Crystal Detector
project of developing a FM receiver?
iam persuing b.e. ece .iam doing summer training nd they have given me a project of developing a FM receiver .i have just given my 4th sem. exams so i didnt have knowledge abt FM modulation nd stuff lyk that. i have done lots of searching on internet there r mne complex ckts available on net. Firstly i dont know whether to make a fm receiver wid using ic(tda) or not. I have tried to read abt tda 7000 but its of no use. secondly if iam making fm receiver without using ic still i dont able to understand the ckts given on the net even the simplest ones (as there r no explanations r given). i only have sme knowledge abt crystal detectors but they r not widely used and lots of noise problem also arises frm that kinds of ckts and the mainly ckts that r given on net r suprregenerative . pls suggest me a book or what topic i should read so that i could able to understand abt FM receivers.or smeone help me in understanding a ckts given on net. plz help me out.
FM receiver for audio.
Modulating frequency = max 10 to 15 kHz.
Deviation ratio = deviation / modulating frequency. Typicaly deviation is 50 kHz on either side. So an 88 MHz carrier deviates from centre value by 50 kHz on either side.
This is the maximum deviation, be the modulating frequency 10 kHz or 100 hz.
So use an FM modulator (typically an oscillator with a varicap where capacitance gets varied by modulating voltage of 100 hz or 10 kHZ).
So you have an FM, where in 88 MHz deviaties by +/- 50 kHZ. Now to receive this: use an IF of 10.7 MHz. So add a loal oscillator that transfers 88 MHz signal to 10.7 MHz. The bandwidth of 10.7MHz filter has to be 100kHz.
Not to demodulate this 10 MHz. Slope demodulation, and others. See
http://en.wikipedia.org/wiki/Foster-Seeley_discriminator
for details. One could also use a PLL, and one way or other, the frequencies between 100 hz and 10kHz is extracted. Then amplify this, feed it to a speaker.
And any word such as “squelch” etc, see wikipedia if you want more details. the receiver will work even without squelch.
In telecommunications, squelch is a circuit function that acts to suppress the audio (or video) output of a receiver in the absence of a sufficiently strong desired input signal.
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