"Anthony Fremont" wrote in message
...
K7ITM wrote:
On Mar 19, 9:23 am, "Anthony Fremont" wrote:
The waveform in a high Q tank that's lightly coupled to the amplifier
should be very nearly sinusoidal. If in addition, the amplifier
remains linear and represents a constant impedance over the whole
cycle of the waveform, then the waveforms should everywhere be
sinusoidal. If the amplifier+tank has barely enough loop gain to
sustain oscillation, then clipping will be minimal, but it's also
possible to detect the level and control the gain of the amplifier.
You could, for example, use a light bulb like HP did in their original
audio oscillator. Beware, though, that best oscillator performance in
other regards may not be achieved the same way you achieve lowest
harmonic distortion. Be careful that you optimize the right things
for your application.

After reading the other replies, it seems aparent that the shape of the
signal from the first stage is not that critical, it is stability and
phase noise that are most important. So, I should put things back where
there is clipping to be sure that the oscillator oscillates and then clean
up the signal in subsequent stages.

I have never seen clipping. These things are supposed to limit in cutoff,
not saturation. As the signal build up, the conduction angle gets smaller
and smaller until the device runs out of gain. That is another way of saying
that the DC value of the gate voltage gets more negative the bigger the
amplitude. This works out automatically with a JFET. You need about 10K -
100K DC resistance from gate to ground. Using a bipolar transistor is not a
good idea.

Tam