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.

Personally, I prefer to use a square wave, or at least a strong sine wave
into any mixer....

The mixer itself is non-linear and so it will usually produce a number of
LO harmonics.

Driving the LO at a high level reduces the length of time the mixer is in
the 'semi-on/off' region as the LO transits through 0 VAC.

Generally speaking, that time near 0 VAC LO is where the mixer is most
prone to being overloaded and intermod products being created so a strong LO
drive usually results in somewhat better dynamic range for the mixer.

I plan on the filters before the mixer to keep signals at the LO harminoc
out of the mixer, and as noted in another posting, a simple lowpass will do
the trick in an upconverting Rx design.


Jim Pennell
N6BIU

Personally, I prefer to use a square wave, or at least a strong sine wave
into any mixer....

The mixer itself is non-linear and so it will usually produce a number of
LO harmonics.

Driving the LO at a high level reduces the length of time the mixer is in
the 'semi-on/off' region as the LO transits through 0 VAC.

Generally speaking, that time near 0 VAC LO is where the mixer is most
prone to being overloaded and intermod products being created so a strong LO
drive usually results in somewhat better dynamic range for the mixer.

I plan on the filters before the mixer to keep signals at the LO harminoc
out of the mixer, and as noted in another posting, a simple lowpass will do
the trick in an upconverting Rx design.


Jim Pennell
N6BIU