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!
 Breadboard |
 Module |
 RC Oscillator |
 1MHz Quartz-LO |
 Overmodulation |
 80% Modulation XY |
 80% Modulation Yt |
 Transmitter 1 |
 Transmitter 2 |
 Schematic |
