DSB-Modulator as simple AM test transmitter


For test and measurement purposes, a small AM modulated RF signal generator is a very useful tool for tuned the resonant circuits in my do-it-yourself receivers or in the old receivers from the tube era. The intentionally weak transmitter signal is fed directly into the test object without an antenna and is not transmitted further.
While searching for such a simple transmitter I came across the ancient mixer IC NE602 (SA602 or NE612/SA612) in one of my many tinkering boxes. It contains a Gilbert Cell Multiplier and an oscillator. With this and some transistors it should be possible to build a useful modulator. In the internet I could find some variants for a circuit as a basis for own experiments.

For first experiments I built up an RC phase shift oscillator with a frequency of 1 kHz and used it to drive the balanced input of the NE602 via a transistor. As LO the internal oscillator was used which oscillates to 1 MHz due to the external circuitry. The modulated AM signal is decoupled via a HF transformer. The signal was now displayed by an oscilloscope in XY mode as a modulation trapezoid. This makes it easy to check the signal quality and adjust the modulation level. Also in Yt mode the 1 MHz signal can be viewed unmodulated and modulated. Considering the very simple circuit it looks quite good. For AM modulation (with fully carrier) as required in broadcast receivers, the balanced NE602 input must be operated slightly off-center (setting with 20 kOhm trimming potentiometer).

A further modulation possibility was created by an audio input. For buffering the NE602, a small RF amplifier with low impedance output follows at the output. The whole circuit is built up on a breadboard. That was it!


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Photo and Diagram

RC Oscillator
1MHz Quartz-LO
80% Modulation XY
80% Modulation Yt
Transmitter 1
Transmitter 2


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