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Entry  Mon Oct 23 20:51:07 2017, Craig, awade, DailyProgress, FSS, North TTFSS Common Path Op-Amp Replaced, works 
    Reply  Thu Oct 26 11:08:05 2017, awade, Craig, DailyProgress, FSS, 50 MHz oscillations in TTFSS rev. C field boxes 
       Reply  Thu Oct 26 11:21:50 2017, Koji, DailyProgress, FSS, 50 MHz oscillations in TTFSS rev. C field boxes 
          Reply  Thu Oct 26 15:57:37 2017, awade, DailyProgress, FSS, 50 MHz oscillations in TTFSS rev. C field boxes 
Message ID: 1961     Entry time: Thu Oct 26 15:57:37 2017     In reply to: 1960
Author: awade 
Type: DailyProgress 
Category: FSS 
Subject: 50 MHz oscillations in TTFSS rev. C field boxes 

Ok. Doesn't sound great, but is common to both the units we have.

The AD797 is configured with a feedback capacitor of 1.5 nF parrallel to to a 3.08 kΩ.  This is for a 34 kHz pole.  Could it be an issue with the choise of capacitor type?  I'll try switching out the one that is in there for another mica one, I would have thought the one that is there would be enough (unless it has bad HF impedance).  

I'll try some things.

Quote:

If you have an oscillation in a circuit, it is no longer linear!

Sounds like a typical AD797 oscillation. GBWs of OP27 and AD797 are 8MHz and 110MHz, respectively. OP27 can't make such high freq oscillation.
I didn't check the circuit diagram. Usually AD797 has ~5pF feedback capacitance to increase the stability. You may be able to stop the oscillation by adding/replacing a mica capacitor  from 10~470pF. But 50% of the time the oscillation is not fixed with this kind of modification. You need a cut-and-try.

 

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