On Mar 19, 9:23 am, "Anthony Fremont" wrote:
Pictures available in ABSE

The top trace (yellow) is taken between C4 and R2. The bottom trace (cyan)
is taken at the base of the transistor. There is a switchercad file, but
the simulation will show allot of distortion that really isn't present in
the prototype circuit, because of lots of circuit capactance I suspect. R1
was something I was playing with to try and tame the voltage across L1/C3
being applied to the base.

Hello all,

I was tinkering with this LC oscillator (Colpitts/Clapp) this weekend. I
arrived at the values of C1 and C2 empirically after starting with a crystal
oscillator circuit. The values in the original circuit created a horrid
waveform that looked allot like the simulation. After much tinkering around
and simulating, I come to the conclusion that getting a perfect waveform is
nearly impossible, especially with big swing. It seems that the transistor
likes to take a bite out of the right half of the peak of the wave.

What is the secret to beautiful waveforms? Do I need another LC resonator
on the output to fix it up? I mean, I'm getting a pretty nice wave now, but
there is still some distortion that you can just see at the top of the peaks
on the yellow trace.

How do you control the peak voltages of an LC resonattor without mangling
the waveform? The waveform at the junction of L1/C3 is of course quite
beautiful, how do I get it from there to the output? ;-)

I realize that I will need a buffer stage(s) before I can make any real use
of the signal, but I want the input to the buffer to be as perfect as
possible.

Thanks :-)


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.

In the work I do, I need to measure distortion, and the generators I
use don't have low enough distortion in their outputs to be directly
useful. The distortion levels in the "raw" outputs are generally
about -40 to -50dBc. I use filters to make things better, and can get
to -140dBc distortion levels fairly easily. If it's low harmonic
distortion you want, I'd suggest that it may be better to just put a
filter on the output of the oscillator that has only moderately low
harmonic output, and not worry so much about that aspect of oscillator
performance. Filters work well when the oscillator frequency range is
about 1.5:1 or less. Much more than that and you'd need to switch in
different filters depending on the oscillator frequency.

Cheers,
Tom