OZ2OE Technical Home Page - X-tal signal generator.

Updated August 17'th 2000

Build a simple signal generator with very low phasenoise.

The finished generator.


A good receivers must be able to receive weak signals in the presence of strong nearby stations - provided of course that these strong signals are clean and without splatter.
In order to test receiver performance you need a signal generator with a very clean output. There is no need to have a variable frequency generator, as you can tune the receiver around the signal, instead of tuning the frequency of the signal.
A simple overtone x-tal oscillator can provide an excellent low sideband noise signal.

This is a 48 MHz overtone oscillator where the collector is tuned to the 3'rd harmonic on 144 MHz. All the components are mounted in a small metal box (no PCB). Collector coil is 3 3/4 turn of 0.7 mm Cu wire on a 5,0 mm form with tuning slug. Collector tap is 2 1/4 turn from the cold end. Transistor BFX89 can be substituted with other common VHF/UHF small signal types. Output may vary, so the -14 dB attenuation section can be changed to give the wanted output.

Output spectrum of the oscillator. It's rich in harmonics, but if your receiver has any degree of selectivity that should remove the unwanted products.
As the harmonic level will vary from unit to unit, you'll have to initially measure your harmonic level with a spectrum analyzer.
Regarding sideband noise I have not been able to measure it. Based on other work I've done on phase noise from x-tal oscillators, I expect it to be -120 dBc or less in 2.5 kHz bandwidth at 10 kHz or more from the carrier.

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