Here is the circuit of a medium power crystal based amplitude modulation (AM) transmitter that can deliver 100-150mW of radio frequency (RF) power.
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| Circuit diagram of 150mW Crystal AM Transmitter : Click on image to enlarge |
The heart of this circuit is a crystal oscillator. A 10MHz crystal is used to produce highly stable carrier frequency. The sound signal from the condenser mic is amplified by the amplifier circuit consist of transistors Q1, Q2 and Q3. The amplified audio signal modulates the RF carrier generated by the crystal oscillator built around transistor Q4. Here, modulation is done via the power supply source line. The amplitude modulated (AM) signal is obtained at the collector of oscillator transisitor Q4.
The use of matching dipole antenna and co-axial cable, the transmission range can be increased. The external wire antenna radio receiver can be used to obtain maximum range. The circuit works off a 9V-12V battery.
For oscillator coil L1: wind 14turns of 30SWG wire round an 8mm diameter radio oscillator coil former witha ferrite bead.
For Modulation transformer X1: You can use audio output transformer found in old transistor radios. Alternatively, you can make it from E/I core transformer lamination with inner winding having 40 turns of 26SWG wire and outer winding having 200 turns of 30SWG.
For oscillator coil L1: wind 14turns of 30SWG wire round an 8mm diameter radio oscillator coil former witha ferrite bead.
For Modulation transformer X1: You can use audio output transformer found in old transistor radios. Alternatively, you can make it from E/I core transformer lamination with inner winding having 40 turns of 26SWG wire and outer winding having 200 turns of 30SWG.
Assemble the circuit on a general purpose PCB and enclose within a suitable cabinent.
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