtering effects of the receiver, these signals generally produced a click or thump, which were audible but made determining dot or dash difficult.

In 1905, Canadian inventor Reginald Fessenden came up with the idea of using two Alexanderson alternators operating at closely spaced frequencies to broadcast two signals, instead of one. The receiver would then receive both signals, and as part of the detection process, only the beat frequency would exit the receiver. By selecting two carriers close enough that the beat frequency was audible, the resulting Morse code could once again be easily heard even in simple receivers. For instance, if the two alternators operated at frequencies 3 kHz apart, the output in the headphones would be dots or dashes of 3 kHz tone, making them easily audible.

Fessenden coined the term "heterodyne", meaning "generated by a difference" (in frequency), to describe this system. The word is derived from the Greek roots hetero- "different", and -dyne "power".

Regeneration[edit]

Morse code was widely used in the early days of radio because it was both easy to produce and easy to receive. In contrast to voice broadcasts, the output of the amplifier didn't have to closely match the modulation of the original signal. As a result, any number of simple amplification systems could be used. One method used an interesting side-effect of early triode amplifier tubes. If both the plate (anode) and grid were connected to resonant circuits tuned to the same frequency and the stage gain was much higher than unity, stray capacitive coupling between the grid and the plate would cause the amplifier to go into oscillation.

In 1913, Edwin Howard Armstrong described a receiver system that used this effect to produce audible Morse code output using a single triode. The output of the amplifier taken at the anode was connected back to the input through a "tickler", causing feedback that drove input signals well beyond unity. This caused the output to oscillate at a chosen frequency with great amplification. When the original signal cut off at the end of the dot or dash, the oscillation decayed and the sound disappeared after a short delay.

Armstrong referred to this concept as a regenerative receiver, and it immediately became one of the most widely used systems of its era. Many radio systems of the 1920s were based on the regenerative principle, and it continued to be used in specialized roles into the 1940s, for instance in the IFF Mark II.

RDF[edit]

There was one role where the regenerative system was not suitable, even for Morse code sources, and that was the task of radio direction finding, or RDF.

The regenerative system was highly non-linear, amplifying any signal above a certain threshold by a huge amount, sometimes so large it caused it to turn into a transmitter (which was the entire concept behind IFF). In RDF, the strength of the signal is used to determine the location of the transmitter, so one requires linear amplification to allow the strength of the original signal, often very weak, to be accurately measured.

To address this need, RDF systems of the era used triodes operating below unity. To get a usable signal from such a system, tens or even hundreds of triodes had to be used, connected together anode-to-grid. These amplifiers drew enormous amounts of power and required a team of maintenance engineers to keep them running. Nevertheless, the strategic value of direction finding on weak signals was so high that the British Admiralty felt the high cost was justified.

Superheterodyne[edit]

One of the prototype superheterodyne receivers built at Armstrong's Signal Corps laboratory in Paris during World War I. It is constructed in two sections, the mixerand local oscillator(left)and three IF amplification stages and a detector stage (right). The intermediate frequency was 75 kHz.

Although a number of researchers discovered the superheterodyne concept, filing patents only months apart (see below), Armstrong is often credited with the concept. He came across it while considering better ways to produce RDF receivers. He had concluded that moving to higher "short wave" frequencies would make RDF more useful and was looking for practical means to build a linear amplifier for these signals. At the time, short wave was anything above about 500 kHz, beyond any existing amplifier's capabilities.

It had been noticed that when a regenerative receiver went into oscillation, other nearby receivers would start picking up other stations as well. Armstrong (and others) eventually deduced that this was caused by a "supersonic heterodyne" between the station's carrier frequency and the regenerative receiver's oscillation frequency. When the first receiver began to oscillate at high outputs, its signal would flow back out through the antenna to be received on any nearby receiver. On that receiver, the two signals mixed just as they did in the original heterodyne concept, producing an output that is the difference in frequency between the two signals.

For instance, consider a lone receiver that was tuned to a station at 300 kHz. If a second receiver is set up nearby and set to 400 kHz with high gain, it will begin to give off a 400 kHz signal that will be received in the first receiver. In that receiver, the two signals will mix to produce four outputs, one at the original 300 kHz, another at the received 400 kHz, and two more, the difference at 100 kHz and the sum at 700 kHz. This is the same effect that Fessenden had proposed, but in his system the two frequencies were deliberately chosen so the beat frequency was audible. In this case, all of the frequencies are well beyond the audible range, and thus "supersonic", giving rise to the name superheterodyne.

Armstrong realized that this effect was a potential solution to the "short wave" amplification problem, as the "difference" output still retained its original modulation, but on a lower carrier frequency. In the example above, one can amplify the 100 kHz beat signal and retrieve the original information from that, the receiver does not have to tune in the higher 300 kHz original carrier. By selecting an appropriate set of frequencies, even very high-frequency signals could be "reduced" to a frequency that could be amplified by existing systems.

For instance, to receive a signal at 1500 kHz, far beyond the range of efficient amplification at the time, one could set up an oscillator at, for example, 1560 kHz. Armstrong referred to this as the "local oscillator" or LO. As its signal was being fed into a second receiver in the same device, it did not have to be powerful, generating only enough signal to be roughly similar in strength to that of the received station.[a] When the signal from the LO mixes with the station's, one of the outputs will be the heterodyne difference frequency, in this case, 60 kHz. He termed this resulting difference the "intermediate frequency" often abbreviated to "IF".

In December 1919, Major E. H. Armstrong gave publicity to an indirect method of obtaining short-wave amplification, called the super-heterodyne. The idea is to reduce the incoming frequency, which may be, for example 1,500,000 cycles (200 meters), to some suitable super-audible frequency that can be amplified efficiently, then passing this current through an intermediate frequency amplifier, and finally rectifying and carrying on to one or two stages of audio frequency amplification.[2]

The "trick" to the superheterodyne is that by changing the LO frequency you can tune in different stations. For instance, to receive a signal at 1300 kHz, one could tune the LO to 1360 kHz, resulting in the same 60 kHz IF. This means the amplifier section can be tuned to operate at a single frequency, the design IF, which is much easier to do efficiently.

Development[edit]

The first commercial superheterodyne receiver,[3]the RCA Radiola AR-812, brought out March 4, 1924, priced at $286 (equivalent to $4,320 in 2020). It used 6 triodes: a mixer, local oscillator, two IF and two audio amplifier stages, with an IF of 45 kHz. It was a commercial success, with better performance than competing receivers.

