It doesn't matter whether you are a short wave listener, an A.M. radio dx'er, into hobby electronics or amateur radio design, the broad basic principles will still apply. Here we will briefly discuss the radio receiver basics as they apply to:
Basic crystal set.
A T.R.F. Receiver.
A Superhetrodyne Receiver.
The Reflex Receiver.
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1. The first receiver built by a hobbyist is usually the plain old crystal radio set. If you are unfamiliar with the design then check out the crystal radio set page.
2. The earliest receivers built were of the tuned radio frequency TRF variety. Here all the stages were made to tune simultaneously to the received frequency. Some tuned radio frequency TRF receivers we very elaborate but suffered a number of disadvantages overcome by the superhetrodyne principle.
3. A superhetrodyne receiver works on the principle the receiver has a local oscillator called a variable frequency oscillator or V.F.O.
This is a bit like having a little transmitter located within the receiver. Now if we still have our T.R.F. stages but then mix the received signal with our v.f.o. we get two other signals. (V.F.O. + R.F) and (V.F.O. - R.F).
In a traditional a.m. radio where the received signal is in the range 540 Khz to 1650 Khz the v.f.o. signal is always a constant 455 Khz higher or 995 Khz to 2105 Khz.
Several advantages arise from this and we will use our earlier example of the signal of 540 Khz:
(a) The input signal stages tune to 540 Khz. The adjacent channels do not matter so much now because the only signal to discriminate against is called the i.f. image. At 540 Khz the v.f.o. is at 995 Khz giving the constant difference of 455 Khz which is called the IF frequency. However a received frequency of v.f.o. + i.f. will also result in an i.f. frequency, i.e. 995 Khz + 455 Khz or 1450 Khz, which is called the i.f. image.
Put another way, if a signal exists at 1450 Khz and mixed with the vfo of 995 Khz we still get an i.f. of 1450 - 995 = 455 Khz. Double signal reception. Any reasonable tuned circuit designed for 540 Khz should be able to reject signals at 1450 Khz. And that is now the sole purpose of the r.f. input stage.
(b) At all times we will finish up with an i.f. signal of 455 Khz. It is relatively easy to design stages to give constant amplification, reasonable bandwidth and reasonable shape factor at this one constant frequency. Radio design became somewhat simplified but of course not without its associated problems.
We will now consider these principles in depth by discussing a fairly typical a.m. transistor radio of the very cheap variety.
I have chosen to begin radio receiver design with the cheap am radio because:
(a) nearly everyone either has one or can buy one quite cheaply. Don't buy an A.M. / F.M. type because it will only confuse you in trying to identify parts. Similarly don't get one of the newer I.C. types.
Just a plain old type probably with at least 3 transformers. One "red" core and the others likely "yellow" and "black" or "white". Inside will be a battery compartment, a little speaker, a circuit board with weird looking components, a round knob to control volume.
(b) most receivers will almost certainly for the most part follow the schematic diagram I have set out below.
(c) if I have included pictures you know I was able to borrow either a digital camera or had access to a scanner.
Important NOTE: If you can obtain discarded or broken "tranny's" (Australian for transistorised am radio receiver) by all means do so because they are a cheap source of valuable parts. So much so that to duplicate the receiver as a kit project for learning purposes costs about $A70 or $US45. Incredible. That is why colleges in Australia and elsewhere can not afford to present one as a kit for students to construct.
Figure 1 - a.m. bcb radio schematic
There are no parts values shown as this schematic is purely for illustration puposes.
Such a receiver includes a reflex amplifier and is one which is used to amplify at two frequencies - usually both the intermediate and audio frequencies.
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crystal radio set
am radio receivers
radio receiver basics
tuned radio frequency TRF receivers
reflex radio receivers
regenerative radio receivers
superhetrodyne radio receivers
fm radio receivers
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Updated 15th September, 2000