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I finally got FM, but it isnt loud enough.

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mstechca

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Thanks to NEETS (at tpub.com), I found how how to turn my receiver into an FM receiver. NEETS mentioned (and showed) the use of an astable multivibrator using two transistors, and that is what I did. With astable multivibrators, they have an on and off period. How do I determine the output frequency of the receiver? How should I adjust the timing for the on and off periods to get the best reception? and is the astable multivibrator frequency the on frequency + the off frequency? and is my frequency the sum of the frequency of the signal and the oscillator frequency?
My receiver does not use a mixer. I will upload a circuit later.
 

Scubasteve

New Member
I can't get your circuit off of that link, which doesn't help much.

You seem a bit confused on how the radio works and what exactly frequency is, you should review some basics.

A FM radio system consists of several components. The antenna brings in the FM spectrum primarily, but many others as well. Then you have a low noise amplifier in line with this, usually consisting of an automatic gain control loop that prevents the reciever from saturating and losing the signal under different conditions. Then a basic bandpass filter follows.. You are using a superheterodyne receiver, which means that you are converting the RF down to an intermediate frequency that you can work with. This is done by means of a local oscillator (usually converts to 10.7MHz) which is then fed into a FM discriminator tank, which detects the frequency changes and converts it to a voltage. This voltage is the audio that was modulated in.

There are too many design hints and etc. that can follow this, but remember your basic theory and take a close careful look at the circuit, and you should be able to see it. The quality of the frequency in regards to stability is very important.. In most tank circuits the temperature co-efficients of the components will make you lose your signal over a slight degree of temperature. You can do other things in the future if you want to improve your radio design.. Use a Phase locked loop, direct digital synthesis.. and the list goes on..

Steve
 

mstechca

New Member
try the link again. It does work. It worked now.
 
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