Re: Oscillator Distortion

On 2024-10-20 5:24 a.m., albert@spenarnc.xs4all.nl wrote:

In article <veguu6$ofj1$3@dont-email.me>,
Cursitor Doom  <cd999666@notformail.com> wrote:

Gentlemen,
>
Last week I got an old (1968) pulse generator out of mothballs and managed
to get it fully functional again. However, before replacing the case, I
(true to form) dropped it on the bench and something on the PCB must have
shorted out against the metal tools it fell on, because it no longer works
properly.
I've found an issue with the principal oscillator. It's generating
distorted sine waves. It's a wien bridge type using BJTs as the gain
element and fine tungsten filaments as thermistors, so should produce near
perfect sine waves before they're chopped and shaped by subsequent
circuitry, but since the fall, it's not.

The whole idea of making a sine oscillator is positive feedback with an AC gain  =1  while DC biased with negative feedback and not boosting to a pulse with high gain. So something must limit the gain smoothly like a hot bulb with lower impedance driven by a lower voltage.
Below the inverting gain can be  R1/(R2 + (R(Q1)) =2 maximum unless Q2 is slowly turned off. and quickly turned on ;) otherwise known as fast attack , slow decay.
   The non-inverting side is unity gain for AC signals. So you got an oscillator and the diode voltage turns off the PFET or Pch JFET. The output amplitude is controlled by the gate control voltage V(AGC) which can be attenuated to boost output voltage to 10Vpp with a series R around 4 Meg to the 1 Meg shunt.  The FET threshold of 0.5V and the diode voltage with 1Meg is only 0.4V so slightly less than 1Vp is achieved.
  With a -voltage below ground must meet the FET threshold  to control gain with a ground reference.   This was copied directly from LTSpice examples > education.  If you understand any of what I said then you recognize the differences with Hewlett Packard's old design.
Whatever is boosting the gain of your circuit or NOT cutting the gain with high R must be fixed.
Cheers
Tony Stewart, near Toronto
EE since 1975
Learning how to retire since age 54.