FM transmitter as Receiver

[mr_byte31]

Hi All,

I found this interesting video about converting the FM Tx as Rx : http://www.youtube.com/watch?v=zY3R8vT0798

I tried to do the same concept to my FM Tx : **broken link removed**

I tried to simulate the circuit , if R2 resistance is low the circuit doesn't oscillate.
Lets assume I used a high input impedance amplifier to fix this problem , the circuit will oscillate but I can't see the 32KHz modulated signal on R2 on multisim!

any suggestion ?

 

[Audioguru]

Why is your capacitor value for C5 as high as 3 Farads!! In the video it is trillions of times less at only 3pF. It forms a voltage divider with your C2 and C4 in parallel (the video has a single 100pF capacitor) so the transistor base gets almost nothing.

If the signal generator is FM modulated at 32kHz (not AM modulated) then its RF is forcing the transistor's frequency to follow it up and down with the FM. As the frequency passes the high impedance resonant frequency of the tuned RC on the collector, there is less collector load so output voltage swing increases. As the frequency passes the lower or upper peak then there is more collector load causing the output voltage swing to decrease. This is called "AM slope modulation" which is how a similar FM regenerative receiver works. So the output of the circuit is the carrier with AM and FM modulation at 32kHz.

The 32kHz FM modulation is such a small frequency compared to the 98MHz carrier that you cannot see it. You probably also cannot see the AM modulation.

 

[mr_byte31]

I am already aware that it function as FM and AM same time.

it was a mistake to put 3F on simulation, I fixed it to 3pF but couldn't find any change to simulation results.

so how would i verify its functionality as FM decoder ?

 

[Audioguru]

A regenerative FM radio decodes when it is tuned to one side of the station's frequency. The slope detection changes the FM into amplitude changes. Maybe a sim program never learnt about it.