Armstrong put his ideas into practice, and the technique was soon adopted by the military. It was less popular when commercial radio broadcasting began in the 1920s, mostly due to the need for an extra tube (for the oscillator), the generally higher cost of the receiver, and the level of skill required to operate it. For early domestic radios, tuned radio frequency receivers (TRF) were more popular because they were cheaper, easier for a non-technical owner to use, and less costly to operate. Armstrong eventually sold his superheterodyne patent to Westinghouse, which then sold it to Radio Corporation of America (RCA), the latter monopolizing the market for superheterodyne receivers until 1930.[4]

Because the original motivation for the superhet was the difficulty of using the triode amplifier at high frequencies, there was an advantage in using a lower intermediate frequency. During this era, many receivers used an IF frequency of only 30 kHz.[5] These low IF frequencies, often using IF transformers based on the self-resonance of iron-core transformers, had poor image frequency rejection, but overcame the difficulty in using triodes at radio frequencies in a manner that competed favorably with the less robust neutrodyne TRF receiver. Higher IF frequencies (455 kHz was a common standard) came into use in later years, after the invention of the tetrode and pentode as amplifying tubes, largely solving the problem of image rejection. Even later, however, low IF frequencies (typically 60 kHz) were again used in the second (or third) IF stage of double or triple-conversion communications receivers to take advantage of the selectivity more easily achieved at lower IF frequencies, with image-rejection accomplished in the earlier IF stage(s) which were at a higher IF frequency.

In the 1920s, at these low frequencies, commercial IF filters looked very similar to 1920s audio interstage coupling transformers, had similar construction, and were wired up in an almost identical manner, so they were referred to as "IF transformers". By the mid-1930s, superheterodynes using much higher intermediate frequencies (typically around 440–470 kHz) used tuned transformers more similar to other RF applications. The name "IF transformer" was retained, however, now meaning "intermediate frequency". Modern receivers typically use a mixture of ceramic resonators or surface acoustic wave resonators and traditional tuned-inductor IF transformers.

"All American Five" vacuum-tube superheterodyne AM broadcast receiver from 1940s was cheap to manufacture because it only required five tubes.

By the 1930s, improvements in vacuum tube technology rapidly eroded the TRF receiver's cost advantages, and the explosion in the number of broadcasting stations created a demand for cheaper, higher-performance receivers.

The introduction of an additional grid in a vacuum tube, but before the more modern screen-grid tetrode, included the tetrode with two control grids; this tube combined the mixer and oscillator functions, first used in the so-called autodyne mixer. This was rapidly followed by the introduction of tubes specifically designed for superheterodyne operation, most notably the pentagrid converter. By reducing the tube count (with each tube stage being the main factor affecting cost in this era), this further reduced the advantage of TRF and regenerative receiver designs.

By the mid-1930s, commercial production of TRF receivers was largely replaced by superheterodyne receivers. By the 1940s, the vacuum-tube superheterodyne AM broadcast receiver was refined into a cheap-to-manufacture design called the "All American Five" because it used five vacuum tubes: usually a converter (mixer/local oscillator), an IF amplifier, a detector/audio amplifier, audio power amplifier, and a rectifier. Since this time, the superheterodyne design was used for almost all commercial radio and TV receivers.

Patent battles[edit]

French engineer Lucien Lévy filed a patent application for the superheterodyne principle in August 1917 with brevet n° 493660.[6] Armstrong also filed his patent in 1917.[7][8][9] Levy filed his original disclosure about seven months before Armstrong's.[1] German inventor Walter H. Schottky also filed a patent in 1918.[6]

At first the US recognised Armstrong as the inventor, and his US Patent 1,342,885 was issued on 8 June 1920.[1] After various changes and court hearings Lévy was awarded US patent No 1,734,938 that included seven of the nine claims in Armstrong's application, while the two remaining claims were granted to Alexanderson of GE and Kendall of AT&T.[1]

Principle of operation[edit]

Block diagram of a typical superheterodyne receiver. Redparts are those that handle the incoming radio frequency (RF) signal; greenare parts that operate at the intermediate frequency (IF), while blueparts operate at the modulation (audio) frequency. The dotted line indicates that the local oscillator and RF filter must be tuned in tandem.
How a superheterodyne radio works. The horizontal axes are frequency f. The blue graphs show the voltages of the radio signals at various points in the circuit. The red graphs show the transfer functionsof the filters in the circuit; the thickness of the red bands shows the fraction of signal from the previous graph that passes through the filter at each frequency. The incoming radio signal from the antenna (top graph)consists of the desired radio signal S1plus others at different frequencies. The RF filter (2nd graph)removes any signal such as S2at the image frequencyLO - IF, which would otherwise pass through the IF filter and interfere. The remaining composite signal is applied to the mixer along with a local oscillator signal (LO) (3rd graph). In the mixer the signal S1combines with the LO frequency to create a heterodyne at the difference between these frequencies, the intermediate frequency (IF), at the mixer output (4th graph). This passes through the IF bandpass filter (5th graph)is amplified and demodulated (demodulation is not shown). The unwanted signals create heterodynes at other frequencies (4th graph), which are filtered out by the IF filter .

The diagram at right shows the block diagram of a typical single-conversion superheterodyne receiver. The diagram has blocks that are common to superheterodyne receivers,[10] with only the RF amplifier being optional.

The antenna collects the radio signal. The tuned RF stage with optional RF amplifier provides some initial selectivity; it is necessary to suppress the image frequency (see below), and may also serve to prevent strong out-of-passband signals from saturating the initial amplifier. A local oscillator provides the mixing frequency; it is usually a variable frequency oscillator which is used to tune the receiver to different stations. The frequency mixer does the actual heterodyning that gives the superheterodyne its name; it changes the incoming radio frequency signal to a higher or lower, fixed, intermediate frequency (IF). The IF band-pass filter and amplifier supply most of the gain and the narrowband filtering for the radio. The demodulator extracts the audio or other modulation from the IF radio frequency. The extracted signal is then amplified by the audio amplifier.

Circuit description[edit]

To receive a radio signal, a suitable antenna is required. The output of the antenna may be very small, often only a few microvolts. The signal from the antenna is tuned and may be amplified in a so-called radio frequency (RF) amplifier, although this stage is often omitted. One or more tuned circuits at this stage block frequencies that are far removed from the intended reception frequency. To tune the receiver to a particular station, the frequency of the local oscillator is controlled by the tuning knob (for instance). Tuning of the local oscillator and the RF stage may use a variable capacitor, or varicap diode.[11] The tuning of one (or more) tuned circuits in the RF stage must track the tuning of the local oscillator.

Local oscillator and mixer[edit]

The signal is then fed into a circuit where it is mixed with a sine wave from a variable frequency oscillator known as the local oscillator (LO). The mixer uses a non-linear component to produce both sum and difference beat frequencies signals,[12] each one containing the modulation contained in the desired signal. The output of the mixer may include the original RF signal at fRF, the local oscillator signal at fLO, and the two new heterodyne frequencies fRF + fLO and fRF − fLO. The mixer may inadvertently produce additional frequencies such as third- and higher-order intermodulation products. Ideally, the IF bandpass filter removes all but the desired IF signal at fIF. The IF signal contains the original modulation (transmitted information) that the received radio signal had at fRF.

