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VHF Center Frequency FM Transmitter

VHF Center Frequency FM Transmitter

The circuit was designed to function on the middle of the FM band, 98 MHz, since the FM frequency ranges from 88 MHz to 108 MHz.
The circuit works as the transmitted signal is a Frequency Modulatedwhich means that the carrier’s amplitude stays constant and its frequency varies according to the amplitude variations of the audio signal. When the input signal’s amplitude increases the frequency of the carrier increases too, and vice versa. For this circuit to be tuned at the center frequency (98 MHz) of the FM band, it utilizes an inductor rated at 0.112 uH with a variable capacitor VC1 in the center value of 33 pF. The inductor has a standard wire gauge of 22, made of enameled copper wire that created its winding 5 times around a 3/16” diameter and 5mm long bolt. Otherwise, an adjustable molded coil can be used to achieve a fixed capacitor value of 33 pF from the variable capacitor. Without setting the variable capacitor to a fixed value, the tuning can provide the range between 85 to 125 MHz.

To produce a repetitive electronic signal, to op-amp oscillators were used in the circuit. The electret microphone used was an omnidirectional type while the antenna is a single vertical wire above a ground plane known as whip, stub or monopole, around 76 cms of 22 swg copper wire. The combination of the transistor and LC oscillator (a variable capacitor in series with an inductor) produces a clapp oscillator on Q1 which prevents the circuit to have a variable feedback voltage and cause it to less likely achieve oscillation over a portion of the desired frequency range.

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