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My company got a new vector network analyzer recently and I'm using it to try to learn more about microwave circuits, especially the resonant cavities that we use in our measurement systems. I've seen some unexpected behavior in simple situations and I'd like to know whether this is a gap in my understanding or if the VNA could be broken or miscalibrated.

The VNA is a Rigol RSA5065N (DS), a combination spectrum/network analyzer with a bandwidth of 6.5 GHz. Our systems operate in the 4.0 - 4.5 GHz range. It's not a full two-port analyzer; it only measures S11 and S21. (I'm mainly interested in S11.) We have an AppliedEM 6 GHz calibration kit (DS) and a Times Microwave CLU18-SMNM-03.00F N to SMA cable (DS).

The analyzer lets me set coefficients for each standard -- C0/1/2/3 and an offset length for the open, L0/1/2/3 and an offset length for the short, a resistance for the load, and an offset length and an attenuation for the thru. The cal kit comes with the C and L coefficients but provides offset delays instead of lengths. I wasn't sure how to translate those so I adjusted them until the post-calibration measurements matched the values calculated from the C and L coefficients at the center frequency of 4.25 GHz, giving 0mm for the open and 0.54mm for the short. I'm not sure if that was the right thing to do or if I should be adding the offset delay to those values to adjust the phase.

Below, I've provided screenshots of various situations along with what I expected to see on the Smith chart. Some of them make sense and some of them don't. I'm hoping someone can explain the mismatch between the two. Most of these show frequencies between 4.0 - 4.5 GHz, but some show the full span of 100 kHz - 6.5 GHz.

S11: After system power-up but before calibration; cable left unterminated
What I expect: A 0dB trace spread around the unit circle.
What I see: A trace of -20dB/varying phase below the center frequency and 0dB/0 degrees above the center frequency. Well, it's not calibrated, so I guess the error cancellation can do weird things. Initial trace

S11: After calibration, cable terminated with calibration open
What I expect: A trace along the unit circle with capacitance values matching those given by the C coefficients.
What I see: Exactly that.
Post-cal open

S11: After calibration, cable terminated with calibration short What I expect: A trace spread along the unit circle with inductance values matching those given by the L coefficients.
What I see: Exactly that, although I lost the screenshot for it.

S11: After calibration, cable terminated with calibration load
What I expect: A trace that's basically a dot in the middle of the Smith chart with return loss >36dB.
What I see: Exactly that.
Post-cal load

S11: After calibration, cable unterminated
What I expect: An arc very near the unit circle but with a different phase from the open standard.
What I see: A short wiggly line moving inward and capacitive starting near the short position. Return loss increases a bit with frequency, but is still less than ~3dB. Could this be radiation loss? I know it's hard to make a good open circuit at microwave frequencies. Unterminated cable

S11 and S21: Cable terminated with a 22mm-long 12.4GHz F-F SMA adapter (Amphenol 901-9986-RFX, DS)
What I expect: Something similar to the above but with a different phase.
What I see: A consistent 5dB return loss with about a 45-degree phase shift. An S21 measurement of this same adapter showed losses between +/-0.5dB with a 45-degree phase shift across this frequency range. I suppose if there are radiation losses too this makes sense. I also tried a shorter ~1cm Rosenberger F-F adapter (32K101-K00L5, DS) and it looked very similar to the calibration open.
F-F adapter S11
F-F adapter S21
Shorter F-F adapter S11

S11: Cable terminated with a 6in-long 6GHz SMA-F to MMCX-M adapter cable (Cinch Connectivity 415-0071-006, DS)
What I expect: Something similar to the above two opens.
What I see: The trace departs the Smith chart entirely, showing an |S11| of more than +20dB at the center frequency. I didn't think this was physically possible.
WTF is going on here

S11 full-span: Cable left unterminated
What I expect: A trace that goes around the unit circle.
What I see: A trace that starts at the open point and immediately departs the Smith chart, eventually arriving back inside near where the unterminated cable was at 4.25 GHz. |S11| shows a peak of around +7.5dB near 2.5 GHz.
Full-span male calibration open through F-F adapter

I tried adding port extension delays to the above. They dramatically change the shape of the trace without affecting the magnitude at all. The delay produces a variable phase shift, so I think it makes sense, but it also makes me think my calibration offset settings aren't the problem. Here's a 100-picosecond delay, for instance:
100-picosecond delay

Ultimately, what I want to do is understand the impedance characteristics of our cavity resonators. But sometimes when I look at a resonator I get something like this, and I have no idea how to interpret it:
Cavity resonator trace meanders outside the Smith chart

Can anyone tell me what's going on?