The frequency of the local oscillator fLO is set so the desired reception radio frequency fRF mixes to fIF. There are two choices for the local oscillator frequency because the dominant mixer products are at fRF ± fLO. If the local oscillator frequency is less than the desired reception frequency, it is called low-side injection (fIF = fRFfLO); if the local oscillator is higher, then it is called high-side injection (fIF = fLOfRF).

The mixer will process not only the desired input signal at fRF, but also all signals present at its inputs. There will be many mixer products (heterodynes). Most other signals produced by the mixer (such as due to stations at nearby frequencies) can be filtered out in the IF tuned amplifier; that gives the superheterodyne receiver its superior performance. However, if fLO is set to fRF + fIF, then an incoming radio signal at fLO + fIF will also produce a heterodyne at fIF; the frequency fLO + fIF is called the image frequency and must be rejected by the tuned circuits in the RF stage. The image frequency is 2 fIF higher (or lower) than the desired frequency fRF, so employing a higher IF frequency fIF increases the receiver's image rejection without requiring additional selectivity in the RF stage.

To suppress the unwanted image, the tuning of the RF stage and the LO may need to "track" each other. In some cases, a narrow-band receiver can have a fixed tuned RF amplifier. In that case, only the local oscillator frequency is changed. In most cases, a receiver's input band is wider than its IF center frequency. For example, a typical AM broadcast band receiver covers 510 kHz to 1655 kHz (a roughly 1160 kHz input band) with a 455 kHz IF frequency; an FM broadcast band receiver covers 88 MHz to 108 MHz band with a 10.7 MHz IF frequency. In that situation, the RF amplifier must be tuned so the IF amplifier does not see two stations at the same time. If the AM broadcast band receiver LO were set at 1200 kHz, it would see stations at both 745 kHz (1200−455 kHz) and 1655 kHz. Consequently, the RF stage must be designed so that any stations that are twice the IF frequency away are significantly attenuated. The tracking can be done with a multi-section variable capacitor or some varactors driven by a common control voltage. An RF amplifier may have tuned circuits at both its input and its output, so three or more tuned circuits may be tracked. In practice, the RF and LO frequencies need to track closely but not perfectly.[13][14]

In the days of tube (valve) electronics, it was common for superheterodyne receivers to combine the functions of the local oscillator and the mixer in a single tube, leading to a savings in power, size, and especially cost. A single pentagrid converter tube would oscillate and also provide signal amplification as well as frequency mixing.[15]

IF amplifier[edit]

The stages of an intermediate frequency amplifier ("IF amplifier" or "IF strip") are tuned to a fixed frequency that does not change as the receiving frequency changes. The fixed frequency simplifies optimization of the IF amplifier.[10] The IF amplifier is selective around its center frequency fIF. The fixed center frequency allows the stages of the IF amplifier to be carefully tuned for best performance (this tuning is called "aligning" the IF amplifier). If the center frequency changed with the receiving frequency, then the IF stages would have had to track their tuning. That is not the case with the superheterodyne.

Normally, the IF center frequency fIF is chosen to be less than the range of desired reception frequencies fRF. That is because it is easier and less expensive to get high selectivity at a lower frequency using tuned circuits. The bandwidth of a tuned circuit with a certain Q is proportional to the frequency itself (and what's more, a higher Q is achievable at lower frequencies), so fewer IF filter stages are required to achieve the same selectivity. Also, it is easier and less expensive to get high gain at a lower frequencies.

However, in many modern receivers designed for reception over a wide frequency range (e.g. scanners and spectrum analyzers) a first IF frequency higher than the reception frequency is employed in a double conversion configuration. For instance, the Rohde & Schwarz EK-070 VLF/HF receiver covers 10 kHz to 30 MHz.[14] It has a band switched RF filter and mixes the input to a first IF of 81.4 MHz and a second IF frequency of 1.4 MHz. The first LO frequency is 81.4 to 111.4 MHz, a reasonable range for an oscillator. But if the original RF range of the receiver were to be converted directly to the 1.4 MHz intermediate frequency, the LO frequency would need to cover 1.4-31.4 MHz which cannot be accomplished using tuned circuits (a variable capacitor with a fixed inductor would need a capacitance range of 500:1). Image rejection is never an issue with such a high IF frequency. The first IF stage uses a crystal filter with a 12 kHz bandwidth. There is a second frequency conversion (making a triple-conversion receiver) that mixes the 81.4 MHz first IF with 80 MHz to create a 1.4 MHz second IF. Image rejection for the second IF is not an issue as the first IF has a bandwidth of much less than 2.8 MHz.

To avoid interference to receivers, licensing authorities will avoid assigning common IF frequencies to transmitting stations. Standard intermediate frequencies used are 455 kHz for medium-wave AM radio, 10.7 MHz for broadcast FM receivers, 38.9 MHz (Europe) or 45 MHz (US) for television, and 70 MHz for satellite and terrestrial microwave equipment. To avoid tooling costs associated with these components, most manufacturers then tended to design their receivers around a fixed range of frequencies offered, which resulted in a worldwide de facto standardization of intermediate frequencies.

In early superhets, the IF stage was often a regenerative stage providing the sensitivity and selectivity with fewer components. Such superhets were called super-gainers or regenerodynes.[16] This is also called a Q multiplier, involving a small modification to an existing receiver especially for the purpose of increasing selectivity.

IF bandpass filter[edit]

The IF stage includes a filter and/or multiple tuned circuits to achieve the desired selectivity. This filtering must have a band pass equal to or less than the frequency spacing between adjacent broadcast channels. Ideally a filter would have a high attenuation to adjacent channels, but maintain a flat response across the desired signal spectrum in order to retain the quality of the received signal. This may be obtained using one or more dual tuned IF transformers, a quartz crystal filter, or a multipole ceramic crystal filter.[17]

In the case of television receivers, no other technique was able to produce the precise bandpass characteristic needed for vestigial sideband reception, such as that used in the NTSC system first approved by the US in 1941. By the 1980s, multi-component capacitor-inductor filters had been replaced with precision electromechanical surface acoustic wave (SAW) filters. Fabricated by precision laser milling techniques, SAW filters are cheaper to produce, can be made to extremely close tolerances, and are very stable in operation.

Demodulator[edit]

The received signal is now processed by the demodulator stage where the audio signal (or other baseband signal) is recovered and then further amplified. AM demodulation requires the simple rectification of the RF signal (so-called envelope detection), and a simple RC low pass filter to remove remnants of the intermediate frequency.[18] FM signals may be detected using a discriminator, ratio detector, or phase-locked loop. Continuous wave and single sideband signals require a product detector using a so-called beat frequency oscillator, and there are other techniques used for different types of modulation.[19] The resulting audio signal (for instance) is then amplified and drives a loudspeaker.

When so-called high-side injection has been used, where the local oscillator is at a higher frequency than the received signal (as is common), then the frequency spectrum of the original signal will be reversed. This must be taken into account by the demodulator (and in the IF filtering) in the case of certain types of modulation such as single sideband.

