On Sun, 14 Mar 2004 23:35:56 GMT, Gary Morton wrote:

I'm experimenting with various VCO configurations/designs for a general
coverage short wave receiver. The output of the VCO will drive a MOSFET mixer.

I am able to vary the gain of the oscillator. I get the best looking sine wave
at the point that oscillation just starts.

That is the nature of oscillators. As you then increase the gain, the amplitude
increases until some non-linearity prevents it - usually the available voltage
swing in a device. This non-linearity causes the harmonic content to appear and
rise to fairly significant levels, as you have observed.

Whether the harmonic levels you noted are of great significance depends on the
mixer attributes.

One technique often used is to ensure the limiting non-linearity occurs in a
separate gain stage following the oscillator. Gain control derived from that
stage is then fed back to the oscillator to keep it in that "just oscillating"
region where harmonic content is lowest. A separate take-off from the
oscillator through a second buffer stage is then used as the actual output.