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.