Multiple conversion[edit]

Double conversion superheterodyne receiver block diagram

To overcome obstacles such as image response, some receivers use multiple successive stages of frequency conversion and multiple IFs of different values. A receiver with two frequency conversions and IFs is called a dual conversion superheterodyne, and one with three IFs is called a triple conversion superheterodyne.

The main reason that this is done is that with a single IF there is a tradeoff between low image response and selectivity. The separation between the received frequency and the image frequency is equal to twice the IF frequency, so the higher the IF, the easier it is to design an RF filter to remove the image frequency from the input and achieve low image response. However, the higher the IF, the more difficult it is to achieve high selectivity in the IF filter. At shortwave frequencies and above, the difficulty in obtaining sufficient selectivity in the tuning with the high IFs needed for low image response impacts performance. To solve this problem two IF frequencies can be used, first converting the input frequency to a high IF to achieve low image response, and then converting this frequency to a low IF to achieve good selectivity in the second IF filter. To improve tuning, a third IF can be used.

For example, for a receiver that can tune from 500 kHz to 30 MHz, three frequency converters might be used.[10] With a 455 kHz IF it is easy to get adequate front end selectivity with broadcast band (under 1600 kHz) signals. For example, if the station being received is on 600 kHz, the local oscillator can be set to 1055 kHz, giving an image on (-600+1055=) 455 kHz. But a station on 1510 kHz could also potentially produce an image at (1510-1055=) 455 kHz and so cause image interference. However, because 600 kHz and 1510 kHz are so far apart, it is easy to design the front end tuning to reject the 1510 kHz frequency.

However at 30 MHz, things are different. The oscillator would be set to 30.455 MHz to produce a 455 kHz IF, but a station on 30.910 would also produce a 455 kHz beat, so both stations would be heard at the same time. But it is virtually impossible to design an RF tuned circuit that can adequately discriminate between 30 MHz and 30.91 MHz, so one approach is to "bulk downconvert" whole sections of the shortwave bands to a lower frequency, where adequate front-end tuning is easier to arrange.

For example, the ranges 29 MHz to 30 MHz; 28 MHz to 29 MHz etc. might be converted down to 2 MHz to 3 MHz, there they can be tuned more conveniently. This is often done by first converting each "block" up to a higher frequency (typically 40 MHz) and then using a second mixer to convert it down to the 2 MHz to 3 MHz range. The 2 MHz to 3 MHz "IF" is basically another self-contained superheterodyne receiver, most likely with a standard IF of 455 kHz.

Modern designs[edit]

Microprocessor technology allows replacing the superheterodyne receiver design by a software defined radio architecture, where the IF processing after the initial IF filter is implemented in software. This technique is already in use in certain designs, such as very low-cost FM radios incorporated into mobile phones, since the system already has the necessary microprocessor.

Radio transmitters may also use a mixer stage to produce an output frequency, working more or less as the reverse of a superheterodyne receiver.

Advantages and disadvantages[edit]

Superheterodyne receivers have essentially replaced all previous receiver designs. The development of modern semiconductor electronics negated the advantages of designs (such as the regenerative receiver) that used fewer vacuum tubes. The superheterodyne receiver offers superior sensitivity, frequency stability and selectivity. Compared with the tuned radio frequency receiver (TRF) design, superhets offer better stability because a tuneable oscillator is more easily realized than a tuneable amplifier. Operating at a lower frequency, IF filters can give narrower passbands at the same Q factor than an equivalent RF filter. A fixed IF also allows the use of a crystal filter[10] or similar technologies that cannot be tuned. Regenerative and super-regenerative receivers offered a high sensitivity, but often suffer from stability problems making them difficult to operate.

Although the advantages of the superhet design are overwhelming, there are a few drawbacks that need to be tackled in practice.

Image frequency (fIMAGE)[edit]

Graphs illustrating the problem of image response in a superheterodyne. The horizontal axes are frequency and the vertical axes are voltage. Without an adequate RF filter, any signal S2 (green)at the image frequency {\displaystyle f_{\text{IMAGE}}}is also heterodyned to the IF frequency {\displaystyle f_{\text{IF}}}along with the desired radio signal S1 (blue)at {\displaystyle f_{\text{RF}}}, so they both pass through the IF filter (red). Thus S2 interferes with S1.

One major disadvantage to the superheterodyne receiver is the problem of image frequency. In heterodyne receivers, an image frequency is an undesired input frequency equal to the station frequency plus (or minus) twice the intermediate frequency. The image frequency results in two stations being received at the same time, thus producing interference. Reception at the image frequency can be combated through tuning (filtering) at the antenna and RF stage of the superheterodyne receiver.

{\displaystyle f_{\mathrm {IMAGE} }={\begin{cases}f+2f_{\mathrm {IF} },&{\text{if }}f_{\mathrm {LO} }>f{\text{   (high side injection)}}\\f-2f_{\mathrm {IF} },&{\text{if }}f_{\mathrm {LO} }<f{\text{  (low side injection)}}\end{cases}}}

For example, an AM broadcast station at 580 kHz is tuned on a receiver with a 455 kHz IF. The local oscillator is tuned to 580 + 455 = 1035 kHz. But a signal at 580 + 455 + 455 = 1490 kHz is also 455 kHz away from the local oscillator; so both the desired signal and the image, when mixed with the local oscillator, will appear at the intermediate frequency. This image frequency is within the AM broadcast band. Practical receivers have a tuning stage before the converter, to greatly reduce the amplitude of image frequency signals; additionally, broadcasting stations in the same area have their frequencies assigned to avoid such images[citation needed].

The unwanted frequency is called the image of the wanted frequency, because it is the "mirror image" of the desired frequency reflected about f_{o}\!. A receiver with inadequate filtering at its input will pick up signals at two different frequencies simultaneously: the desired frequency and the image frequency. A radio reception which happens to be at the image frequency can interfere with reception of the desired signal, and noise (static) around the image frequency can decrease the receiver's signal-to-noise ratio (SNR) by up to 3dB.

Early Autodyne receivers typically used IFs of only 150 kHz or so. As a consequence, most Autodyne receivers required greater front-end selectivity, often involving double-tuned coils, to avoid image interference. With the later development of tubes able to amplify well at higher frequencies, higher IF frequencies came into use, reducing the problem of image interference. Typical consumer radio receivers have only a single tuned circuit in the RF stage.

Sensitivity to the image frequency can be minimized only by (1) a filter that precedes the mixer or (2) a more complex mixer circuit [20] to suppress the image; this is rarely used. In most tunable receivers using a single IF frequency, the RF stage includes at least one tuned circuit in the RF front end whose tuning is performed in tandem with the local oscillator. In double (or triple) conversion receivers in which the first conversion uses a fixed local oscillator, this may rather be a fixed bandpass filter which accommodates the frequency range being mapped to the first IF frequency range.

