Tim Shoppa wrote:

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.

It seams reasonable that if I can look at the junction with a scope and the
wave looks good, I should be able to tap it with a secondary JFET without
destroying it. Yet I see no examples of that being done. I guess it's just
easier to accomplish the waveform repair by using a tank on the output of
the oscillator and not loading down the primary tank circuit.

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!

This sounds like what Chris Jones was talking about. Do you have a link so
I could check it out?

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).

Clever stuff. :-)

On Mar 20, 7:43 pm, "Anthony Fremont" wrote:
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!

This sounds like what Chris Jones was talking about. Do you have a link so
I could check it out?

A "classic" oscillator with AGC is the Sulzer Oscillator. Very nice
pics and schematics at

http://leapsecond.com/museum/sul25-1/

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).

Clever stuff. :-)

For really nitty-gritty stuff about low-distortion oscillators, see
Jim Williams' examples in Linear Technologies appnote AN-43. (Go to
http://www.linear.com/ and do a search for "AN43" without the hyphen).
Most of the examples there are bridges, and distortions in the
sub-0.1% category are achieved AND THEN IMPROVED UPON BY FACTORS OF
HUNDREDS! Also google "Meacham Bridge" and "Wien Bridge".

Tim.

On Mar 20, 7:43 pm, "Anthony Fremont" wrote:
This sounds like what Chris Jones was talking about. Do you have a link so
I could check it out?

In addition to the low-noise-low-distortion-high-stability URL's I
pointed you towards in my other followup, recent ARRL Handbooks have
some really clever low-noise VFO circuits using a multitude of
approaches, including explicit AGC circuitry. I have been slowly
working my way through the cookbook examples and every example has its
merits.

In typical ham use, for better or worse, stability and reliability to
start-up are often the most important criteria. What you are
complaining about when you see a distorted output, is something that
is actually a design goal of oscillators that are followed by
multipliers.

One very common method over the years of decoupling the frequency-
determining tuned circuits from other frequencies generated in a radio
is to run the oscillator grid tank at half the output frequency and
depend on distorition to make the desired output frequency. In the
simplest case a balanced or push-pull oscillator is a "No-No" because
you WANT the second harmonic. The electron-coupled oscillator that was
in the 50's/60's/early 70's handbooks is a classic design.

Tim.