I had a customer call me the other day to tell me what piece of s@!%*^ my company's
amplifier is because it had such terrible IP3 performance. I ran through the usual list
of questions about how she was testing it - power supply, coaxial cable & connector
integrity, spec an setup, etc. All seemed well, so I dared to ask about following ESD
precautions and caught an earful for impugning her skills as a lifelong technician.
Just as I was about to give up and send a replacement (and have to admit as she
implied that, yes, I was yet another male that cannot admit being wrong), I remembered
her reading off the power supplies being used. It was two 6V supplies in series that
had a 6V center tap to a separate controller board. After a little more questioning,
I figured out she had the negative line of the PA plugged into the center tap of the
two supplies, but was measuring the voltage with the DMM connected between the ground
reference and the second series 6V supply. So the DMM read 12V while the PA saw only
The coax cables didn't pull the PA reference to ground because the DC connections
are isolated from the case. Kind of a strange situation, but it's one I'll be sure to
ask from now on.