All HF bands low distortion sinusoidal XO

It is very often that you need a quick oscillator for testing purposes. If you have crystals in different frequencies, the lowest parts-count oscillator and the quickest to build, is the Pierce crystal oscillator.








The Pierce crystal oscillator is a well known circuit that oscillates at the fundamental mode of the crystal. Normally, there are no tuning elements and the only component that needs to be changed from band to band, is the crystal itself. Indeed this circuit accepts crystals from about 1MHz to 30MHz without any tuning.

However, as tested on a 1Mohm oscilloscope probe, the output signal is mostly distorted, with exception on some crystals in higher frequencies, where they cause the output signal to be low enough. Accidentally, I have found that a small variable capacitor from the gate to the GND can set the output level but also greatly defines the waveform shape to be a low distortion sinusoidal. Variable resistors on the gate/source/drain or alterations on the VCC did not seem to have a great effect on the waveform shape. It seems that the only component that really shapes the waveform, is the variable capacitor. The side effect of setting the capacitor is that the frequency of operation can change a little, but this is of less importance, stability and low distortion are more important in QRP operation.

This capacitor needs to be re-set every time a different frequency or type of crystal is used. I have tested many HF crystals and it seems that the optimum setting is somewhere between keeping the oscillator starting reliably on power-up and setting the output signal level at the maximum that allows a low distortion sinewave waveform to present at the output. Based on my crystals, the capacitor was set so that the output level of the oscillator was about 2.5-3Vpp when measuring with the 1Mohm oscilloscope probe. This level ensured reliable start up and low distortion sinewave at all HF bands! The oscillator is capable of greater output level at lower bands and still giving a low distortion sinewave, but I usually set the level to 2.5-3Vpp, as a means of keeping the level constant at all bands.

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