Asymmetric VNA behavior -- is it normal? - Page 1
Forum section: electronics / rf-microwave

author: blueskull
author_rank: Unknown
timestamp: April 29, 2019, 10:48:27 pm
content: I got my self a good quality 50R SMA cable from Caltest, rated up to 18GHz.I used this cable to run S11, S12, S21 and S22 on my VNA.S12 and S21 look similar, but S11 and S12 look a bit different (in both tests, one end of the cable goes to port 1, the other end goes to port 2).S11 has sharper notches, and S22 has shallower notches, hence average S11 is a bit lower than S22.With open ended cable (not plugging into the other port), S11 and S22 look similar.So, is it normal to see such behavior?

author: hendorog
author_rank: Super Contributor
timestamp: April 29, 2019, 10:59:03 pm
content: I would think this is normal.You are measuring the return loss of this chain: connector => cable => connector => 'other' port.Even if you reversed the cable to ensure the reflections were the same, the two ports will not have identical return loss.You should see something more sensible if you compare the S11 and S22 with a good load on the end of the cable. Then you should see something comparable - as long as the cable isn't so incredibly good that you are hitting the effective directivity of the calibrated VNA port.

author: TheSteve
author_rank: Unknown
timestamp: April 29, 2019, 11:52:08 pm
content: I'd also agree it is normal but for fun if you do reverse the cable do the sharper notches follow the port or the cable?Are you using the port savers yet and are you using a torque wrench?

author: hendorog
author_rank: Super Contributor
timestamp: April 30, 2019, 12:14:36 am
content: Quote from: blueskull on April 29, 2019, 11:16:55 pmIs that after a OSL calibration, and are you putting it directly on the port? It sounds like its either not calibrated with that amount of error or the 18GHz load isn't great.If you put the same load you used for calibration back on the port again you should see a dot in the centre - my old 8753C VNA assumes the load is a perfect 50 ohms, but this might not be the case for yours.I didn't see you mention a cal kit so you might not have one yet? Having a characterised cal kit is critical as otherwise you are measuring the VNA internals and VNA coupler as well as your DUT.

author: TheSteve
author_rank: Unknown
timestamp: April 30, 2019, 12:23:13 am
content: Time to buy yourself a nice 85033D or E.

author: TheSteve
author_rank: Unknown
timestamp: April 30, 2019, 12:34:37 am
content: An ECAL would be awesome. At times the 85033D/E does go for reasonable pricing on ebay.

author: hendorog
author_rank: Super Contributor
timestamp: April 30, 2019, 12:54:31 am
content: Quote from: blueskull on April 30, 2019, 12:19:51 amThe load may or may not be that bad. The problem is that you are measuring everything in the VNA as well as the load.If you can accurately measure its DC resistance then you will get an idea of how close it is to 50ohms at low frequency at least.If you have access to a calibrated VNA somewhere else, then you can measure and therefore characterise your load and short and voila you have a cal kit.

author: DaJMasta
author_rank: Super Contributor
timestamp: April 30, 2019, 01:16:51 am
content: 6.5GHz top end means you can get cal kits more reasonably, but SMA/3.5mm kits are in demand, so it's not necessarily cheap. I think you're not going to be getting really meaningful data from it unless it's calibrated, though.The usual configuration is port 1 -> good quality cable -> connector saver -> DUT -> connector saver -> good quality cable -> port 2, then you run your OSLT calibration between the connector cables, then if you want to test a cable, it takes the place of the DUT. If you calibrated directly at the ports, you could probably get a reasonable result with just the one cable, but if you had DUT that is separate, you'd need to run your cal on, for example: port 1 -> connector saver, and then port 2 -> cable -> connector saver, then connect the DUT directly to the saver on port 1 and the saver on the cable from port 2. That also means that if the VNA is relying on a stored internal calibration, that includes any equipment attached to it the last time it was calibrated, so if that were the case, it could have been that your cable is in fine shape but the cables and adapters used in the last cal were slightly worn on one port's side.VNAs are designed to be really sensitive around their nominal impedance, so having the cal done and doing it with the full set of connecting hardware minus the DUT can make a pretty significant difference, since it can pick up on those tiny parasitics adding to the reading.

