.png)
A FM transmitter is a device that uses the principles of frequency modulation to broadcast sound supplied at its input. Typical FM transmitter design’s usually follow the block diagram below;
The signal strength of audio inputs into the transmitter is usually low therefore an amplifier
is usually built to bring the signal level up. Based on the desired frequency for transmission
(which is usually between the FM frequency band, 88MHz to 108MHz),
the carrier frequency is generated using an oscillator circuit and mixed with audio signal to
create the modulated signal. The modulated signal is then passed through a power amplifier at
the transmission stage to create low impedance which is matched with the antenna.
Component Required:
2N2222 NPN Transistor x2
Condenser mic/ audio jack
100nf Ceramic capacitor x1
10nf ceramic capacitor x1
4 pf ceramic capacitor x1
100 ohms resistor x1
10k resistor x 3
1k resistor x 1
100k resistor x1
1M resistor x1
Variable Capacitor 20pf
Gauge 18 - 22 copper wire
9v battery
9v battery Cap
Circuit Diagram:
Connect the components as shown in the FM transmitter schematic below
Procedure:
The audio output signal from the microphone is usually small, the first transistor thus
performs the job of amplifying that signal to a level good enough for transmission.
After amplification as described earlier, the next stage of the FM transmitter is modulation.
At this stage the amplified audio signal is then mixed with the carrier frequency at with which
the signal is to be transmitted. This carrier frequency can be varied using the 20pF variable
capacitor connected with the inductor, and the typical frequency band of this particular design
is between the 88MHz to 108MHz and since there is no visual output to recognize the exact
frequency at which the transmitter is working, you will need to adjust your FM receiver radio
within the range of the frequencies mentioned to get the frequency at which the transmitter is
transmitting. After modulating the Audio signal with the carrier frequency, the signal is then
sent out through the antenna.
The air core inductor is made by winding a 8 to 10 turns of 18 – 22 gauge wire around a ¼ inch former which can be represented by a pencil. The values of components used for these tutorials are not stringent and for learning purpose, you can play around with the resistor and capacitor values to optimise the performance of the transmitter.
Comments