Adding to the IMD testing thread:
When generating a dual tone signal in the computer, outputting it to one
radio to generate the drive signal, and then using the same computer to
do spectrum analisis on the output signal of a monitoring receiver, it's
easy to run into trouble with either ground loops or with the computer
doing strange things due to resource conflicts between the software
generating the dual tone and the other software doing the analysis.
To avoid this problem, I generated a 5 minute long dual tone signal on
the computer, put it into an uncompressed WAV file, and pushed that file
over to a cheap MP3 player in pendrive format. This MP3 player, like
most, will play WAV files just as well as MP3. Then the MP3 player,
playing the dual tone file in loop mode, becomes a stand-alone, very
high quality dual tone generator, connected to the exciter radio, and
the computer is free to run just the analysis.
Most people these days could do that trick with their cellphones,
instead of using a dedicated MP3 player.
Indeed the big limiting factor is the IMD of the exciter. With my
Kenwood TS-450, IMD is down between -40 and only -18dB (!), depending on
power and band. Worst case is full power on 10 meters. This seriously
limits my ability to measure IMD on good amps. But I don't have two
exciters good enough for this kind of use...
If two exciters are used, the combiner needs to be very good, to keep
the signal from each exciter from getting into the other. Because if
that happens, it's likely that the IMD produced will be worse than when
using a single exciter with a dual tone signal! The advice to check the
IMD of the drive signal is a very good one, in any case.
Instead of a full fledged receiver to get the amp's output signal into
the computer, a basic oscillator (or a DDS) and mixer is plenty. There
was an article in a ham magazin a few years ago, about such a
contraption, and kits to build it are being sold. I got one, and it
works fine. The price is right. And in many cases a DDS isn't really
needed. All it would take is a crystal oscillator and a mixer, and then
the exciter is set to the proper frequency and the test is run at
whatever frequency the crystal runs at. Should be good enough in most cases.
Regarding spectrum analysis software, Spectrum Lab by DL4YHF is really
powerful and free, but many other options exist.
The dampening note to this matter is that one might ask what's the point
in even measuring IMD, when the bands are full of hams who disable the
ALC of their radios, add an audio preamplifier/compressor, and set all
knobs to max, then enjoy watching the power meter's needle stay put at
2.5kW, while they talk, or even while they DON'T talk, just from
background noise alone. Just a few hours ago, there was a net control
operator here transmitting like that. He was using the entire band and
then some.
Manfred
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