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 6V.
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.