This is an FM transmitter circuit found in Internet Link. Before doing ,I'm simulating in multisim. Since there is no mic input,I have given sine wave generator

1).What is frequency of wave must be given in the place of MIC(Audio signal range).

As per the information , here BB109 is used as fm modulator.

2).Can anyone Explain me about BB109 operation as fm modulator.?.

3).Can we give the solar energy as source ?.

4).How to set fequency of transmitting signal as 102 Mhz or anything else ?

FM Transmitter


1 Answer 1


Use 1kHz for the input sinwave.... Probably about 50mV peak to peak. It depends on the electret mic so you should check what the mic tells you in the specification.

BB109 is a varicap diode. It is reversed bias normally in operation. The more reverse bias you provide the lower the capacitance it gives across its terminals. The oscillation frequency is mainly governed by L1, C9, C4 and the BB109 - they form a "tuned circuit".

VR1 can set the bias on the BB109 and hence it can tune the nominal centre frequency of oscillation.

The microphone input produces a small voltage that superimposes itself on the BB109 bias - this therefore modulates the BB109 capacitance and therefore the frequency of the oscillator is modulated hence FM.

  • \$\begingroup\$ Can you give answers for 3 and 4th one \$\endgroup\$ Apr 9, 2013 at 18:24
  • \$\begingroup\$ @AbiRAM02 - Answer 3 requires a solar panel that is capable of supplying about 0.5W (I reckon) at the specified line voltages on the circuit. If you didn't use the ouput stage to boost power to antenna you'd need about 100 to 150mW from the solar panel but it's usable transmit distance would be a hundred or so metres. Answer 4 I did answer - use VR1 for adjustment of centre frequency. \$\endgroup\$
    – Andy aka
    Apr 9, 2013 at 18:41

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