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 secret to a beautiful waveform is -- you usually don't need it
straight from the oscillator.

There are a lot of things that you want out of an LC oscillator. Low
phase noise, frequency stability, consistently strong oscillation, pure
tone, etc. Of these, the only two that you can't clean up later in the
following amplifier chain is low phase noise and frequency stability.
Concentrate on those, & don't sweat the nice waveform.

Frequency stability and phase noise performance are often achieved by
intentionally designing the amplifier so the active element operates in
class C, without ever going into voltage saturation. This keeps it's
drain (or collector) impedance high, yet delivers a large voltage swing
to the gate (or base) to keep phase noise low. It also gives you a more
or less consistent standing voltage in the tank, which helps the design
of the following buffer stages.

If you absolutely positively must tap the World's Most Beautiful Sine
Wave off of the oscillator section, consider a parallel-tuned tank
that's loosely coupled to the active element. Then loosely couple your
output tap to that -- it's your best chance.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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