Post
by Albert H » Tue Aug 23, 2016 1:36 pm
The FM receiver is a fairly complicated beast, but it can be broken down into several pieces, to make understanding it a little easier:
Let's start at the aerial. Any piece of metal will act as an aerial. Ideally, it should have a length proportional to the wavelength of what you're going to receive, but in most cases it isn't - it's just a random length.
The next bit is the input filter and amplifier. Since the aerial isn't actually related in length to the wavelength (in most cases), the input stage will provide a crude (and inaccurate) impedance transformation to feed the input filter. Some cheaper receivers get away with omitting that amplifier, but it really can help if you're just using a telescopic aerial. This amplifier works at Band II and doesn't provide much gain - it's more to do with matching. Some more sophisticated receivers use a filtered, tuned amplifier here, for the very best sensitivity, but most don't bother.
The next two parts are intimately connected together, and in the case of really cheap receivers, may be done in just one transistor. We have a "local" oscillator (LO), which is running either 10.7MHz above or 10.7MHz below the incoming signal. This LO is usually PLL-tuned these days, but older receivers used VFOs (and some clever tricks to stabilise them). The mixer combines the incoming signal with the oscillator signal, to produce a number of mixing products. The strongest of these will be the 10.7MHz difference signal, and this is selected by a tuned circuit filter which is set to a fixed frequency of 10.7MHz.
This frequency conversion is done because it's easier to greatly amplify a signal at one frequency, rather than a range of frequencies. The incoming signal will be frequency-modulated - varying slightly in frequency to represent the audio - and this variation is maintained in the 10.7Mhz signal. 10.7MHz was chosen in the late '50s as the "industry standard" "Intermediate Frequency" - IF - because there was a conveniently unused bit of shortwave there and it was a low enough frequency to allow the use of cheap amplification components. Also, 10.7MHz filters can be reasonably small sized, allowing compact receivers.
Three things go on in the IF stages - lots of gain, filtering and - very importantly - "limiting". Usually, the signal is amplified to the point at which a diode clipper can go into heavy conduction. This slices the top and bottom off the signal, but the FM information is still there in the squarewaves that come out of the limiter. The best part is that it also slices the interference (the AM components) off the signal, leaving pure FM.
The last bit is the "discriminator". This turns FM back into audio (or stereo multiplex). Basically there are three ways of recovering the audio - the most popular is the "Foster-Seeley Discriminator" which uses a tightly-tuned 10.7MHz coil, and a couple of diodes measuring the amplitude of the signal across the coil - deriving a variable voltage that is the audio. The next option is to use a PLL (phase-locked loop) which is tuned to 10.7MHz and follows the incoming IF. The error voltage from the PLL is the audio! The last version is least frequently used and is called the "pulse-counting detector". This charges a capacitor at a rate determined by the incoming frequency, and measuring that voltage will provide the audio.....
Construction of good receivers used to be quite difficult, but the advent of mass-produced hi-fi gear meant that ICs for various parts of a receiver can be bought cheaply. My basic Band I or IV receiver uses a dual-gate FET input stage, dual-gate FET mixer, bipolar transistor crystal oscillator, a bipolar transistor for the first IF amplifier (and to match the ceramic filters I use), followed by a CA3189 IF amplifier / limiter / discriminator chip. The final stage is an op-amp to provide a good low-impedance modulation output. I also add an audio frequency PLL looking for either a 19kHz stereo pilot or a sub-audio tone to operate a relay to switch the main rig.
"Why is my rig humming?"
"Because it doesn't know the words!" 
