[CamperBob2]

That doesn't sound right at all. I mostly use mine as a TDR (50 GHz with 83651B) but I can generally reload saved calibrations from months ago, less than 5 minutes after a cold start, and still see < 50 dB return loss through 20 GHz if I look at the same load.

I'd go through the receiver channel tests in the manual and try to isolate the problem to the test set or IF box. Does S21 drift as well?

[jjoonathan]

Yes, I'd expect better (though maybe not necessarily 50dB+). Troubleshooting:

1. First suspect is always a bad calibration -- they tend to be unstable in addition to incorrect, but if you have anything with known(-ish) S parameters you can check for correctness too.

2. Second step is to put it in TDR mode and watch to see where the TDR changes. That's where your problem is.

3. A trip through the test set with a torque wrench is a good way to not just check the connections but also calibrate your intuition about the couplers/mixers, which should help interpret #2. You can loosen connectors in sequence and watch them spike on TDR to zero in on the actual problem.

As it happens, I was comparing my 8510C + 8517B to a FieldFox recently and I took some drift measurements, although those were on a short rather than a load. The 8510C blew the FieldFox out of the water, lol. Given the TDR, this might be because the standard itself was temperature sensitive and the FieldFox ports are piping hot, but still.

https://jjoonathan.github.io/plot_drift.html

In case they are useful, here are a bunch of different standards measured by the two instruments.

https://github.com/jjoonathan/cal-std-8510-fieldfox/blob/mai...

EDIT: Also worth mentioning, I recently upstreamed my nice 8510C driver into scikit-rf!

https://scikit-rf.readthedocs.io/en/latest/api/vi/generated/...

[james_a_craig]

Thanks for the suggestions - I'll definitely have a poke at it in TDR mode, that's a good plan.

Thanks also for the scikit-rf driver - I've an 8753ES as well, and I've used scikit-rf with that, but not yet with the 8510.

[james_a_craig]

Thanks, it's really useful to have a reference point. Having played about a bit with which device is left to warm up, I'm fairly sure it's the 85102 (IF box) that's drifting. I've also tried my old 8350B sweeper instead of the 8341B and although the pattern's very different (different band-switch points) it drifts about equally badly. The odd bit is that it looks like it's related to the sweep position - I get a sawtooth effect as it drifts, like so: https://www.jamiecraig.com/wp-content/uploads/2024/06/image....

[jjoonathan]

Oh, you're in RAMP mode! Yeah, that drifts too much to hold a load calibration. This is expected behavior, it's designed to trade away precision to get speed. This will be worst on the 8350, which is completely unsynthesized, and on the upper end of each band on the 8340, which is also unsynthesized. You should see much better stability at the low end of each ramp band on the 8340 (which is synthesized) and you should be able to get that good stability everywhere if you use the 8340 source and put it in STEP mode to synthesize every point.

If you can get your hands on an 8360 source, it will synthesize both the start and end of RAMP sweeps (starting on the second sweep -- it looks to see how far off it was and corrects it), and you will also have significantly faster STEP sweeps.