[Amps] Measuring IMD

[Karl-Arne Markström]

A quite little known IMD test set is the Racal 9058 Selective Analyser.
 
This is a small self-contained unit consisting of a two-tone generator and a manually tuned direct conversion 
spectrum analyser covering 1 to 100 MHz.
 
It was designed for quick field checks of manpack and vehicle SSB sets when carrying a complete IMD measuring 
setup was considered impractical. 
 
http://www.milcom.co.nz/items/item024.html

73/
Karl-Arne
SM0AOM

----Ursprungligt meddelande----
Från: jim at audiosystemsgroup.com
Datum: 2013-11-02 08:45
Till: <amps at contesting.com>
Ärende: Re: [Amps] Measuring IMD

On 11/1/2013 9:31 PM, Bill Turner wrote:
> Aside from buying a very expensive spectrum analyzer, is there a way for the
> average ham to do it in his shack?

Yes, with some effort and ingenuity.  First, you need two clean audio 
oscillators to modulate the transmitter, and a way to cleanly combine 
them. You could do that with a couple of vintage HP oscillators and a 
decent audio mixer as simple as a Mackie 1202. You feed that to the 
transmitter audio input, making sure that you don't overload it.  
Vintage HP oscillators are common at hamfest flea markets for cheap. The 
K3 has a 2-tone generator built in, you simply activate it from the menu.

Second, you need a spectrum analyzer. The Rigol unit (don't recall model 
number, but I have one), for about $1400 will get you there, and do a 
lot of other useful stuff. The DG8SAQ VNWA is a fine network analyzer, 
and can also do spectrum analysis, costs about $750 shipped to the US, 
and runs on a Windoze computer USB port. The dynamic range of this unit 
is limited in spectrum analyzer mode, I don't remember if it's good 
enough for this.

There are LOTS of audio FFT analyzers that run in Windoze, take the 
audio out of an RX and give you the audio spectrum. That will give you 
magnitude of the two tones, the magnitude of difference frequency, and 
it will give you the magnitude of the sum frequency if it is within the 
audio bandwidth of the RX. You can even do this with many sound editing 
programs like Audicity and WavePad, which are free or cheap. For best 
quality, they should use a decent USB sound card for I/O.  A Tascam USB 
card that sells for about $125 is plenty good enough.

The Elecraft P3 spectrum display can be tuned to almost any IF, 
including the output of a TX, and can be set for any scan width between 
2 kHz and 200 kHz, so it could also be a direct detector (small piece of 
wire for an antenna), or hooked up to the IF of a good RX.  It's very 
flexible, wide range of scales and sensitivity. The one thing it lacks 
is a cursor that reads the amplitude at a frequency -- you've got to 
interpolate from the vertical axis.

HP gear of various sorts also shows up on the auction sites.  I paid 
about $1800 for an 8590D tht has a frequency calibration issue, but 
other wise works fine. Before that, I owned a tube version with modular 
plug-ins and CRT display that was a real arm-stretcher, but worked fine 
and cost about half as much.

Also, ask around your local ham club -- you may be surprised by what 
test gear lurks that you can borrow. I'm happy to loan my stuff to locals.

So depending on what you have laying around and what you might use for 
other things, IMD measurements can be made for no more than a monthly 
mortgage payment!

73, Jim K9YC
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