WA4LAV writes
> An oscillator, good mixer and a sound card running FFT should
> work. You want to mix the RF down to around 10 kHz ( center of
> the sound card frequency range) and observe sidebands around it.
There are a couple of issues in a "poor man's spectrum analyzer"
using sound card.
1) the mixer needs to be clean and handle a 60+ dB dynamic
range. If it is overloaded it can generate IMD itself.
2) the output needs to be very carefully matched to the sound
card dynamic range. Most sound cards are specified based
on a 4.5 volt p-p input signal (their internal A-D is
referenced to the computer 5 volt supply). The 100 dB
dynamic range claimed for some 24 bit cards cannot be
achieved in most systems due to noise floor problems caused
by the power supply and digital sources in the computer case.
> You can use a sound card to also produce the two tone test
> signal to insert into the SSB exciter.
As pointed out previously, a two tone signal does not test
IMD caused by (the lack of) power supply and/or bias regulation.
If IMD testing is performed with a two tone audio signal and
transceive, the transceiver IMD will also effect the overall
measurements.
None of this is to say that useful results can't be obtained
by an amateur with home made equipment ... just that it is
not as easy as it might seem. The most reliable results are
probably obtained with a spectrum analyzer, three very clean
high power RF generators (Fo, Fo + 5 Hz, and Fo + 2 KHz) and
proper combiners.
73,
... Joe, W4TV
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