[Help] FM receiver problem

[mr_byte31]

Hi All,

i tried to make this board :
**broken link removed**

ref : **broken link removed**

The only difference I have on my small kit , the source is 3.5 v

my board :
**broken link removed**

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 ?

 

[BradtheRad]

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.

 

[mr_byte31]

I will try this.

what scares me is this sentence :

On Pic.3.43 you can se the electronic circuit of an extremely simple direct FM receiver. The T2 transistor together with the R1 resistor, the coil L the variable capacitor C and internal capacitances of the T1 transistor, comprises the so-called Kolpitz oscillator.

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.

- - - Updated - - -

I tried to simulate the circuit on multisim
the circuit doesn't oscillate at all !!!!

 

[Audioguru]

It is a Mickey Mouse circuit that probably does not work.

 

[jiripolivka]

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.

 

[betwixt]

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.

 

[Audioguru]

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.

 

[mr_byte31]

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.

 

[Audioguru]

Yes, don't waste time on garbage circuits.