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. The 2nd harmonic is around -40dbC.
The voltage as scoped at the tank is around 1V ptp. If the gain is increased
the 2nd harmonic raises to -30dbC and the tank voltage to 4V ptp. I think this
is about the right drive level for the MOSFET mixer (yet to be built).

Just what difference will the level of 2nd harmonic make? Does it matter
whether I have -40 or -30dbC? I'm not sure what effect it will have on the
performance of the final receiver, but my gut feeling is that the better the
input to the mixer the less unwanted components coming out from the mixer.
Then again this is not meant to be a high performance receiver.

I have seen designs which take the output from the source of the FET (for the
design under test). The waveform here varies from bad to attrocious - not the
kind of thing that should IMHO be fed into the mixer, although of course it
will still work.

The variable gain is necessary to obtain oscillation as the tuning capacitance
varies. Later I want to add an AGC circuit.

I have also experimented with buffering the tank, as this is the place with
the cleanest waveform. A fet source follower didn't seem to work as expected
and I now think that this type of circuit is only good for low level signals,
let's say 1V ptp if the pinch voltage is around -3 or -4V. Comments?

In a previous circuit I used an emitter follower using two 100k resistors to
bias the transistor base to Vdd/2. It seemed to work and certainly I
understand far better the voltage swing limitations of this circuit. The tank
will be loaded by 50k (depending upon beta and emitter load), however an
experiment with the tank loaded with a 12k resistor showed that this had
little detrimental effect on the circuit, although the gain control needed to
be increased. The general opinion that I have read appears to recommended
loading the tank as lightly as possible, although the advantages as not
clearly spelt out (at least to me!).

regards...

--Gary