Image rejection is an important factor in choosing the intermediate frequency of a receiver. The farther apart the bandpass frequency and the image frequency are, the more the bandpass filter will attenuate any interfering image signal. Since the frequency separation between the bandpass and the image frequency is 2f_\mathrm{IF}\!, a higher intermediate frequency improves image rejection. It may be possible to use a high enough first IF that a fixed-tuned RF stage can reject any image signals.

The ability of a receiver to reject interfering signals at the image frequency is measured by the image rejection ratio. This is the ratio (in decibels) of the output of the receiver from a signal at the received frequency, to its output for an equal-strength signal at the image frequency.

Local oscillator radiation[edit]

Further information: Electromagnetic compatibility

It can be difficult to keep stray radiation from the local oscillator below the level that a nearby receiver can detect. If the receiver's local oscillator can reach the antenna it will act as a low-power CW transmitter. Consequently, what is meant to be a receiver can itself create radio interference.

In intelligence operations, local oscillator radiation gives a means to detect a covert receiver and its operating frequency. The method was used by MI5 during Operation RAFTER.[21] This same technique is also used in radar detector detectors used by traffic police in jurisdictions where radar detectors are illegal.

Local oscillator radiation is most prominent in receivers in which the antenna signal is connected directly to the mixer (which itself receives the local oscillator signal) rather than from receivers in which an RF amplifier stage is used in between. Thus it is more of a problem with inexpensive receivers and with receivers at such high frequencies (especially microwave) where RF amplifying stages are difficult to implement.

Local oscillator sideband noise[edit]

Local oscillators typically generate a single frequency signal that has negligible amplitude modulation but some random phase modulation which spreads some of the signal's energy into sideband frequencies. That causes a corresponding widening of the receiver's frequency response[dubious – discuss], which would defeat the aim to make a very narrow bandwidth receiver such as to receive low-rate digital signals. Care needs to be taken to minimize oscillator phase noise, usually by ensuring[dubious – discuss] that the oscillator never enters a non-linear mode.

Terminology[edit]

First detector, second detector
The mixer tube or transistor is sometimes called the first detector[citation needed], while the demodulator that extracts the modulation from the IF signal is called the second detector. In a dual-conversion superhet there are two mixers, so the demodulator is called the third detector.
RF front end
Refers to all the components of the receiver up to and including the mixer; all the parts that process the signal at the original incoming radio frequency. In the block diagram above the RF front end components are colored red.

See also[edit]

Notes[edit]

  1. ^Although, in practice, LOs tend to be relatively strong signals.

References[edit]

  1. ^ abcdKlooster, John W. (2009). Icons of Invention: The Makers of the Modern World from Gutenberg to Gates. ABC-CLIO. p. 414. ISBN . Retrieved 2017-10-22.
  2. ^Leutz, C. R. (December 1922). "Notes on a Super-Heterodyne". QST. Hartford, CT, USA: American Radio Relay League. VI (5): 11–14 [11].
  3. ^Malanowski, Gregory (2011). The Race for Wireless: How Radio Was Invented (or Discovered?). Authorhouse. p. 69. ISBN .
  4. ^Katz, Eugenii. "Edwin Howard Armstrong". History of electrochemistry, electricity, and electronics. Eugenii Katz homepage, Hebrew Univ. of Jerusalem. Archived from the original on 2009-10-22. Retrieved 2008-05-10.
  5. ^Bussey, Gorden (1990). Wireless: the crucial decade - History of the British wireless industry 1924–34. IEE History of Technology Series. 13. London, UK: Peter Peregrinus Ltd. / Institution of Electrical Engineers. p. 78. ISBN . ISBN 978-0-86341-188-5. Archived from the original on 2021-07-11. Retrieved 2021-07-11. (136 pages)
  6. ^ abKoster, John (2016-12-03). "Radio Lucien Lévy". Vintage Radio Web. Retrieved 2017-10-22.
  7. ^Howarth, Richard J. (2017-05-27). Dictionary of Mathematical Geosciences: With Historical Notes. Springer. p. 12. ISBN . Retrieved 2017-10-22.
  8. ^"The History of Amateur Radio". Luxorion. Retrieved 2011-01-19.
  9. ^Sarkar, Tapan K.; Mailloux, Robert J.; Oliner, Arthur A.; Salazar-Palma, Magdalena; Sengupta, Dipak L. (2006). History of Wireless. John Wiley and Sons. p. 110?. ISBN .
  10. ^ abcdCarr, Joseph J. (2002). "Chapter 3". RF Components and Circuits. Newnes. ISBN .
  11. ^Hagen, Jon B. (1996-11-13). Radio-frequency electronics: circuits and applications. Technology & Engineering. Cambridge University Press. p. 58, l. 12. ISBN . Retrieved 2011-01-17.
  12. ^The art of electronics. Cambridge University Press. 2006. p. 886. ISBN . Retrieved 2011-01-17.
  13. ^Terman, Frederick Emmons (1943). Radio Engineers' Handbook. New York, USA: McGraw-Hill. pp. 649–652.. (NB. Describes design procedure for tracking with a pad capacitor in the Chebyshev sense.)
  14. ^ abRohde, Ulrich L.; Bucher, T. T. N. (1988). Communications Receivers: Principles & Design. New York, USA: McGraw-Hill. pp. 44–55, 155–164. ISBN .. (NB. Discusses frequency tracking, image rejection and includes an RF filter design that puts transmission zeros at both the local oscillator frequency and the unwanted image frequency.)
  15. ^Langford-Smith, Fritz, ed. (November 1941) [1940]. Radiotron Designer's Handbook(PDF) (4th impression, 3rd ed.). Sydney, Australia / Harrison, New Jersey, USA: Wireless Press for Amalgamated Wireless Valve Company Pty. Ltd. / RCA Manufacturing Company, Inc. p. 102. Archived(PDF) from the original on 2021-02-03. Retrieved 2021-07-10. (352 pages) (Also published as Radio Designer's Handbook. London: Wireless World, 1940.)
  16. ^"A Three Tube Regenerodyne Receiver". Retrieved 2018-01-27.
  17. ^"Crystal filter types". QSL RF Circuit Design Ideas. Retrieved 2011-01-17.
  18. ^"Reception of Amplitude Modulated Signals - AM Demodulation"(PDF). BC Internet education. 2007-06-14. Retrieved 2011-01-17.
  19. ^"Chapter 5". Basic Radio Theory. TSCM Handbook. Retrieved 2011-01-17.
  20. ^Kasperkovitz, Wolfdietrich Georg (2007) [2002]. "United States Patent 7227912 Receiver with mirror frequency suppression".
  21. ^Wright, Peter (1987). Spycatcher: The Candid Autobiography of a Senior Intelligence Officer. Penguin Viking. ISBN .

