Handbook, chapter 8. The chain: from antenna to sound
Between the antenna and the speaker stand a dozen blocks, and textbooks draw them all the same way: rectangles with arrows. The diagram is correct and explains nothing — it does not show the main question: why does a receiver need an oscillator of its own.
The answer is short. A filter that can cut out three kilohertz at fourteen megahertz is an expensive and unreliable thing. A filter that cuts the same three kilohertz at four hundred and fifty kilohertz is a cheap slab of quartz. So the signal is first moved down, and only then processed.
The same chain, but with frequencies
An oscillator at 14,655 MHz shifts 14,200 MHz down to 455 kHz. But 15,110 MHz lands there too — its difference from the oscillator is the same. Only the input circuit, ahead of the mixer, can reject the image.
Turn the receive frequency and watch the third block. Whatever you are receiving — eighty metres or ten — after the mixer it is always one and the same frequency. That is the whole idea.
The mixer: it adds and subtracts frequencies
A mixer is neither an amplifier nor a filter. It multiplies two signals, and four frequencies appear at its output: both inputs, their sum and their difference.
It is the difference that is wanted. Receive 14.200 MHz, put the local oscillator on 14.655 — the difference is 455 kHz. Move to 3.650 MHz, put the oscillator on 4.105 — 455 kHz again. The tuning knob on a receiver turns neither the filter nor the front end, but the local oscillator.
From there the whole chain works at one frequency. One filter, one amplifier, aligned once and for all — and that is where all the set's sensitivity and all its selectivity come from.
A receiver like this is called a superheterodyne, and in a hundred years nothing better has come into general use.
The image: the price of the convenience
A difference of 455 kHz comes not from one signal but from two. An oscillator at 14.655 gives the intermediate frequency both from 14.200 (our signal) and from 15.110 — because the difference is the same, only the sign differs.
That second frequency is called the image, and the receiver has no way at all of telling it from the wanted one: after the mixer they are absolutely identical.
From which there is one conclusion, and it shapes the whole architecture: the image has to be removed before the mixer. That is the job of the front end — the very first block, the one that looks optional. The higher the intermediate frequency, the further the image sits from the signal and the easier it is to cut off; the lower it is, the better the filter that can follow. All the double- and triple-conversion designs grew out of that compromise.
A cheap receiver without a proper front end hears a strong station at several places on the dial at once. That is not a fault and not interference: it is the image and other spurious responses. If a station «appears» on a band where it cannot possibly be, suspect the receiver, not the propagation.
Direct conversion: the same thing without an IF
There is a simpler route. Put the local oscillator right on the signal frequency and the difference is zero plus the audio frequency. That is, audio comes straight out of the mixer and no intermediate frequency is needed at all.
A receiver like that goes together in an evening from a dozen parts and works beautifully. The price of the simplicity: the only selectivity is the audio filter, and signals on both sides of the oscillator are heard at once.
This is the circuit to start soldering with, and we will come back to it in a chapter of its own — with the parts, the board and the sound of a first reception.
A transmitter is the same chain backwards
The microphone gives audio. A balanced modulator turns it into a signal around the intermediate frequency, a filter leaves one sideband (why just one is a matter of saving power), a mixer moves that to the working frequency, and an amplifier adds watts.
Hence «transceiver» rather than «a receiver and a transmitter in one box»: they really do share the oscillator, the filter and most of the chain. The set simply switches the direction.
What to take away from this
Three things; the rest will follow:
- The tuning knob turns the local oscillator, not the filter.
- After the mixer the frequency is always the same. So the selectivity of a receiver is a property of one single filter.
- The image is removed before the mixer, and nothing downstream can take it out afterwards.
Next — chapter 9, the antenna: why height matters more than the type of antenna.