Is a 2-tone IMD measurement possible with the tinySA Ultra?

I no longer have my old Siglent, Rigol or other spectrum analyzers I once had access to - the ones used (among other things) to measure IMD of SSB rigs.

But I do have everything else I need: 2-tone generator, directional coupler, fixed attenuators and a tinySA Ultra.

With Hans G0UPL making great progress in developing firmware that will allow SSB operation with QMX/QMX+ transceivers, I wanted to know if the tinySA Ultra will allow measurement of IMD.

Hans' method of SSB generation is unique and it will be interesting to see how the transmitter output looks compared to more traditional rigs.

The original tinySA does not have the resolution bandwidth needed to make IMD measurements, but the newer Ultra version, with 200 Hz minimum RBW, does.

My test subject was the KX2 on 20m phone.

 

For the test:

I dialed the KX2's power down to 1 watt, then fed the audio output of Elecraft's 2-tone generator into the mic input of the KX2, one tone at a time, using the generator's pots to balance the two tones in terms of how much RF output they caused the KX2 to produce.

The KX2's RF output was sent to the tinySA via an Elecraft directional coupler and some additional external attenuation.

With the tones now equalized, I set the 2-tone generator to inject both tones together then used the delta marker function of the tinySA Ultra to measure the resulting 3rd order IMD.

As can be seen, 3rd order IMD is 40dB down (on 20 meters - this will vary by band). 

ARRL measurement

The ARRL's review of the KX2 shows -36dB in their measurement so I'm happy with the result of my test. It shows that the tinySA Ultra is at least in the ballpark and may, in fact, be right on the money.

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