On 12/23/2024 4:46 PM, Michael Tope wrote:
My point was that by switching to CW or sending a pure tone on SSB, it
would be easy to see hum sidebands on a waterfall or spectrum analyzer.
As to whether any particular tube amplifier will generate hum sidebands,
others have commented on the fact that class AB amplifiers have good
anode supply rejection ratio compared to class C amplifiers. That isn't
intuitively obvious to me, but I trust it is correct.
I do agree with you that using an SDR is a great way to assess ones
signal quality.
Yes, and several of their capabilities combine to make very inexpensive
SDR receivers vastly superior in some respects to the traditional
frequency analyzers we've considered a gold standard for a century. The
SVGA module for the Elecraft P3 spectrum display can achieve fractional
Hz resolution. Using a lot of averaging, I would expect to be able see
and even measure audible levels of hum at 60 and/or 120 Hz.
Here's work I did in 2015 with the P3/SVGA. Although I documented it in
Power Point because it was easy to incorporate the screen grabs and
comments, it was not intended for presentation.
http://k9yc.com/P3_Spectrum_Measurements.pdf
Here's work I did in 1986 using Time Delay Spectrometry, a measurement
system invented in 1969 by the late Richard Heyser when he was at JPL.
http://audiosystemsgroup.com/AESPaper-TDS.pdf
Some of the more technically exploring minded in the pro audio world
picked up on his invention, and implemented it with a stack of HP lab
gear in the late '70s. In 1982, Gerald Stanley at Crown (the power amp
people) developed a dedicated instrument to do both frequency analysis
and time analysis using TDS, and I bought one of first units to come off
the production line. Almost immediately TDS opened powerful windows in
acoustic analysis of systems, transducers, and rooms, revolutionizing
many aspects of pro audio and room acoustics!
Dick consulted with me on the project documented by the Paper, confirmed
that my method would work, and told me he liked the paper when he heard
it via a remote hookup to his hospital room, where he was to present his
last paper on the same session. He was being treated for cancer, and
died not long after. My test signal was embedded in the program material
(an acoustic big band) at such a low level that you didn't hear it if
you didn't know it was there.
73, Jim K9YC
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