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Anritsu MS4630B 10Hz-300MHz network analyser teardown/repair/review
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TurboTom:
Initially, we (my collegue -- the "real" EE -- and me, the "crazy" physicist...) also questioned our setup and what errors we might have done when we found this discrepancy. Truth is, the BNC cables we used are good enough at the frequency range of that device that we shouldn't see any deviations of the delay. I checked the cables with my HP 8753C and they perform flawlessly up to > 1GHz delay-wise, above that some tiny wiggles can be observed, but still no significant deviation from the general phase delay, up to the 8753C's limit of 3GHz. Btw, the 8753 doesn't show the "funny" incline of the delay plot in logarithmic frequency mode.

Of course, the delay will be introduced by the length difference of the two cables, that was the intended effect. I also agree that the 4630B (like probably most VNAs) primarily evaluate phase (by measuring the balance between the I and Q signals) but can easily calculate phase delay by dividing continuous phase by (360°*frequency). If the DUT (cable's dielectric) hasn't got any significant dispersion in the observed frequency range and there aren't any resonances otherwise, this figure should be fairly constant as it actually is, provided the linear frequency scale is selected (see attached photo).

For comparison, I attached the phase measurement of our two cables as well, both with linear and with logarithmic frequency scale. These figures are both correct. To me, it actually seems that Anritsu erroneously used the frequency reading from the linear scale to calculate the delay in the logarithmic display, which would also explain that right at the end of the logarithmic delay plot, the reading drops back to the approximately correct range.

Nico, what's the firmware date and revision of your 4630B if i may ask?

Thanks and all the best,
Thomas



Edit: Added the delay plot with logarithmic time scale for better comparison.
nctnico:
I'm getting similar weird readings on mine. Also my MS4630B only wants to measure the delay up to 100MHz  :-//
Omicron:
I have one of these as well. However I think mine is faulty. The traces are really noisy in the region from 10Hz to 50KHz or so, jumping up and down by about 10dB. Calibration does not flatten out the trace in this band as it does for frequencies above 50KHz and the calibrated trace is just as noisy. I'm assuming there must be a fault in the LF part of the analyser. Unless I'm missing something obvious? I've only spent about an hour playing with the unit.

Has anyone stumbled upon a service manual for these things?

[Edit] Ah! My bad! I need to select a longer sweep speed if I want to see the lower end of the spectrum. Obvious in retrospect! :-)
R_G_B_:
I have the predecessor to this network analyzer.

And it shows something similar in the low frequency band
Not sure if it's could be the resolution bandwidth settings Or sweep time settings.

R_G_B_
Omicron:
If I turn the sweep speed down the trace clears up. When I set it to 100 seconds for example then I get a pretty clean trace all the way down to 10Hz. It makes sense if you think about it, the sweep needs to be slow enough for it to be able to capture a few complete sine wave cycles at each measurement point. So the lower you go the more time it needs.

I'm still learning how to use this device, but I'm pretty sure now mine is working as intended.
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