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From: chuck <donnyduck@gmail.com>
Newsgroups: sci.electronics.design,sci.electronics.repair
Subject: Re: Oscillator Distortion
Date: Sun, 17 Nov 2024 12:01:52 -0500
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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.