> Can you point me to a text which describes the 3-tone test, how
> to set it up, etc?
Five people asked about three tone tests. I want to post this here
because it's quicker and someone else might have a suggestion or two
to improve the test, since it is my own system.
The problem is the changing load on tube supplies causes them to
waddle around, but if they have enough bypassing (energy storage)
they will look stable in a two tone test.
Let's say the two tone test has a five KHz spacing. The loading on
the supplies changes at a five KHz rate. So the filter caps, bypasses
and so on prevent the supplies from moving.
I use two transceivers on CW driving a combiner, and feed slowly
varying (at a speech syllable rate of a few Hz) ALC voltage to each
transceiver. This modulates the two tone signal with a "third tone",
the levels are adjusted so the maximum null just hits zero.
Now you can reference the display on an analyzer to the PEP value,
and get an idea of true speech type IMD.
Another problem is the reference amplitude is always high, and so
that masks cross-over or low level distortion. ( For example, a class
C PA with a stiff low impedance bias source might have -10 dB IMD at
one watt drive, -20 dB at 10 watts drive, and -30 dB with 100 watts
drive.
To do this test you can manually step the signal, or use the
slowly changing ALC voltage, compared to a reference voltage in a
pair of op-amps (one up and one down slope), to trigger the
sweep on the analyzer to give you a snapshot of one drive level at a
time while the supplies are being perturbed by the slow amplitude
variation.
Some very clean PA's with normal fixed l;evel two tone tests can be
rather dirty at some power levels or with slow amplitude changes in
the two tone test.
If anyone has any suggestions please tell me, because I have no
references or outside input on this method.
73, Tom W8JI
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