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 :-)

In some LC oscillators, the amplitude of the oscillation is controlled by a
feedback loop. For example, a rectifier can be used to create a DC voltage
proportional to the oscillation amplitude on the LC tank, and then an
op-amp can be used to compare the rectifier output signal to a reference
voltage. The output from the op-amp can be filtered and then used to
control the current in the oscillator core. It is difficult to do all of
this in a way that keeps the phase noise low, but given the right
simulation tools (e.g. SpectreRF which is rather expensive), good results
can be obtained. In particular, a well-defined oscillation amplitude can
help to keep the KVCO well controlled, which is useful in PLLs.

Chris