A tutorial on the principles of radio receiver design. It continues on with a discussion of the possible construction of a short wave receiver from scrounged radio parts. I suggest using a fixed tuned IF as a receiver with tunable converters.

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Wednesday, 18-Mar-2015 17:14:14 PDT

AM RADIO RECEIVERS - Part 3


 

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What are the basics of AM radio receivers - Part 3?

In relation to am radio, a frequent email request I receive is for an "am radio kit". Previously on this page I said such a kit wasn't feasible, now it is.

AM/FM RADIO PROJECT KIT

AM/FM radio kit and training course contains 14 transistors and 5 diodes, a 52 page manual is divided into 9 lessons. Superheterodyne receiver of standard AM and FM broadcast frequencies. Makes an excellent classroom project.

This image is copyright © by Ian C. Purdie VK2TIP - photo of the AM/FM radio project kit

Photo courtesy Parts Express

Scroll down this Parts Express Electronics Project Kits Page for the AM/FM Radio Project Kit. A site I'm affiliated with.

Alternatively what you can and should do of course is:

(a)    scrounge as many unserviceable radios from relatives and friends as you can lay your hands on. Not the real old valve types, they're too valuable.
(b)    cannibalise those radios for parts.
(c)    design and build your own radio from those parts.
 

A Shortwave Receiver

Everything here assumes you have access to suitable soldering / desoldering equipment and are reasonably experienced. Basic test equipment is also necessary. If not then this is all a purely theoretical discussion.

O.K. let's consider the prospect of building our own short wave receiver from some scrounged bits and pieces. Please note this will NOT be a high performance unit because:

(a)    we can't afford high class crystal filters for the selectivity.
(b)    we're not going to have a high class digital synthesiser as a tuning aid.
(c)    we will not have a digital frequency display.
(d)    and for practical reasons we with be limited to one band

Let's look at what we can do with our shoestring budget as against what we would like, i.e. what compromises to we need to make? Obviously we will need to buy some extra bits and pieces. For this purpose we will work backwards (often a smart way to go).

Speaker

got plenty of them from spares.

Headphone or earphone

Surely we can scrounge one.

Audio amp

Obviously we can use that portion from a scrounged radio. We only need 200 - 500 mW of audio power output.

Volume control

Well the little transistor radio ones are a pain to use so we'll consider scrounging or buying one of the regular potentiometers (log type only NOT linear).

I.F. Amplifier

Now it's going to get a bit tricky. We want good gain with reasonable A.G.C.

Mixer

Active or passive mixer?

R.F. Amplifier

Below about 15 Mhz we don't want or need one, above that maybe.

Bandwidth

How wide a band is our single band receiver going to cover? I'd suggest 500 Khz is probably too much but we'll look at that prospect.

Case

How are we going to package this technological marvel of ours? Should look reasonably nice to show off to relatives and friends.

Summarising

There you go, I've left more questions hanging in the air than I have answered. See if we can refine our goal a bit. We need to reconcile what we would like against what we can do for relatively next to nothing.

I think the way to go is to build a fixed band tunable receiver and use converters to get to other bands. Now there are a lot of drawbacks with this approach but hey, an inferior radio is better than none at all.

Assuming we use one good cheap yet functional AM transistor radio as a fixed permanent I.F. we can do quite a lot. I did this back in 1957, but that was with a valve set.

We will for the moment assume we will be permanently setting this fixed I.F. receiver at 1500 Khz, nearly the end of the AM radio band. That figure is not sacred, you can change it to whatever you want - JUST MAKE SURE THERE IS NO RADIO STATION OPERATING IN YOUR AREA ON THAT FREQUENCY - DAY OR NIGHT.

Frequency Coverage

For the moment we'll consider the band 5 - 5.5 Mhz. With our fixed I.F. of 1500 Khz we need a local oscillator which covers 3.5 Mhz to 4 Mhz. This is a frequency ratio of 4/3.5 or 1.1428:1 which when squared comes to 1.306:1 this is the ratio our tuning capacitance will vary.

 

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Principles of Transistor Circuits -
Introduction to design of amplifiers, receivers and digital circuits - S.W. Amos, M.R. James - 416 pages

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