[Amps] Checking for IMD

[Joe Subich, W4TV]

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