Introduction to
Naval Weapons Engineering1. Know how a superheterodyne receiver works and what its advantages are.
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).
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.
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.
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:
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).
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.
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.