Joe Subich, W8IK wrote:
>
>> Three-tone is good for testing distortion in conventional amplifiers,
>> however, if the screen voltage remains constant during the three-tone
>> test in a G2DAF amp. (which seems likely) it would not pick this sort of
>> thing up. The best test is probably ordinary speech since it produces
>> wide changes in the screen potential in the G2DAF circuit. .
>
>
>However, if the three tone test is PROPERLY designed (with two of the
>tones very close together) the screen voltage will vary at a semi-sylabic
>rate and produce the wide variations in screen potential. Similarly,
>the properly desigsed three tone test will stress most power supplies
>and document IM and other problems due to poor power supply regulation.
>
The problem with any two or three fixed tones is that the test signal
doesn't explore all possible weaknesses of the power supplies. Although
it's an improvement to add a third, lower tone which modulates the other
pair, that doesn't solve the whole problem.
Speech has the advantage of being realistic, and by using something like
a .WAV file it is now possible to create a speech test signal that will
be bit-for-bit identical wherever and whenever it is used.
Alternatively it should now be possible to program a sound card to
generate two or three pure tones that are randomly swept in frequency to
cover the whole range of possibilities.
In both cases it seems best to set the spectrum analyser to peak-hold in
order to record the worst case. (I wish my own analyser could do that.)
With this method, instead of a line spectrum you see something like a
pyramid. The top of the pyramid is the spectrum occupancy of the
legitimate SSB speech signal, and the stuff down each side is the worst-
case IMD that has occurred at any instant during the test. In other
words, this method shows you something related to total spectrum
occupancy when the whole system (exciter, PA and power supply) is
exercised with a real speech signal.
Returning to the G2DAF question, the spectra that Lars has e-mailed on
request were recorded in this way. In each case the spectrum occupancy
with the PA is overlaid on top of the same plot for the driver alone.
The spectra with/without the PA seem to be identical in shape, and the
spectrum with the PA is simply lifted up by about 15dB which is the gain
of the amplifier. This suggests that the PA is not contributing gross
distortion, but beyond that it's hard to judge.
In order to get a clear picture of what's really happening, it probably
needs conventional two-tone spectra as well as the composite speech
spectrum.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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