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    \$\begingroup\$ A few hints here. Beware when converting Length to Delay, there may be a factor of 2 lurking in the definitions - is it one way or there-and-back. You might need to ask both Rigol and AppliedEM or reverse-engineer it somehow. For "only" 4.5 GHz I think you'll get better results than you have so far, just setting all coefficients to zero and living with the C. \$\endgroup\$
    – tomnexus
    Commented Apr 20, 2023 at 6:40
  • \$\begingroup\$ @tomnexus Do you mean using only the offset delay/length for calibration? \$\endgroup\$
    – Adam Haun
    Commented Apr 21, 2023 at 16:15
  • \$\begingroup\$ No, the link was just for some background. While you can use port extension through a short cable, at 4 GHz it will be pretty rough. Try it. I meant that at 3 or 4 GHz I got pretty good results with three SMA F connectors on a keyring, short open and load in the same plane, and all coefficients zero. Your kit probably doesn't have them in the same plane, Open probably has no pin while short is a few mm longer, so this might not work. So concentrate on getting the right lengths for the kit and don't worry so much about the polynomial coefficients. \$\endgroup\$
    – tomnexus
    Commented Apr 21, 2023 at 17:05
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    \$\begingroup\$ Remember a VNA will regurgitate whatever you typed in for the standard, while it's connected - that's what you asked for - so proves nothing. If its "factory" cal to its front panel is fairly good, then try using that, with just one N-SMA adapter and only a port extension, to get some independent data on your cal standards. Also - to 1st order you don't really need to have the reference plane correct on each, just the delta-delay between Short and Open, and you will get that easily. Finally, use my knife-short-circuit trick to move the measuring plane to where you want it at/in the DUT. \$\endgroup\$
    – tomnexus
    Commented Apr 21, 2023 at 17:12
  • \$\begingroup\$ @tomnexus After removing the cable for calibration and then playing with port extension delays, I think I'm seeing consistent results between my male and female SMA standards, at least. I'm still not sure what an S11 > 1 is supposed to mean, but I suppose I can chalk it up to loss compensation being a bit off. If you want to put your advice into an answer, I'll be happy to accept it. \$\endgroup\$
    – Adam Haun
    Commented Apr 28, 2023 at 19:13

1 Answer 1

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It sounds like you have a good network analyser, top quality cables and a solid calibration kit. I think the coefficients are wrong.

In my experience, a 4 GHz impedance measurement with a 3 foot cable should be fairly easy, at least down to the +-0.5 dB level - I get this at 4 GHz with a NanoVNA and some $2 SMA cables from Amazon (just don't bend them). |S11| should never be >> 1.0 unless you remove some cables or adapters after calibration.


Your first graph looks very odd. -20 dB is too low for S11 through such a short cable at this frequency, it should be -3 dB. The step in the middle is terrible. I can only assume there is some old stored calibration applied for 4.0-4.2 and then it runs out, creating the step, but the Cor icon is grey. No idea.

The next two graphs are what you'd expect but they prove nothing. Remember a VNA will regurgitate whatever you typed in for the standard, while that standard is connected - that's what you asked for. To test a set of cal standards, (or their coefficients), you need another known standard.

The open cable graph makes me think you mixed up open and short, or used some very large delay errors. Open is on the right and removing a few mm of adapter shouldn't make it a short.

S11+S21 graph - shows some bad calibration in S11. If the ports are connected together, you expect S11 to be small, basically 50 ohms, because it sees port 2 which is a fairly good load. That |S11| is still -10 dB is a problem. The S21 shows about the right magnitude for the loss of two cables, -5 dB, so this looks correct (unless you also calibrated Thru, then S21 should show 0 dB...). Adapters do contribute phase but their loss will be < 0.1 dB at these frequencies, and it's mostly by reflection from the discontinuities, not absorption, definitely not radiation.


Ways forward:

Try reverse-engineering the cal kit ?
Is the "factory" cal of the VNA fairly good (or can you reset it)? You should see a reasonable open circuit with a small tail, with nothing connected and all calibration turned off.
Try using that to get some independent data for your standards. Use just one N-SMA adapter and a port extension to compensate for it.

Try using zero coefficients?
At 3 or 4 GHz I got pretty good results with three cheap SMA F connectors on a keyring, one shorted, one cut flat as an open (in the same plane) and one load made with two 100 ohm SMD resistors, all coefficients set to zero. So (some of) the coefficients you've entered are at least 10 times away from their true values.

Beware when converting Length to Delay, there may be a factor of 2 lurking in the definitions - is it one way or there-and-back? You might need to ask both Rigol and AppliedEM to clarify, or figure it out yourself. Also to 1st order you don't really need to have the reference plane correct on each standard, just the delta-delay between Short and Open, and you will measure that easily.

Finally, when you are calibrated to the end of the measurement cable, use my knife-short-circuit trick to move the measuring plane to where you want it at the DUT.

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