author: virtualparticles
author_rank: Regular Contributor
timestamp: May 03, 2019, 08:35:56 pm
content: Without calibration your S11 measurement is going to see the directivity error (leakage through the bridge), Source match error (Source isn't exactly 50 ohms), Load match error (The other port isn't exactly 50 ohms), and Reflection tracking error (The frequency response of the cable and port and bridge system). This will definitely be asymmetrical as those errors will be different from one side to the other.Using a home-made calibration kit might be better than nothing but it will probably fall apart at high frequencies unless you can find a way to characterize the open and short properly. If you have a friend with high-end VNA they might agree to measure your short and open and create data-base files.

author: DaJMasta
author_rank: Super Contributor
timestamp: May 03, 2019, 09:27:13 pm
content: I would definitely bring it up with the seller, but while there's a chance it's indicative of mishandling in the past, I don't think that will cause an issue, especially at the frequencies you're looking at. Since it's not on the connector and since the remainder of the connector should still do well for grounding and shielding, I don't think that damage will matter electrically, though there's a chance the burrs on the damage could wear out mating connectors prematurely.

author: TheSteve
author_rank: Unknown
timestamp: May 03, 2019, 09:27:39 pm
content: Well I certainly would not be happy with that.Considering the connectors are the most important part and that connector care is "critical for achieving specified performance".I wonder how that would respond when properly torqued. Any chance it will damage the male threads on the mating connector?If you don't get consistent mating with a connector then calibration is rather pointless.Unless it was crazy cheap I'd be sending that back.Things get a lot tougher to get good results up to 26.5 GHz.

author: TheSteve
author_rank: Unknown
timestamp: May 03, 2019, 09:59:10 pm
content: Looks about right to me. The open and short are 180 degrees out of phase.

author: TheSteve
author_rank: Unknown
timestamp: May 03, 2019, 10:09:35 pm
content: After the cal you can look at log mag of the 3 components - load should show amazing return loss(even if not true) and open/short should be flat. Your smith chart looks fine but I still like looking at log mag.You can then look at phase of the open and short and they should look very nice and be 180 degrees apart of course, no curved lines.If any connectors weren't making good contact it will show up instantly.

author: KE5FX
author_rank: Super Contributor
timestamp: May 03, 2019, 10:17:16 pm
content: Quote from: TheSteve on May 03, 2019, 10:09:35 pmCareful there, the linesshouldbe curved because the open and short standards aren't perfect. The VNA should be set up to take the actual properties of the standards into account during calibration, which means that measuring the same standards should reveal their dispersive effects.That's basically what he's seeing on the Smith Chart. We can tell that some non-ideal standard definitions were in place at the time of calibration, but of course we can't tell if they were actually the correct ones for that cal kit. Presumably that's not a problem with e-cal hardware.

author: TheSteve
author_rank: Unknown
timestamp: May 03, 2019, 10:22:20 pm
content: Quote from: KE5FX on May 03, 2019, 10:17:16 pmTrue - I was just meaning you shouldn't see funky arcs. I've been experimenting with generating calibration co-efficients for an unknown cal kit and have seen all kinds of stuff when my values are wrong.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 03, 2019, 10:33:20 pm
content: Quote from: blueskull on May 03, 2019, 08:54:49 pmthe rating is when it was new. overtime they will lost their "metrology grade" (as someone termed it) and not up to the rating anymore. my recent purchase with gore cable (rated 18GHz) i have issue with one of the cable connector, i have to wiggle my load cal to get flat 50 ohm line, otherwise i have some ringing. the other cable is fine. i tried not to blame the cable since maybe its something with my cheap (not so cheap) cal kit, but it only happened to one sma connector of the cable. looking at the connector of the cable (and my cal kit too) even using loupe, there's nothing wrong about it they look perfectly fine. so i ended up labelling the cable with connector issue as cable #2, the other cable with no issue is cable #1. my test only upper than 3-4GHz to see the ringing of slight mis-wiggling. at 26GHz and that botched connector, i dont think there is a chance. but how much price you paid for it? because we get what we paid for.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 03, 2019, 10:48:23 pm
content: Quote from: blueskull on May 03, 2019, 10:42:12 pmfor the load alone? thats quite pricey, its about twice the cost i paid for my complete cal kit from Kirkby. if i were you i will only be willing to pay that money if i can verify its performance up to the said 26GHz to know how much price i deserve for refund if its not performing. but ymmv.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 03, 2019, 11:20:14 pm
content: ok thats cheapdoes it come with their coefficients/parameters files?