Further reading[edit]

  • Whitaker, Jerry (1996). The Electronics Handbook. CRC Press. p. 1172. ISBN .
  • US 706740, Fessenden, Reginald A., "Wireless Signaling", published September 28, 1901, issued August 12, 1902 
  • US 1050441, Fessenden, Reginald A., "Electric Signaling Apparatus", published July 27, 1905, issued January 14, 1913 
  • US 1050728, Fessenden, Reginald A., "Method of Signaling", published August 21, 1906, issued January 14, 1913 
  • Witts, Alfred T. (1936). The Superheterodyne Receiver (2nd ed.). London, UK: Sir Isaac Pitman & Sons.

External links[edit]

Sours: https://en.wikipedia.org/wiki/Superheterodyne_receiver
Super Heterodyne Receiver basics, working, block diagram \u0026 Image Frequency by Engineering Funda

Heterodyne

Signal processing technique

This article is about waveform manipulation. For other uses, see Heterodyne (disambiguation).

Frequency mixer symbol used in schematic diagrams

A heterodyne is a signalfrequency that is created by combining or mixing two other frequencies using a signal processing technique called heterodyning, which was invented by Canadian inventor-engineer Reginald Fessenden.[1][2][3] Heterodyning is used to shift one frequency range into another, new frequency range, and is also involved in the processes of modulation and demodulation.[2][4] The two input frequencies are combined in a nonlinear signal-processing device such as a vacuum tube, transistor, or diode, usually called a mixer.[2]

In the most common application, two signals at frequencies f1 and f2 are mixed, creating two new signals, one at the sum of the two frequencies f1 + f2, and the other at the difference between the two frequencies f1 − f2.[3] The new signal frequencies are called heterodynes. Typically, only one of the heterodynes is required and the other signal is filtered out of the output of the mixer. Heterodyne frequencies are related to the phenomenon of "beats" in acoustics.[2][5][6]

A major application of the heterodyne process is in the superheterodyne radio receiver circuit, which is used in virtually all modern radio receivers.

History[edit]

Fessenden's heterodyne radio receiver circuit. The incoming radio frequency and local oscillator frequency mix in the crystal diode detector.

In 1901, Reginald Fessenden demonstrated a direct-conversion heterodyne receiver or beat receiver as a method of making continuous waveradiotelegraphy signals audible.[7] Fessenden's receiver did not see much application because of its local oscillator's stability problem. A stable yet inexpensive local oscillator was not available until Lee de Forest invented the triode vacuum tube oscillator.[8] In a 1905 patent, Fessenden stated that the frequency stability of his local oscillator was one part per thousand.[9]

In radio telegraphy, the characters of text messages are translated into the short duration dots and long duration dashes of Morse code that are broadcast as radio signals. Radio telegraphy was much like ordinary telegraphy. One of the problems was building high power transmitters with the technology of the day. Early transmitters were spark gap transmitters. A mechanical device would make sparks at a fixed but audible rate; the sparks would put energy into a resonant circuit that would then ring at the desired transmission frequency (which might be 100 kHz). This ringing would quickly decay, so the output of the transmitter would be a succession of damped waves. When these damped waves were received by a simple detector, the operator would hear an audible buzzing sound that could be transcribed back into alpha-numeric characters.

With the development of the arc converter radio transmitter in 1904, continuous wave (CW) modulation began to be used for radiotelegraphy. CW Morse code signals are not amplitude modulated, but rather consist of bursts of sinusoidal carrier frequency. When CW signals are received by an AM receiver, the operator does not hear a sound. The direct-conversion (heterodyne) detector was invented to make continuous wave radio-frequency signals audible.[10]

The "heterodyne" or "beat" receiver has a local oscillator that produces a radio signal adjusted to be close in frequency to the incoming signal being received. When the two signals are mixed, a "beat" frequency equal to the difference between the two frequencies is created. By adjusting the local oscillator frequency correctly, the beat frequency is in the audio range, and can be heard as a tone in the receiver's earphones whenever the transmitter signal is present. Thus the Morse code "dots" and "dashes" are audible as beeping sounds. This technique is still used in radio telegraphy, the local oscillator now being called the beat frequency oscillator or BFO. Fessenden coined the word heterodyne from the Greek roots hetero- "different", and dyn- "power" (cf. δύναμις or dunamis).[11]

Superheterodyne receiver[edit]

Block diagram of a typical superheterodyne receiver. Redparts are those that handle the incoming radio frequency (RF) signal; greenare parts that operate at the intermediate frequency (IF), while blueparts operate at the modulation (audio) frequency.

An important and widely used application of the heterodyne technique is in the superheterodyne receiver (superhet), which was invented by U.S. engineer Edwin Howard Armstrong in 1918. In the typical superhet, the incoming radio frequency signal from the antenna is mixed (heterodyned) with a signal from a local oscillator (LO) to produce a lower fixed frequency signal called the intermediate frequency (IF) signal. The IF signal is amplified and filtered and then applied to a detector that extracts the audio signal; the audio is ultimately sent to the receiver's loudspeaker.

The superheterodyne receiver has several advantages over previous receiver designs. One advantage is easier tuning; only the RF filter and the LO are tuned by the operator; the fixed-frequency IF is tuned ("aligned") at the factory and is not adjusted. In older designs such as the tuned radio frequency receiver (TRF), all of the receiver stages had to be simultaneously tuned. In addition, since the IF filters are fixed-tuned, the receiver's selectivity is the same across the receiver's entire frequency band. Another advantage is that the IF signal can be at a much lower frequency than the incoming radio signal, and that allows each stage of the IF amplifier to provide more gain. To first order, an amplifying device has a fixed gain-bandwidth product. If the device has a gain-bandwidth product of 60 MHz, then it can provide a voltage gain of 3 at an RF of 20 MHz or a voltage gain of 30 at an IF of 2 MHz. At a lower IF, it would take fewer gain devices to achieve the same gain. The regenerative radio receiver obtained more gain out of one gain device by using positive feedback, but it required careful adjustment by the operator; that adjustment also changed the selectivity of the regenerative receiver. The superheterodyne provides a large, stable gain and constant selectivity without troublesome adjustment.

The superior superheterodyne system replaced the earlier TRF and regenerative receiver designs, and since the 1930s most commercial radio receivers have been superheterodynes.

Applications[edit]

Heterodyning, also called frequency conversion, is used very widely in communications engineering to generate new frequencies and move information from one frequency channel to another. Besides its use in the superheterodyne circuit found in almost all radio and television receivers, it is used in radio transmitters, modems, satellite communications and set-top boxes, radar, radio telescopes, telemetry systems, cell phones, cable television converter boxes and headends, microwave relays, metal detectors, atomic clocks, and military electronic countermeasure (jamming) systems.

Up and down converters[edit]

In large scale telecommunication networks such as telephone network trunks, microwave relay networks, cable television systems, and communication satellite links, large bandwidth capacity links are shared by many individual communication channels by using heterodyning to move the frequency of the individual signals up to different frequencies, which share the channel. This is called frequency division multiplexing (FDM).

