NQ5T:
>It isn't a 339A but it is a true RMS meter
>I redid the measurement, this time with an HP 331A Distortion Analyser
Grant, I don't believe the 331A is True RMS.
It uses a tunable notch filter to eliminate the
fundamental and measures the harmonic content of
what remains with an average-responding meter.
The 339A is much newer and uses a different
technique based on True RMS. True RMS meters
you are most likely to find used are the HP 3400A
or the Ballantine 323. BTW, I worked at the HP
Loveland (Colo) Division responsible for these
meters for about 30 years including 11 years as
Division GM.
Let's just think about your conclusion.
8-10 dB S/N improvement over the 100 Hz DSP BW
implies a 10-15 Hz filter bandwidth due to noise
bandwidth reduction. Since even a 12 WPM CW
signal has a bandwidth of ~30 Hz, it is not
believable that we could copy signals using a
10-15 Hz filter without extreme ringing, which
would preclude copy. Of course even weak DX
CW signals on EME or 160 are more normally in
the 20 WPM range which requires even higher
bandwidth for non-ringing copy.
http://www.ka7oei.com/qrss1.html
I cannot find anything on Google to
confirm that the 331A isn't True RMS, but
that's what I recall. If anyone has an old
HP catalog, the 331/2/3/4A description will
describe its measurement technique. I suspect
not using a True RMS meter accounts for the
inconsistencies in your measurements.
73, Bill W4ZV
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