G3RUH

PARASITIC OSCILLATIONS CURE

applies to the following modems:

  G3RUH        9600 Baud Packet Radio Modem PCB Issue 2 onwards
  PacComm      NB-96 and family
  Kantronics   DE9600
  MFJ          MFJ9600
  Tasco (JA)   TMB-965 (all surface mount, hard to mod!)


I have noticed parasitic oscillations on the G3RUH 9600 modem. They started
around the opamp TL084 U16 pins 1 2 3. As soon as the oscillation started
only switch off or shorting pins 2 and 3 stopped the oscillation.

Changing C4 from 100 nF to a 1 uF Tantal cured the problem. The oscillation
spoiled the whole board, a triangular 200 mVtt around 1 Mhz even on the
outputs of the DAC's and indeed on the Vref-line.