3V FM Transmitter for Short Distance - 88MHz to 108MHz

Description:

Features of the transmitter:
Supply Voltage:+3V DC
Current: 21mA
Output Frequency: 88MHz - 108MHz
Modulation: FM
Frequency Shift: ±30KHz
Input: Electret microphone or other input audio source

The important part of the circuit is formed of the Colpitts type oscillator. C3,C4,C5,C6,CD1-CD2 and L1 determines the frequency. BF982 and dual gate MOSFET are active parts in oscillator. When the input impedance of the MOSFET gate inputs are high, LC tank is not affected. However transistors force the LC tank and cause phase shift.

Two driver stages are added to isolate the antenna from oscillator. First stage (BF199) amplifies the low signal of the oscillator and works as a constant load. The second stage (BFR90) amplifies the signal going through the antenna some more. A short copper wire can be used as an antenna here. Attaching a large antenna to this circuit is unnecessary because the output power is low.

Notes: 

1. Coil 1: Winding wire must be 1 mm thick and isolated. Number of turns: 3.5 .  5mm core must be used and the distance between each turn must be 1mm.
2. You can use BF199 instead of BFR90.
3. If you can't find the varicap diode, you can use two BB405 instead. 

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Comments (2)

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Deepak aagri said:

i have no idia about output supplie.

February 16, 2008

Nermin said:

This seams to be very nice transmitter,actually this is finest transmitter for wireless microphone that I saw.Mosfet is very stable and there are two more stages to amplifie the output power.I had workes with such a transistors and they are great.With some small changes, and for example adding PLL,this could be very nice local FM broadcast station,with correct other parts,like stereo-coder,proper power unit,shielding ,casing and antenna.I will try some work with this.Good luck

May 17, 2008