author: TheSteve
author_rank: Unknown
timestamp: May 03, 2019, 11:23:55 pm
content: Quote from: Mechatrommer on May 03, 2019, 11:20:14 pmIt's a Keysight 85521A cal kit so you can download the parameters from the website and they are included in some VNA's(such as the FieldFox).

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 03, 2019, 11:30:17 pm
content: Quote from: TheSteve on May 03, 2019, 11:23:55 pmyeah its amazing how they can machine that standard to such precision, the C & L coefficients are even available in the datasheet (no need custom coeff for each cal kit)...https://literature.cdn.keysight.com/litweb/pdf/85521-90001.pdf?id=2232814so it should be easy to verify that unit with any compatible VNA available.

author: OwO
author_rank: Super Contributor
timestamp: May 04, 2019, 02:50:11 am
content: Looking at the graphs it's pretty similar to the performance I get with a $300 VNA, homemade cal kit, and ordinary SMA cables from mouser (up to 4GHz).

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 04, 2019, 03:05:54 am
content: retry calibrating your VNA with some descent quality SOL kit and then measure the openness, closeness, and 50 ohm loadness of your other/diy cal kit... and then add your sma cable to the calibration plane + load used to calibrate the plane at the end of the sma cable. do you get satisfactory result?

author: KE5FX
author_rank: Super Contributor
timestamp: May 05, 2019, 02:05:39 am
content: Quote from: blueskull on May 05, 2019, 01:50:52 amNot an issue, you'll never notice the difference. 3.5 mm is just SMA with an air dielectric on the center pin, and the assorted SMA-based cable and devices you measure will probably vary more from one to the next than the best of them will differ from your 3.5mm standards.I don't think HP even sold an "SMA" cal kit, did they?

author: virtualparticles
author_rank: Regular Contributor
timestamp: May 28, 2019, 11:30:55 pm
content: The plots you show look about right. A "short" is actually a short on the end of a small 50 ohm transmission line. The "open" is an open on the end of a small transmission line. The result is a clockwise arc on the Smith chart starting at 0 or 180 degrees due to rotation by twice the distance to the short or open as seen by the VNA. Looking at this doesn't actually tell you anything though. If you tell the VNA through the calibration definitions that the short looks like "this" and the open looks like "that" and the load is a dot in the middle, you can attach a piece of wood to the connectors and the VNA will force the display to look like each of those things. (Slight exaggeration)The only way to check a calibration is to use an independent calibration piece.

author: virtualparticles
author_rank: Regular Contributor
timestamp: May 28, 2019, 11:43:44 pm
content: If you have access to a calibrated VNA you can make a couple of semi-rigid cables. The longer one should be 20 degrees long at the lowest frequency you want to measure. The second should be 20 degrees long at a slightly lower frequency than where the first cable is 160 degrees long. Repeat this once again if you need more frequency range. You'll need to know the time delay through each of these cables. They don't have to be perfect and they can overlap quite a bit. Each of those cables will be characterized by a start and stop frequency and a delay. These will be your "lines"The next thing you need is a "bullet", a really short SMA-SMA or 3.5mm to 3.5mm. You'll need to know it's delay as well. That will be your "Thru"Then create a short. You'll need to measure it to find out the delay from the connector to the actual short. With all that data in hand you can create a TRL cal kit. No load is needed. Most shorts are also characterized by a third order polynomial to model spurious inductance but in practice this adds only a fraction of a degree to the accuracy. This will be your "short"