FM TRANSMITTER
OBJECTIVE:TO BUILD ,DESIGN AND TEST FM TRANSMITTER
THEORY :
This
Mini FM Transmitter is a low power transmitter for VHF FM band, which transmits
sound signals in form of electronic waves. These waves are received by the
aerial of any FM receiver of the range 88 MHz to about 108 MHz. Speech and music etc. are sent to thousands of kilometers away by a radio transmitter. The scene in front of a television camera is also sent many kilometers away to viewers. Similarly a Moon probe or a Venus probe checking its environments, sends the information it gathers millions of kilometers through space to receivers on earth. In all these cases, the carrier is the high-frequency radio wave. The intelligence, i.e. sight, sound or other data collected by the probe is impressed on the radio wave and is carried along with it to the destination.
Modulation is the process of combining the low-frequency signals with a high-frequency radio wave called carrier wave (CW). The resultant wave is called modulated carrier wave. This job is done at transmitting station.
Demodulation is a process of separating or recovering the signal from the modulated carrier wave. It is just the opposite of modulation and is performed at the receiving end.
For successful transmission and reception of intelligence (code voice, music.), by the use of radio waves, two proceses are essential:
(i) Modulation and
(ii) Demodulation.
CIRCUIT DIAGRAM
Description
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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.
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.
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.
CIRCUIT
CALIBRATION:
Place the transmitter about 10 feet from a FM radio. Set the radio to somewhere about 89 - 90 MHz. Walk back to
the Fm Tx and turn it on. Spread the winding of the coil apart by approximately 1mm from each other. No coil winding should be touching another winding. Use a small screw driver to tune the trim cap. Remove the screwdriver from the trim screw after every adjustment so the LC circuit is not affected by stray capacitance. Or use a plastic screwdriver. If you have difficulty finding the transmitting frequency then have a second person tune up and down the FM dial after every adjustment. One full turn of the trim cap will cover its full range of capacitance from 6pF to 45pF. The normal FM band tunes in over about one tenth of the full range of the tuning cap. So it is best to adjust it in steps of 5 to 10 degrees at each turn. So tuning takes a little patience but is not difficult. The reason that there must be at least 10 ft. separation between the radio and the Tx is that the Tx emits harmonics; it does not only emit on one frequency but on several different frequencies close to each other.
Place the transmitter about 10 feet from a FM radio. Set the radio to somewhere about 89 - 90 MHz. Walk back to
the Fm Tx and turn it on. Spread the winding of the coil apart by approximately 1mm from each other. No coil winding should be touching another winding. Use a small screw driver to tune the trim cap. Remove the screwdriver from the trim screw after every adjustment so the LC circuit is not affected by stray capacitance. Or use a plastic screwdriver. If you have difficulty finding the transmitting frequency then have a second person tune up and down the FM dial after every adjustment. One full turn of the trim cap will cover its full range of capacitance from 6pF to 45pF. The normal FM band tunes in over about one tenth of the full range of the tuning cap. So it is best to adjust it in steps of 5 to 10 degrees at each turn. So tuning takes a little patience but is not difficult. The reason that there must be at least 10 ft. separation between the radio and the Tx is that the Tx emits harmonics; it does not only emit on one frequency but on several different frequencies close to each other.
LIST OF COMPONENTS
RESISTORS
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CAPACITORS
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TRANSISITORS
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MICE
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OTHERS
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68K
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22n
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PN2222-TWO TRANSISTORS ARE REQUIRED
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1-MICE ARE REQUIRED
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1-COPPER CLAD REQUIRED
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1M
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100n
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1-9V BATTERY
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||
10K
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2-10p variable capacitor
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1-ANTTENA
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47K
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39p
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|||
470R
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10p
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|||
1n
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||||
22n
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