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UNDERSTANDING FM TRANSMITTER CIRCUIT HELP

Question asked by Jayzee217 on Jul 24, 2013
Latest reply on Jul 25, 2013 by KJBob

Hi everyone, this is my first post in this forum. I've bought one fm transmitter hobby project kit. The circuit diagram is shown below.

 

 

I did some calculations of voltage (shown in second image). However I'm not certain about some calculation cos when I bring the oscillosope probe near the transistor c1959 there's lot of noise.

 

 

So here are my questions

 

 

- why they put a voltage divider (which is connected to ground), which comprise of 150k and 4k7 (x2) resistors at the base of 1st transistor c945

 

 

- I think the first transistor c945 amplifies the voltage and current and inverts it. Is this correct? This voltage was 1.9v (with osc.) and it was a dc with small signal coming from the mic

 

 

- the second transistor of c945 again amplifies this signal and inverts again. which is in phase with the modulating signal. Is this correct?

 

 

- The output of the circuit, from the second c945 transistor is the modulating signal right?

 

 

- I don't understand how does the oscillator circuit, consisting of 2 C1959 transistors, produces oscillations (the carrier). Can any body tell me how this oscillation is produced?

 

 

- The pcb coils values were not given in the schematic, cos it was in the pcb and there was no inductance. Can i calculate the inductance in any way?

 

 

- the output frequency was 92.4 Mhz. So i used the resonant freqeuency formula, excluding the inductive resistance and capacitive resistance, the ideal formula (fr = 1/2pi root LC) to roughly calculate the inductance. Which was roughly 9nH. Any suggestions?

 

 

- I calculated the voltage difference in c1959. I don't know how that works. Can anyone plz tell me how this transistor operates and produce oscillations in this circuit?

 

- The Antenna has + and - connections. Is this a Dipole Antenna?

 

If anyone needs any requirements to answer these questions, plz let me know.

 

Thanks

fm transmitter.png

fm.png

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