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cs200939

FM Transmitter help!

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hey guys I've been working on this FM transmitter for like a month and eventually my efforts payed off and i was able to make it work and it works awesome.Now as a matter of fact (which may be a little matter of shame too),I dont know a single bit how it works!.I Know theres and amplifier and an oscilattor used, but i dont know for sure and i also dont know what all the other capacitors and resistors,inductors doing there.I mean I am totaly nill about this project and it was just a luck!.So kindly tell me what each part of this circuit does and since i really want to study this FM phenomanen ,tell me any relative theories or articles that i should study in order to get in depth knowledge about this circuit's working and the theory behind it.Your help is highly demanded and appreciated.thanks

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Sorry.
Since you don't know anything about electronics then it might take me a few months to teach you what each part does in the extremely simple circuit.
Maybe you can find a tutorial in Google.

Basically R3, Q2, L1, C5, C6, the battery voltage and the distance of anything to the antenna set the FM radio frequency. A more complicated circuit has a stable radio frequency that does not wander all over the place like this one does.

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Read up on transistors and amplifier configurations.

I'll have a go at explaining it but you won't understand if you're new to electronics.

Q1 is configured as a common emitter amplifier with shunt feedback provided by R1 which also biases it at around half the supply voltage. This amplifies the signal from the microphone.

Q2 is the RF oscillator which is a tuned common base amplifier with positive feedback.  In a common base amplifier, the input is the emitter, the output is the collector and the base is connected to ground, as far as the signal is concerned. In this case, the base is DC biased around half the supply voltage by R3 but at RF it's connected 0V via C4, the collector load is L1 and C5 which form a resonant tank, which means it will have a high gain at the resonant frequency and nearly no gain at other frequencies and C6 connects the input of the amplifier to the output. With the input connected to the output the amplifier will oscillate, just like an audio amplifier will when the microphone is held too need the speaker. In this case the frequency of oscillation will be the same as the tank circuit's resonant frequency.

The audio amplifier is AC coupled to the RF oscillator via C3 which means the voltage on Q2's base will vary with the audio signal. This will have two effects:

  • The gain of Q2 will vary with the audio frequency so the amplitude of the output waveform will vary with the audio frequency so you'll get amplitude modulation.
  • Because the capacitance of Q2's junction varies depending on the collector voltage, the resonant frequency of the tank will alter as the transistor's capacitance adds to the value of C5. This is frequency modulation which is what the FM receiver picks up.


This circuit is not very good for the following reasons:
  • The frequency will alter as the power supply voltage varies.
  • The frequency will alter depending on the loading which means objects placed near the aerial will cause it to drift.
  • You get both AM and FM which is not idea.
  • And most importantly, it lacks pre-emphasis so the sound quality won't be great, fine for voice but not good enough for music. Commercial radio stations boost higher audio frequencies before transmitting to minimise the effects of high frequency noise. Radio receivers include a an RC filter to cut the gain at higher frequencies to restore the original sound quality which means there will be no treble when listening to your receiver.


If you don't understand any of this then read up on transistors, amplifiers etc.

Here are some links:
http://arcarc.xmission.com/PDF_Electronics/
http://en.wikipedia.org/wiki/Transistor

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