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R&S SMIQ as a replacement for a general signal generator?
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G0HZU:
One (optional?) feature of the E4433 is that you get raw baseband IQ outputs at the rear of the instrument.

So if I set one (the E4433B) to give out a 4 tone SSBSC signal using the AWG and then feed the IQ out into the analogue IQ inputs of the E4433A I can look at how well the two perform when daisychained like this.

In the plot below you can see that the 4 image terms are now visible on the left side of the suppressed carrier. The suppressed carrier is the tone at the centre of the display. On the E4433B the image terms are much lower and the carrier is at about -50dBc rather than -45dBc in the image of the E4433A below.

But it's still pretty good if you consider that this is being driven from the baseband generator from one generator and fed into the IQ inputs of another. I did a User Cal on the E4433A IQ modulator prior to feeding in this signal and also the ALC is turned off as the ALC action degrades the performance slightly.

PA0PBZ:
.... the 4 image terms are now visible on the left right side of the suppressed carrier.

you had me staring at the image there for a while  ;)
G0HZU:
Sorry for any confusion... by image terms I mean the 'unwanted' image terms on the left of the carrier as arrowed in blue below. If there was only 30dB image rejection there would be a set of 4 (unwanted) image tones on the left of the carrier that sit 30dB lower than the (wanted) terms on the right of the carrier. These would appear as per the dotted yellow lines below.

I only have to manually trim the IQ calibration slightly away from the auto cal settings to upset the modulator and make these unwanted terms begin to appear as per the yellow lines :)




PA0PBZ:
Ah, I see, thanks for clarifying that, it's the unwanted sideband you are talking about.
G0HZU:
For a bit of fun I tried capturing a similar 4 tone SSBSC signal with the E4406A and the VSA SW and I then sent the captured IQ data back to the E4433B as an ARB file for playback. So in an ideal world the signal should look the same as the raw signal from the E4433B.

However, to prevent glitches on an analyser that would appear at the loopback (start/stop) point of the captured segment my software can crop the data to make sure there is a fairly seamless crossover at this point. This removes a huge discontinuity at the loopback point and improves the playback.

Without this, the analyser would show jumpy glitches and something like an old school swept analyser would show a huge fan of spiky clicks across the spectrum.

This plot below is my second attempt BTW. The first one had lots more distortion terms. I managed to improve this by setting up the VSA interface in a better way on this second attempt.

The result is pretty good although I think it can be better with a bit more experience on my part. I'd imagine something like the SMIQ (or a decent Anritsu vector sig gen) would give similar performance if I used it instead of the E4433B.

But I guess the system performance here is set by the E4433B and also the combination of the linearity of the RF converter in the E4406A and the performance of its 14 bit ADC/digital IF and also the VSA SW and also the way I process the captured VSA data to make it compatible with the E4433B before sending it to the E4433B for playback via its 14 bit AWG.
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