Rather than the sensitivity of my hybrid to the impedances presented to its ports, it's the VNA's self reported port 2 input impedance of 53.5 ohms that's my concern. That's 7% from nominal. I would like the port 2 input impedance to be within a tenth or two of 50 ohms. People have also mentioned maybe the cables or even the thru are contributing to that, although I have yet to read about anybody claiming that the cables or thru that come with the nano VNA to be anything other than 50 Ohms.
I'm presently dealing with VHF centered around 146 MHz.
This video is quite entertaining but its content is questionable; it doesn't demonstrate how a slightly bad calibration kit affects the calibration.
In the video, the presenter uses open as short, short as open, and 2dB attenuator as load. These are quite extreme and unrealistic examples for representing the effects of a bad calibration kit.
Here's some pics of the loads I made for the hybrid.
This video is quite entertaining but its content is questionable; it doesn't demonstrate how a slightly bad calibration kit affects the calibration.
In the video, the presenter uses open as short, short as open, and 2dB attenuator as load. These are quite extreme and unrealistic examples for representing the effects of a bad calibration kit. Not sure how the down-votes are visible, but this could be the reason.
Well, I'm already kind of on that. I made a some loads just for the hybrid, because I bought some cheap 2 Watt supposedly 50 Ohm RF SMA resistors for the job and then found they varied by over ohm. So for the hybrid, I got some male pcb thu-hole SMA connectors and soldered a couple of 100 Ohm 0.01% resistors on them. I made three. One of them came out bad. I'll have to take it apart and redo it. The other two test within a tenth of a ohm of each other and vary in reactance by just a little bit as far as I can tell.
I have parts to make a female SMA calibration set the same way. I just have yet to get around to doing it. I also have s parameter touchstone files for them, although they are on my computer that has a damaged video card. So, I have to wait to get a new video card before I can get to them.
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Metrology is a wiley bugger.
Hey, my waveguide calibration expert.
... which I am confident is very close to 50 ohms DC resistance. ...
I bought a supposedly better F-F barrel that is a little longer, nickel plated and with a flat for a wrench in the middle. In the course of my testing I swapped the one that came with the kit and the one I bought later. Their performance was very similar.
As far as your results go, I make sense little of the first one, with the ~43.6 Ohm impedances. Is that the uncalibrated one?
The second results kind of makes sense, if the port 2 input impedance is significantly closer to 50 Ohms than the port 2 on my machine.
Because I am working on a new computer I have yet to set up your software on it. I installed VNA-QT, although it displays a discontinuity in the sweep results that I have yet to sort out. I like the VNA-QT for the one reason that it allows me to load s parameter touchstone files for the calibration loads. I can produce them from my simulation software with which I designed and modeled my loads. Converting the S parameter touchstone files to coefficients numbers I have yet to figure out.
You should post more details about your SPICE simulations. I would take the time to follow along.
Some of the last changes I made to my software were to work around problems when importing data collected with the QT software. I believe we were posting about that problem in this thread.
I've thought about adding support for databased standards but have not looked into it enough to know if all the vendors follow a standard. The standards I had access to were from Agilent using the polynomial fit. For the low cost VNAs, I pretty much use the ideal model. The software supports my other VNAs. It's these other VNAs that drove the ability to change the coefficients. It's in the common code, so all of the software goes along for the ride.
We can't simply measure it as the are doing some switching. Thinking it may be an artifact from sweeping, I tried to select the CW mode from their built-in firmware but it has no effect. I repeated this with the V2Plus4 with the same results.
Attached plots show the effect for both low cost VNAs. What is interesting is how the LiteVNA changes about an ohm as it chops, compared with the V2Plus4 at about 2 ohms.
Attached plots show the effect for both low cost VNAs. What is interesting is how the LiteVNA changes about an ohm as it chops, compared with the V2Plus4 at about 2 ohms.
Something else to be aware of.
Apparently it switches back and forth between measuring S11 and S21 for each frequency point.
If I comprehend your post correctly, you're measuring the low cost VNA, with a quality PNA.
I fail to make sense of what your are showing, a switching that is apparently every few MHz or so across the frequency sweep, and we fail to see that when self-measuring the port 2 input impedance of the low cost units. That's too slow for the S11/S21 switching. So, what do you think that is?
Just for giggles, could you use the PNA to measure the output impedance of port 1 on the low cost devices?