On Mar 19, 12:23 pm, "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? ;-)

Oscillators have to have gain greater than one at the frequency of
oscillation.

When turned on, the amplitude builds up until something in the circuit
cuts back the gain. In simple oscillators, that "something that cuts
back the gain" is almost always the active device saturating and
distorting its output.

The higher your gain, the more reliable the oscillator starting up,
but also the higher the distortion.

If you take the output not from the output of the active device, but
from a lightly-coupled tank, then you'll see something much more like
the sine wave you were expecting. This is what you see at the L1/C3
junction. But still you'll get lower distortion there if the active
device isn't driven so far into saturation/distortion. And by
definition you cannot suck much power out of the L1/C3 junction
without decreasing the Q of the tank and making distortion there too.

You can add a few more active devices and not only buffer things but
also put a fairly linear AGC in the loop. This still has distortion,
but this is done intentionally in a rectifier to derive the AGC
control voltage, which is then filtered. The intentional distortion
does not have to appear in the output!

Clever use of devices can make the AGC loop quite beautiful. Look at
the Wien Bridge or Meacham Bridge oscillators that use a light bulb in
the bridge to not only be the loop-control device but also do
filtering (thermal time constant of the filament).

Tim.