Can somebody help how message signal (words spoken near microphone) is modulated using LC circuit and how lc circuit oscillates at particular frequency by applying dc voltage. In the below circuit l1 is 0.1 uH and capacitor is vc10-100 pf.
Actually in frequency modulation increase in voltage of message signal increases the frequency of carrier frequency and decrease in the voltage of message signal decreases the carrier frequency .I want that concept to be linked to the circuit. Thanks in Advance
Answer
Andy is correct but perhaps it would be easier to look at a picture of what happens.
What you don't see in the circuit diagram is the collector-base 'capacitor' which varies according to the audio input signal. This effect is multiplied by the transistor (in the old days this effect was first seen in valves). The change in capacitance follows the audio signal.
In the second diagram we re-arrange the circuit to look at the equivalent AC circuit. C1 shorts out the base and MIC1 to ground and of course the positive and negative rails (to AC) are the same connection. This leaves the miller capacitance in parallel with the tank circuit. Changes in the miller capacitance (following the audio signal) change the resonant frequency of the tank circuit.
This isn't a particularly good circuit (linear, well behaved etc.) but it works for a simple FM bug.
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