Hello all,
my approach to IMD measuring is this:
We need basically two things: A dual tone drive source, and some sort of
spectrum analyzer. I will comment them separately.
The old style of dual tone exciter would comprise two low distortion
audio oscillators, an adder, and feeding the resulting signal into an HF
transmitter (or transceiver).
This method is obsolete, because nowadays there is free software
available that can be used to create the same dual tone signal on the
computer, with much higher quality. Distortion of -80dB, and perfectly
matched levels of the two tones, are easy to obtain.
The signal can either be directly fed into the radio, perhaps using an
existing "soundcard interface", which doesn't need to be much more than
a few straight wires, or you could create an audio file with the dual
tone signal, load it into a portable MP3 player, which are cheap and
easy to get, and there you have your portable, battery powered low
distortion dual tone generator. Just put the player in loop mode, to
play the same file continuously, and use WAV format instead of MP3.
Any of the above approaches is strictly limited in how low an IMD it can
measure, because common SSB transmitters produce lots of IMD themselves.
You cannot measure the IMD of a high quality amp, when driving it from
a poor quality transceiver, because the IMD from the tranceiver will
mask that of the amp.
To get around this, you can use two HF radios, if you have them, and a
combiner, to drive the amp under test. The combiner can be built from
some ferrite cores, some wire, and perhaps a resistor. In this case, no
dual tone generator (hardware nor software) is required. The
transmitters don't even need to be SSB. All you need is CW. Just set
them 1kHz apart, and transmit a steady carrier from each. Match their
power levels.
The combiner should have as good a cross attenuation as possible. This
is both to protect one radio from the other, and to keep the
transmitters from generating IMD by mixing each other's signal.
To play it safe, when using transceivers, use this sequence: Set power
level to zero on both, key them both up, increase power settings to
whatever drive level you need, matching them. To stop transmission, do
the same in reverse. So you don't allow one radio to transmit anything
into the receiver of the other radio, through a leaky combiner.
Now the spectrum analyzer: I'm using the free program by George R.
Steber, with the very inexpensive mixer front end, and a still
reasonably inexpensive DDS signal source. It works pretty well.
But if you want an "entirely free" solution, and you have yet another
radio (receiver), hopefully one with a rather wide bandwidth, you can
use that. Just set the bandwith as wide as possible, the BFO to one edge
of that bandwidth, and feed the receiver output into the computer, using
the free Spectrum Lab software. Tune the receiver so that its bandwidth
is centered on the signal center. Depending on the separation of your
two tones, and the bandwidth of the receiver, you will be able to see
and measure at least 3rd order IMD, probably 5th order too, possibly
even more.
If you can shut off the AGC and manually control the RF gain, you can
tune the receiver off the center of the signal, to look at far-away IMD
products. In that case even a standard, narrow receiver can be used to
fully measure IMD, but it's a bit more laborious than if you can see the
full spectral display at once.
In short: If you have a SSB transmitter, a separate wide SSB receiver,
and a computer, you can already do some meaningful IMD testing without
spending any significant amount of money. With an additional
transmitter, which could be an old AM rig, you can improve the measuring
range. And with a cheap mixer and some frequency source instead of the
receiver above, you can measure IMD up to the upteenth order product.
The accuracy of the results will depend on the equipment used. Be aware
of receiver filters changing relative levels of signals by several dB,
even well within their passband. They all have passband ripple. The
wider and smoother the receiver's response, the better. But even with
pretty basic means, good results can be obtained. Certainly many, many
times better than putting an amp on the air and asking fellow hams about
the perceived amount of splatter! Although that on-air test should also
eventually be used, as a reality check.
Manfred
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http://ludens.cl
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