(tr)uSDX IMD performance

(tr)uSDX uses less common techniques for generation of an SSB telephony signal at modest power (a few watts).

There are aspects of the techniques that might raise questions as to how well they work, questions that do not seem to be addressed by the developers.

Wide signal / distortion reports

Whilst there are lots of videos where users demonstrate making QSOs with the (tr)uSDX, credible evaluation of common reports of wide transmit bandwidth is scarce, though spectrum displays of excessively wide (tr)uSDX signals on air are not so rare.

One simple objective measure of IMD is that of a two tone test with spectrum analysis.

Two tone test with spectrum analysis

A two tone test calls for combining two equal amplitude non-harmonically related pure sine waves and feeding that to the transmitter input, and observing the spectral distribution on a Spectrum Analyser.

Ideally, the output should be just the two input components frequency shifted by the virtual carrier frequency (in the case of USB). Practical transmitters are not perfect, so there will also distortion products, and a common measure is that of the 3rd order mix at  If F1 and F2 are the frequencies of the two tones (at RF), the third-order distortion products occur on both sides of these tones at 2F2 – F1 and 2F1 – F2.

Above is a figure from an Anritsu application note showing a wider scope of IMD. Distortion products are measured wrt to each of the desired signals or ‘carriers’, and this value is often given relative to those signals as dBc. (The ARRL uses a different method, referenced to two tone PEP.)

Above is an example spectrum display from a Wikipedia article on IMD. Note there are two strong signals at F1 and F2, and two much weaker signals at 2F2 – F1 and 2F1 – F2, and they are around -56dBc.

It is commonly accepted that IM3 (3rd order IMD) higher than about -25dBc is very poor to unacceptable, commercial practice is better.

AE5X’s two tone test of (tr)uSDX

AE5X produced a short video demonstrating what appears to be a valid two tone test of a sample (tr)uSDX system IMD.

Above a screen shot from AE5X’s video. The video shows that the output is quite unstable (a problem in itself), but 3rd order IMD is never good. The other slightly, and only slightly, lower peaks are higher order IMD products, 5th, 7th, 9th… etc.

The value of the upper IM3 product is about -5dBc, appalling.

The test was done with the (tr)uSDX powered from USB, the nominal output power is 250mW for each of F1 and F2 or 24dBm each, and that IP3 from the display is -5dB. We can calculate the third order output intercept \(OIP3=P_{out}+|\frac{IM3}{2}|=24+|\frac{-5}{2}|=26.5 \text{ dBm}\).

The results should raise the question about out of band products and compliance with spurious signal requirements that may apply in various jurisdictions. This spectral distribution questions claims of the technique use to amplify an SSB telephony signal with a Class E PA.

A link was posted to the video on the (tr)uSDX support forum some weeks ago, but it goes without comment at this time by the developers or any other persons who might challenge the results, or provide an alternative credible measurement set.

Is it a bad instance that was tested, or is it typical of the (tr)uSDX?