Last week, someone (I think it was Jon) commented about measuring IMD
without a spectrum analyser. In the classic Single Sideband fundamentals and
Circuits, there's a method using a scope and a couple of diode detectors.
You sample a small portion of the input sig to a diode detector, and apply
that to the Horizontal input of the scope. You take a similar level sample
of the output to an identical detector and apply it to the vertical input.
Then with a two tone (or a three tone) you read three voltages off the scope
trace (which ideally is a straight line at 45 degrees, rising from left to
right). e1 is about 1/3 of the max volts, e2 about 2/3, and e3 is the
maximum. Then 3rd order products are down
20 log[ 2(-45e1 + 252e2 + 167e3)]. [ 1/(45{-7e1 - 4e2 + 5e3}) ]
Hope the brackets are sorted out - e-mail is not the best mechanism for
transferring formulae. There's another equation for 5th order products.
If you use a 2 tone with close tone spacing (20 Hz or so) it should
simulate syllabic rate better. Probably the ideal is noise loading, (as per
ITU Rec. SM328), but a 3 tone test with 2 tones very close together
(recommended, I believe, by Tom W8JI) is also a good test on the power
supply. Hope this helps.
Incidentally, I'd suggest that the minimum test gear for any linear amp
builder is some sort of oscilloscope and a two tone generator. The scope
needn't be very fancy, as even if it's low frequency, you've usually plenty
of output sig available to feed straight on to the Y plates.
73
Peter G3RZP
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