For example, a coaxial cable used by a cable television system can carry 500 television channels at the same time because each one is given a different frequency, so they do not interfere with one another. At the cable source or headend, electronic upconverters convert each incoming television channel to a new, higher frequency. They do this by mixing the television signal frequency, fCH with a local oscillator at a much higher frequency fLO, creating a heterodyne at the sum fCH + fLO, which is added to the cable. At the consumer's home, the cable set top box has a downconverter that mixes the incoming signal at frequency fCH + fLO with the same local oscillator frequency fLO creating the difference heterodyne frequency, converting the television channel back to its original frequency: (fCH + fLO) − fLOfCH. Each channel is moved to a different higher frequency. The original lower basic frequency of the signal is called the baseband, while the higher channel it is moved to is called the passband.

Analog videotape recording[edit]

Many analog videotape systems rely on a downconverted color subcarrier to record color information in their limited bandwidth. These systems are referred to as "heterodyne systems" or "color-under systems". For instance, for NTSC video systems, the VHS (and S-VHS) recording system converts the color subcarrier from the NTSC standard 3.58 MHz to ~629 kHz.[12]PAL VHS color subcarrier is similarly downconverted (but from 4.43 MHz). The now-obsolete 3/4" U-matic systems use a heterodyned ~688 kHz subcarrier for NTSC recordings (as does Sony's Betamax, which is at its basis a 1/2″ consumer version of U-matic), while PAL U-matic decks came in two mutually incompatible varieties, with different subcarrier frequencies, known as Hi-Band and Low-Band. Other videotape formats with heterodyne color systems include Video-8 and Hi8.[13]

The heterodyne system in these cases is used to convert quadrature phase-encoded and amplitude modulated sine waves from the broadcast frequencies to frequencies recordable in less than 1 MHz bandwidth. On playback, the recorded color information is heterodyned back to the standard subcarrier frequencies for display on televisions and for interchange with other standard video equipment.

Some U-matic (3/4″) decks feature 7-pin mini-DIN connectors to allow dubbing of tapes without conversion, as do some industrial VHS, S-VHS, and Hi8 recorders.

Music synthesis[edit]

The theremin, an electronic musical instrument, traditionally uses the heterodyne principle to produce a variable audio frequency in response to the movement of the musician's hands in the vicinity of one or more antennas, which act as capacitor plates. The output of a fixed radio frequency oscillator is mixed with that of an oscillator whose frequency is affected by the variable capacitance between the antenna and the musician's hand as it is moved near the pitch control antenna. The difference between the two oscillator frequencies produces a tone in the audio range.

The ring modulator is a type of frequency mixer incorporated into some synthesizers or used as a stand-alone audio effect.

Optical heterodyning[edit]

Optical heterodyne detection (an area of active research) is an extension of the heterodyning technique to higher (visible) frequencies. This technique could greatly improve optical modulators, increasing the density of information carried by optical fibers. It is also being applied in the creation of more accurate atomic clocks based on directly measuring the frequency of a laser beam. See NIST subtopic 9.07.9-4.R for a description of research on one system to do this.[14][15]

Since optical frequencies are far beyond the manipulation capacity of any feasible electronic circuit, all visible frequency photon detectors are inherently energy detectors not oscillating electric field detectors. However, since energy detection is inherently "square-law" detection, it intrinsically mixes any optical frequencies present on the detector. Thus, sensitive detection of specific optical frequencies necessitates optical heterodyne detection, in which two different (close-by) wavelengths of light illuminate the detector so that the oscillating electrical output corresponds to the difference between their frequencies. This allows extremely narrow band detection (much narrower than any possible color filter can achieve) as well as precision measurements of phase and frequency of a light signal relative to a reference light source, as in a laser Doppler vibrometer.

This phase sensitive detection has been applied for Doppler measurements of wind speed, and imaging through dense media. The high sensitivity against background light is especially useful for lidar.

In optical Kerr effect (OKE) spectroscopy, optical heterodyning of the OKE signal and a small part of the probe signal produces a mixed signal consisting of probe, heterodyne OKE-probe and homodyne OKE signal. The probe and homodyne OKE signals can be filtered out, leaving the heterodyne frequency signal for detection.

Heterodyne detection is often used in interferometry but usually confined to single point detection rather than widefield interferometry, however, widefield heterodyne interferometry is possible using a special camera.[16] Using this technique which a reference signal extracted from a single pixel it is possible to build a highly stable widefield heterodyne interferometer by removing the piston phase component caused by microphonics or vibrations of the optical components or object.[17]

Mathematical principle[edit]

Heterodyning is based on the trigonometric identity:

\sin \theta _{1}\sin \theta _{2}={\frac {1}{2}}\cos(\theta _{1}-\theta _{2})-{\frac {1}{2}}\cos(\theta _{1}+\theta _{2})

The product on the left hand side represents the multiplication ("mixing") of a sine wave with another sine wave. The right hand side shows that the resulting signal is the difference of two sinusoidal terms, one at the sum of the two original frequencies, and one at the difference, which can be considered to be separate signals.

Using this trigonometric identity, the result of multiplying two sine wave signals \sin(2\pi f_{1}t)\, and \sin(2\pi f_{2}t)\, at different frequencies f_{1} and f_{2} can be calculated:

\sin(2\pi f_{1}t)\sin(2\pi f_{2}t)={\frac {1}{2}}\cos[2\pi (f_{1}-f_{2})t]-{\frac {1}{2}}\cos[2\pi (f_{1}+f_{2})t]\,

The result is the sum of two sinusoidal signals, one at the sum f1 + f2 and one at the difference f1 − f2 of the original frequencies.

Mixer[edit]

The two signals are combined in a device called a mixer. As seen in the previous section, an ideal mixer would be a device that multiplies the two signals. Some widely used mixer circuits, such as the Gilbert cell, operate in this way, but they are limited to lower frequencies. However, any nonlinear electronic component also multiplies signals applied to it, producing heterodyne frequencies in its output—so a variety of nonlinear components serve as mixers. A nonlinear component is one in which the output current or voltage is a nonlinear function of its input. Most circuit elements in communications circuits are designed to be linear. This means they obey the superposition principle; if F(v) is the output of a linear element with an input of v:

F(v_{1}+v_{2})=F(v_{1})+F(v_{2})\,

So if two sine wave signals at frequencies f1 and f2 are applied to a linear device, the output is simply the sum of the outputs when the two signals are applied separately with no product terms. Thus, the function F must be nonlinear to create mixer products. A perfect multiplier only produces mixer products at the sum and difference frequencies (f1 ± f2), but more general nonlinear functions produce higher order mixer products: nf1 + mf2 for integers n and m. Some mixer designs, such as double-balanced mixers, suppress some high order undesired products, while other designs, such as harmonic mixers exploit high order differences.

Examples of nonlinear components that are used as mixers are vacuum tubes and transistors biased near cutoff (class C), and diodes. Ferromagnetic coreinductors driven into saturation can also be used at lower frequencies. In nonlinear optics, crystals that have nonlinear characteristics are used to mix laser light beams to create optical heterodyne frequencies.

