Pulling together a few sub-threads...
The 11/110MHz (8553B) plugin for the 141T mainframe is stable enough for
IMD testing, if you leave it for long enough to warm up - just don't
expect to do any serious narrow-band work the same evening as you switch
it on!
A big advantage of the old 141T mainframe is that you can use the
storage screen as a 'peak hold' facility, to build up a picture of the
maximum occupied bandwidth as you talk into the microphone. When the
Persistence control is set to max, the trace never decays. This emulates
the 'peak hold' features of much more expensive digital analysers, and
allows you to see the highest signal level ever achieved at any time.
The resolution bandwidth doesn't have to be very narrow for this test.
The main signal plus IMD builds up into a 'dogpile' that is several kHz
wide, so a bandwidth of 100Hz or more will be OK, with the fastest scan
allowable. (Remember we are only interested in the peak height here. The
narrower bandwidths are only needed if we want to resolve individual
tones right down to the baseline.) All of these factors speed up the
test, and make it less sensitive to the display drifting across the
screen.
The peak hold test is a good one because it uses real-life voice
modulation, unlike any test signal constructed using tones. It tests the
whole transmitter, including the dynamic regulation of the power supply.
It is a very tough test because it captures and penalizes even very
short-lived transients; but I think that is only fair, because other
band users hear those transients and call them "splatter".
--
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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