[Help] FM receiver problem
i tried to make this board :
ref : http://www.mikroe.com/old/books/rrbo.../chapter3i.htm
The only difference I have on my small kit , the source is 3.5 v
my board :
I used my audio amplifier from PC but I couldn"t hear any channel when i tuned the variable capacitor.
I use a 40 cm wire as an antenna.
any suggestions for why this circuit doesn't work ?
For audio purposes, the transistors need to be biased so they are at a 'good operating point'. As a general guideline, this is about halfway between the full-on and full-off states.
To do this, I believe you should vary R1. Start at the 12k value, then decrease it in steps. Be careful not to reduce it below a value where components burn up. (Probably 500 or 1000 ohms.)
All this is on the assumption your L and C values will create a frequency that picks up an FM transmission.
I will try this.
what scares me is this sentence :
how can he depend on the internal capacitance of T1 ? it would be very small (less than 1 PF)!
i doubt if it would oscillate.
I tried to simulate the circuit on multisim
the circuit doesn't oscillate at all !
It is a Mickey Mouse circuit that probably does not work.
This circuit may work if you are close to a FM transmitter, or you need a better antenna than a piece of wire.
Good FM receivers need a quite large gain before discriminator, like 60 db, and a noise figure < 6 dB to work with a whip antenna. Your circuit may have ~10 dB gain, so do not expect miracles.
Aside from lack of sensitivity, relying on the junction capacitance of the transistor as part of a tuned circuit is madness. For one thing, it is conducting and even if it wasn't it would vary from transistor to transistor. It may be a 'Mickey Mouse' circuit but I think 'Pluto' did the design work!
Brian.
In addition to its poor sensitivity, when that extremely simple circuit was designed then there were probably only one or a couple of FM radio stations so its very poor selectivity didn't matter. But today my FM dial is full of many FM radio stations and I am glad that my real FM radios have many tuned circuits and crystal filters for good selectivity.
FM circuit should has oscillation circuit so it can demodulate ! I tested this circuit on Multisim but nothing at all !
It is disappointing to find such circuits on mikroe website ! I wasted quite time in it. :(
Yes, don't waste time on garbage circuits.