[regenfreak]

G0HZU_JMR In the case of the Siglent analyser, I don't think you can use the +15dBm input TOI (of the first mixer) for your calculations unless you use really wide tone spacings, eg 20MHz spacing.

This is because the Siglent appears to have been designed with a different gain distribution to a classic spectrum analyser.

The old HP 8566 and 8568 analysers don't have any amplification until after the second mixer. This means the distortion performance on all but the narrowest spans will be dictated by the first mixer. Your Siglent analyser has a lot of amplification after the first mixer and so the input TOI of the second mixer will probably limit the overall input TOI. I think this means that the effective input TOI will be lower than +15dBm with 0dBm input attenuation. The datasheet suggests that it will be about +10dBm.

The noise floor with a 10kHz RBW at 10dB attenuation is going to be about -90dBm and this is equivalent to -130dBm/Hz.

Your SFDR calculation should be done at the chosen RBW setting.

Cheers. I can try to draw a block diagram of the Siglent too based on EEBlog video too in a day or two.
20M spacing seems massive. I am not sure how you got this estimae. My calculation was based on RBW 10KHz

Normally, when measuring the TOI of an external DUT (rather than the TOI of the analyser itself), it's best to do it at a fairly wide tone spacing if allowed and if possible. The HP 8566 and 8568 only have an average input TOI, so one way to get around this is to space the tones a long way apart. eg 1MHz apart. This avoids having high phase noise interference at the 2f2-f1 and 2f1-f2 IMD tone frequencies.

Then set the analyser to a very narrow span of maybe 5kHz so a narrow RBW can be used and then visit 2f1-f2, f1, f2 and 2f2-f1 in turn to measure the amplitude and then work out the TOI.

This method has the advantage of avoiding the worst of the analyser's phase noise and it also allows a very narrow RBW. This means the analyser can be run with a higher input attenuation setting and this helps with the overall measurement uncertainty of the DUT as it improves the input TOI of the HP 8566/8 analyser.

I knew it is a double edged saw. The wide spacing measurement has poorer DANL The narrow spacing measurement has higher phase noise contamination.
Now do you take the worst case out of the two TOI values? Or take the average?

The isolation will be about 10dB worse. However, in the past, I've fitted an RC network followed by a 10dB attenuator at the sum port of the TSC-2-1 and optimised the RC values for max isolation for a chosen band. I've done this at the popular satellite IF frequencies of 70MHz and 140MHz in the past. With the RC network suitably tweaked, it's possible to get a very high isolation over a very narrow bandwidth, maybe across a few MHz or so.

The measurement of TOI of a DUT becomes increasingly like a SAS obstacle course. Ultimately, there must be a middle ground, otherwise the strict, enveloping-pushing requirements on spectrum analyzer, combiner, signal sources and diplexers are so demanding that it means total surrender; ok dude, I will just take the TOI from the manufacturer's datasheet. Having said that understanding how the spectrum analyzer works internally is rewarding and intellectually challenging.

The inside of spectrum analyzer is like a magic box to be explored. I am losing interest in Ham radio. I must confess that I find VNA and Spectrum analyzer on their own are much more interesting than Ham radios.

I've got loads of TSC-2-1 combiners here in a tube and I'll see if I can do this again at 100MHz. It might not be until the weekend though.

Thanks.