Some professional tests use a very narrow spacing between tones - as low as
30Hz - to simulate the syllabic nature of voice, and to exercise the power
supply regulation. Another approach, which, if I remember correctly, has
been advocated here by Tom, W8JI, is to use a three tone test with two of
the tones very closely spaced. As Steve, G8GSQ says, the screen current
varies at a frequency dependent upon the tone spacing.
Because of the peak to average nature of a voice signal, it isn't easy to
relate the performance on a two tone test with that under voice conditions.
The advantage of a two tone test is that it does (or should!) give a number
that can be reproduced in testing by other people. In the days of analogue
FDM telephone circuits, one testing method developed was to load up the
audio channel with noise to the correct PEP: then a a narrow slot in the
frequency domain was inserted in the noise, and the noise power in this
slot at the demodulated tx output was measured. This gave a good approach
to the real IMD performance under practical loading conditions, but did
look at the results of an aggregate signal from many individual voice
channels. However, for a single channel, because of the lack of components
at syllabic rate, it isn't so good. Apparently, for ISB work with voice on
one sideband and multiple data channels on the other, it is a testing
method used extensively by the Russian military, according to a guy I see
at ITU meetings from the Russian administration.
Personally, especially for anything that can have negative screen current,
I like the idea of an electronic shunt regulator. To ensure that the
screen can't over dissipate, this is fed from a constant current source
capable of little more than the maximum rated screen current. As FETs are
pretty cheap ( and things like 6L6s and 807s even cheaper if you have a
good stock!) this seems the way to go.
73
Peter G3RZP
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