Output of a mixer[edit]

To demonstrate mathematically how a nonlinear component can multiply signals and generate heterodyne frequencies, the nonlinear function F can be expanded in a power series (MacLaurin series):

{\displaystyle F(v)=\alpha _{1}v+\alpha _{2}v^{2}+\alpha _{3}v^{3}+\cdots \,}

To simplify the math, the higher order terms above α2 are indicated by an ellipsis (". . .") and only the first terms are shown. Applying the two sine waves at frequencies ω1 = 2πf1 and ω2 = 2πf2 to this device:

v_{\text{out}}=F(A_{1}\sin \omega _{1}t+A_{2}\sin \omega _{2}t)\,
{\displaystyle v_{\text{out}}=\alpha _{1}(A_{1}\sin \omega _{1}t+A_{2}\sin \omega _{2}t)+\alpha _{2}(A_{1}\sin \omega _{1}t+A_{2}\sin \omega _{2}t)^{2}+\cdots \,}
{\displaystyle v_{\text{out}}=\alpha _{1}(A_{1}\sin \omega _{1}t+A_{2}\sin \omega _{2}t)+\alpha _{2}(A_{1}^{2}\sin ^{2}\omega _{1}t+2A_{1}A_{2}\sin \omega _{1}t\sin \omega _{2}t+A_{2}^{2}\sin ^{2}\omega _{2}t)+\cdots \,}

It can be seen that the second term above contains a product of the two sine waves. Simplifying with trigonometric identities:

{\displaystyle {\begin{aligned}v_{\text{out}}={}&\alpha _{1}(A_{1}\sin \omega _{1}t+A_{2}\sin \omega _{2}t)\\&{}+\alpha _{2}\left({\frac {A_{1}^{2}}{2}}[1-\cos 2\omega _{1}t]+A_{1}A_{2}[\cos(\omega _{1}t-\omega _{2}t)-\cos(\omega _{1}t+\omega _{2}t)]+{\frac {A_{2}^{2}}{2}}[1-\cos 2\omega _{2}t]\right)+\cdots \end{aligned}}}
{\displaystyle v_{\text{out}}=\alpha _{2}A_{1}A_{2}\cos(\omega _{1}-\omega _{2})t-\alpha _{2}A_{1}A_{2}\cos(\omega _{1}+\omega _{2})t+\cdots \,}

So the output contains sinusoidal terms with frequencies at the sum ω1 + ω2 and difference ω1 − ω2 of the two original frequencies. It also contains terms at the original frequencies and at multiples of the original frequencies 2ω1, 2ω2, 3ω1, 3ω2, etc.; the latter are called harmonics, as well as more complicated terms at frequencies of 1 + 2, called intermodulation products. These unwanted frequencies, along with the unwanted heterodyne frequency, must be filtered out of the mixer output by an electronic filter to leave the desired frequency.

See also[edit]

Notes[edit]

  1. ^Christopher E. Cooper (January 2001). Physics. Fitzroy Dearborn Publishers. pp. 25–. ISBN .
  2. ^ abcdUnited States Bureau of Naval Personnel (1973). Basic Electronics. USA: Courier Dover. p. 338. ISBN .
  3. ^ abGraf, Rudolf F. (1999). Modern dictionary of electronics (7th ed.). USA: Newnes. p. 344. ISBN .
  4. ^Horowitz, Paul; Hill, Winfield (1989). The Art of Electronics (2nd ed.). London: Cambridge University Press. pp. 885, 897. ISBN .
  5. ^Strange, Allen; Strange, Patricia (2003). The Contemporary Violin: Extended Performance Techniques. Scarecrow Press. p. 216. ISBN .
  6. ^Ingard, Uno (2008). Acoustics. Jones and Bartlett. pp. 18–21. ISBN .
  7. ^Discussion of A History of Some Foundations of Modern Radio-Electronic Technology, Comments by Lloyd Espenschied, Proceedings of the IRE, July, 1959 (Vol. 47, No. 7), pp. 1254, 1256. Critique. ". . . the roots of our modern technology trace back generally to sources other than the Hammond Laboratory." Comment. Many of the roots that nourished the work of the Hammond group and its contemporaries were recorded in our paper: the pioneering work of Wilson and Evans, Tesla, Shoemaker, in basic radiodynamics; . . . of Tesla and Fessenden leading to the development of basic intermediate frequency circuitry.
  8. ^Nahin 2001, p. 91, stating "Fessenden's circuit was ahead of its time, however, as there simply was no technology available then with which to build the required local oscillator with the necessary frequency stability." Figure 7.10 shows a simplified 1907 heterodyne detector.
  9. ^Fessenden 1905, p. 4
  10. ^Ashley, Charles Grinnell; Heyward, Charles Brian (1912). Wireless Telegraphy and Wireless Telephony. Chicago: American School of Correspondence. pp. 103/15–104/16.
  11. ^Tapan K. Sarkar, History of wireless, page 372
  12. ^Videotape formats using 1⁄2-inch-wide (13 mm) tape ; Retrieved 2007-01-01
  13. ^Charles, Poynton (2003). Digital Video and HDTV: Algorithms and Interfaces. San Francisco: Morgan Kaufmann Publishers. pp. 582–3. ISBN .
  14. ^Contract Details: Robust Nanopopous Ceramic Microsensor Platform
  15. ^Contract Details: High Pulsed Power Varactor Multipliers for Imaging
  16. ^Patel, R.; Achamfuo-Yeboah, S.; Light R.; Clark M. (2011). "Widefield heterodyne interferometry using a custom CMOS modulated light camera". Optics Express. 19 (24): 24546–24556. doi:10.1364/oe.19.024546. PMID 22109482.
  17. ^Patel, R.; Achamfuo-Yeboah, S.; Light R.; Clark M. (2012). "Ultrastable heterodyne interferometer system using a CMOS modulated light camera". Optics Express. 20 (16): 17722–17733. doi:10.1364/oe.20.017722. PMID 23038324.

References[edit]

  • US 1050441, Fessenden, Reginald A., "Electric Signaling Apparatus", published July 27, 1905, issued January 14, 1913 
  • Glinsky, Albert (2000), Theremin: Ether Music and Espionage, Urbana, IL: University of Illinois Press, ISBN 
  • Nahin, Paul J. (2001), The Science of Radio with Matlab and Electronics Workbench Demonstrations (second ed.), New York: Springer-Verlag, AIP Press, ISBN 

External links[edit]

  • Hogan, John V. L. (April 1921), "The Heterodyne Receiver", Electric Journal, 18: 116
  • US 706740, Fessenden, Reginald A., "Wireless Signaling", published September 28, 1901, issued August 12, 1902 
  • US 1050728, Fessenden, Reginald A., "Method of Signaling", published August 21, 1906, issued January 14, 1913 
Sours: https://en.wikipedia.org/wiki/Heterodyne

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