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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:
- 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.
- You can use BF199 instead of BFR90.
- If you can't find the varicap diode, you can use two BB405 instead.
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