#77: The Reference Plane

[@wb5mgr]

This was a great video, and I expect eye-opening for many. Lead compensation is very important in quality measurements. As always, you have to decide what your allowable tolerances are in the measurements that you desire to take, but at least when you understand where the sources of error come from you can choose where to focus on preventing them.

[@w0qe]

I agree completely. Larry

[@tomthompson6911]

Another good one, Larry. It really makes you wonder how the UHF radios of the 1950's ever worked.

[@johneslaughter]

Highly informative video. Thanks!

[@w0qe]

You are welcome John. Larry, W0QE

[@ac8vk808]

This is fantastic content. The concept of understanding and getting the most out of the tool we have, is a core philosophy of mine. Understanding these issues is so helpful. Question: Is this type of issue addressed (or reduced in magnitude) by using Calibration and DUT PCB fixtures at are matched? (and my apologies for initially posting this question on the wrong video)

[@w0qe]

AC8VK, Doing a calibration such that there is no need to or only very slightly adjust the reference plane will give more accurate results. However remember that "Perfect is the enemy of getting anything done". SimSmith is a great tool for deciding how close to "perfect" you need to be. I can easily do a video on this topic if folks are interested. 73, Larry W0QE

[@ac8vk808]

@@w0qe Thanks.

[@smahax]

Hi, lot of interesting explanation but there is something that I don’t agree is where you define the calibration plane on the VNA when using an official calkit. maybe because I m using modern network analyzer. If you look to the calkit definition(off the shelves not DIY) the different standards are define at the reference plan which is the connector itself. You can see it when looking to their definition each of them have a delay (except some short on the market) in order that the VNA know where is the cal standard definition plane. Therefor after calibration you don’t need to worry. However I agree with the purpose of the video because when using non standard connection like your alligator clip you need to compensate for this electrical length. This is where some people do the mistake by using the short of the calkit thinking it is the perfect short at the cal reference plane. But the calkit short is define with the ps delay. So on the Schmith chart you can compensate for this delay and voila your short is a short(as your explain it)

[@w0qe]

Smahax, I don't think there is any disagreement here. Chapter 5 in my manual for my HP VNA shows how the analyzer takes the true open and short which is within the calibration standard and moves this location to be at the plane between the connectors. Therefore if you try to measure the open and short after calibration you will see rotation on the Smith chart and they will appear to not be a pure open or short. The 2 HP cal kits I have come with a disk that can be loaded to know the parameters of the cal kit more closely and the cal kit itself is capable of being calibrated. It isn't necessary that the distance correction for the open and short be the same provided that the VNA knows how to correct it. The small VNAs have no such ability and the cal kits are assumed to have the same correction for the open and the short and the VNA does NOT know what this exact value is so the correction must be done manually. Glad you found the video interesting. Larry, W0QE

[@smahax]

@@w0qe thank I got it. I never had the chance to use those small VNA. So know I understand the extend of your explanation. I thought all VNA have the same capability as the old very good hp you mentioned. Thanks

[@sebastianocoppolella7637]

The calibration plane using the professional kit is placed a few millimeters above the junction point between the male and female pins. The short is exactly 180° apart from the open. For example in my VNA if I calibrate a port, directed to the instrument connector I will have zero degree phase with open inserted. While -70° without connector at 4GHz. If I insert only a male pin without a 10mm long connector it returns to zero degrees. In my opinion, moving the offset of the calibration plane is wrong because the male pin must be inserted in the female and with the correct gap! The problem instead is when using trhu adapters in the case of non-insertable dut or adapters after calibration. Those have to be compensated for if precise measurements above 2-3GHz are to be made

[@galileo_rs]

A video on how you made those loads would be interesting. How did you fix the BNC and N connector pins so that they do not move?

[@w0qe]

Dragan, I will make this video soon. I did nothing special to keep the center from moving other than to pick connectors where the center pin was very rigid. The 2 piece captive pin N male connectors came from Array Solutions in Texas. I will see if I can find part numbers for some of the other connectors I used. The BNC male connectors were 30 years old but they may still be made. In the past I made O/S/L loads with connectors where the center pin had some free movement and I mated the connector I was working on to a mate of the other gender when soldering the resistors. Then I used a micro amount of super glue to guarantee the center pin alignment. This worked well. Larry, W0QE

[@galileo_rs]

I guess I will have to stop using cheap connectors from China ;) Looking forward to that video, it is always interesting to see other peoples construction techniques, often I find them quite instructive since I do not have much hands on experience.

[@janrozema]

Thanks for the video Larry

[@w0qe]

You are welcome Jan. Larry

[@singlehanded12]

Awesome, Thanks again.

[@w0qe]

You are welcome! Larry, W0QE

[@johnopalko5223]

Thanks for another great video. 73

[@w0qe]

Glad you enjoyed the video John.

[@vidasvv]

TNX for another GREAT video ! 73 N8AUM

[@w0qe]

Glad that you found the video worthwhile. 73, Larry, W0QE

[@Raoul_Volfoni]

Excellent video. The easiest way to set the port extension time for dumies like me, is to make a short and and display the S11 smith and adjust the delay until you get a (almost) single point at the 0 Ohms point. Once done, open should works also at the infinite point. Some VNA have an auto port extension function that do it automatically.