Simple FM Transmitter circuit diagram and details
Description
|
This FM
transmitter (FM Tx) is about the simplest and
most basic FM Tx it is possible to build and
have a useful
transmitting range. It is surprisingly powerful despite its small component count and 3V operating voltage. It will
easily penetrate over three floors of an apartment building and go over 300 meters in the open air. The circuit
we use is based on a proven Australian design. It may be tuned anywhere in the FM band. Or it may be tuned
outside the commercial M band for greater privacy. (Of course this means you must modify your FM radio to
be able to receive the transmission or have a broad-band FM receiver.) The output power of this FM Tx is below
the legal limits of many countries (eg, USA and Australia). However, some countries may ban ALL wireless transmissions without a license. It is the responsibility of the builder to check the legal requirements for the operation of this circuit and to obey them.
CIRCUIT DESCRIPTION:
The circuit is basically a radio frequency (RF) oscillator that operates around 100 MHz. Audio picked up and
amplified by the electret microphone is fed into the audio amplifier stage built around the first transistor. Output
from the collector is fed into the base of the second transistor where it modulates the resonant frequency of
the tank circuit (the 5 turn coil and the trimcap) by varying the junction capacitance of the transistor. Junction
capacitance is a function of the potential difference applied to the base of the transistor. The tank circuit is
connected in a Colpitts oscillator circuit.
transmitting range. It is surprisingly powerful despite its small component count and 3V operating voltage. It will
easily penetrate over three floors of an apartment building and go over 300 meters in the open air. The circuit
we use is based on a proven Australian design. It may be tuned anywhere in the FM band. Or it may be tuned
outside the commercial M band for greater privacy. (Of course this means you must modify your FM radio to
be able to receive the transmission or have a broad-band FM receiver.) The output power of this FM Tx is below
the legal limits of many countries (eg, USA and Australia). However, some countries may ban ALL wireless transmissions without a license. It is the responsibility of the builder to check the legal requirements for the operation of this circuit and to obey them.
CIRCUIT DESCRIPTION:
The circuit is basically a radio frequency (RF) oscillator that operates around 100 MHz. Audio picked up and
amplified by the electret microphone is fed into the audio amplifier stage built around the first transistor. Output
from the collector is fed into the base of the second transistor where it modulates the resonant frequency of
the tank circuit (the 5 turn coil and the trimcap) by varying the junction capacitance of the transistor. Junction
capacitance is a function of the potential difference applied to the base of the transistor. The tank circuit is
connected in a Colpitts oscillator circuit.
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