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krisheleccom Posted - Jul 02 2010 : 08:37:09 AM
http://www.aaroncake.net/circuits/fmtrans.asp
sir i am an ug student .i was fascinated by the simple fm transmitter in the above url .i was working on that .i found it worked almost .but i was pleased to ask you the design of the above ckt..hoping for your valuable reply...any one can reply to my email id krisheleccom@gmail.com
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audioguru Posted - Jul 02 2010 : 12:40:43 PM
Q2 is a common-emitter preamp transistor. R4 provides AC and DC negative feedback.
Q1 is a common-base Colpitts oscillator. C2 provides positive feedback for the oscillations to continue. C1 grounds the base of Q1 at radio frequencies.
L2 and C5 are the tuned circuit.
R6 powers the electret microphone.

The ouput from Q2 causes Q1 to conduct more and less at audio frequencies. Because Q1 conducts more and less then its collector to emitter voltage drops and increases which changes its capacitance that changes the oscillation frequency which is FM. It also produces some AM that FM radios ignor.

Since the antenna is connected to the tuned circuit then the frequency changes when something moves towards or moves away from the antenna.
Since there is no voltage regulator and the battery voltage drops as it is used then the frequency also changes.
The simple circuit is missing pre-emphasis (treble audio boost) like FM radio stations have so it will sound bad (muffled) when heard on an FM radio because they all have de-emphasis.

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