开云体育

Hello Don

700 / 1900 Hz, standard two tone frequencies. VFO on 7030 kHz (I know, CW segment nor SSB... It's only testing to the spectrum analyzer via 50 ohm attenuators).?

I'm using phase pre-distortion based on a measurement curve (not realtime feedback, which isn't available). See attached. Phase pre-distortion doesn't make much difference to IMD3 but does noticeably improve higher order intermodulation products.?

Amplitude modulation linearity is excellent and I cannot measure any distortion at all within the accuracy limitations of a simple measurement; except for the RF blowthrough from driver through the PA even when PA voltage is zero, which is about 1Vpp at the BNC antenna connector. I am able to zero that by switching off the driver completely but on/off is all the control I have over the driver (74ACT08). Switching off when the output should be below 1Vpp can be called amplitude pre-distortion of a sorr, and appears to make some improvement albeit small.?

It should be noted that the modulation method is quite different from a conventional SSB exciter and PA. A normal linear PA is most disturbed by the non-linearity distortion: a non-straight line of RF output vs RF input, caused normally by the inescapable fact that transistor characteristics are curves not straight lines, and straightening the situation out isn't easy. In the QMX PA the transistors are saturated on/off, there is no use of the transistors curvy characteristics at all. The amplitude modulation is at audio frequencies and the amplitude modulation circuit can be a lot more complex, in QMX it is made up of 4 transistors in a feedback arrangement driving a P-channel MOSFET. This is extremely linear. So the problems with a PA and SSB modulation using EER are quite different to conventional SSB and linear PAs.

In any case the QMX results so far compare very competitively with commercial black box transceivers with 20x the price tag. I reached the stage where as I tinker in ever more detail with DSP adjustments, I'm able to get a dB more of IMD3 at the expense of higher orders, or vice versa, small changes that are debatable which is more important. So it's time now to move on and connect up the microphone signals (digitally).

73 Hans G0UPL