Description
This transmitter is basic but allows transmission of audio to an AM
radio. It consists of an RF oscillator operating in the AM broadcast
band, together with a modulator stage, which mixes the incoming audio
and the RF. A signal appears on the output, which has an AM component
that can be picked up on a nearby AM radio receiver.
Circuit Schematic Diagram
The transmitter consists of oscillator stage Q1 and modulator/buffer
stage Q2. Q1 is biased via R1, R2, and R3. L1, C3, and C4 form the tank
circuit with feedback network C3-C4 providing feedback to the emitter of
Q1. RF voltage at the junction of C3 and L1 drives buffer/modulator
stage Q2. Q2 is biased by base current produced by RF rectification in
the base emitter junction of Q2. C6 is an RF and AF bypass capacitor.
C9, C10, and L2 form the tank circuit for the collector of Q2. RF is
taken from the junction of C9 and C10 and fed to a short-wire antenna.
Audio is fed to modulator Q2 via C8 and isolation resistor R5 and mixes
with the RF signal in the collector circuit of Q2, producing a signal
that has sum and difference frequencies if the RF carrier and AF input
along with the carrier signal.
An AM signal appears at the collector of Q2. Audio with an RMS
voltage equal to about 0.7 times the collector voltage of Q2 is needed
for full modulation of the output. Because of the high level of audio
needed, the modulation obtained from this circuit is somewhat limited
with conventional audio sources because several volts of audio into a
few hundred ohms is needed. The circuit demonstrates the principle of an
AM transmitter, however, and with a suitable audio drive level,
produces a well modulated AM signal.
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