Accurately measuring balun phase delay with nanoVNA (or some other tool)? - Page 1
Forum section: electronics / rf-microwave

author: msat
author_rank: Regular Contributor
timestamp: April 21, 2023, 08:03:32 pm
content: Hello all,I'm working on a little science project that involves some RF transmission/reception, and having accurate phase delay information for all the components is critical. At the TX and RX dipole antenna ends, I plan on using transformer-based baluns that I made (see image). The SMA connectors on the balanced side are for convenience with the center conductor of each connected to one leg of the balanced outputs (and the shell connected to ground). Each dipole half will connect directly to one of the SMA connectors on the balanced side.Firstly, I'd like to specify that this will be operating at only one frequency - around the center of the 900MHz ISM band, and therefore I don't need to concern myself with the group delay - just the phase delay between the unbalanced to balanced legs at a singular frequency.So I took a tip fromhttps://www.w8ji.com/balun_test.htmfigure 1 for current baluns (I also made some PCBs with voltage baluns, but I won't worry about those for the time being). What I did was attach my nanoVNA CH0 to the unbalanced side, and CH1 to one of the balanced legs. Initially, I used the SMA short terminator from my calibration kit and attached it to the other balanced leg. This provided some result, and when I swapped over the balanced ends, I would get a roughly 180 degree phase shift, as is to be expected. What I don't know is the validity of this information, because if I were to replace the short terminator with an open of 50 ohm terminator instead, the indicated phase shift would be substantially different. I understand that improper termination would cause reflections that would have an effect on the indicated phase, but I wouldn't even know where to start on figuring it out on a 3 port device like this.Any suggestions on how to go about testing these would be greatly appreciated.If I can't figure this out with sufficient certainty (at least with the tools I have available to me), I'm thinking of just not using baluns and feeding the dipole directly from a coax. In that case, can I apply the standard phase delay information of the feeder coax?

author: joeqsmith
author_rank: Super Contributor
timestamp: April 22, 2023, 07:38:28 pm
content: So you don't care about the phase error between the two balanced ports, only the delay through it at a single frequency. You want to know only the components delay, not the PCB, connectors and cables. You want to know this with the BALUN loaded the way you plan to use it.What ever the experiment is where the delay through the BALUN is critical, you are already accounting for all of the other errors somehow. It's not something that you can just compensate for with a trimmer?It seems like you would need to consider how to calibrate it and then how to match it. Maybe the attached paper will provide you with some ideas how this could be done.Consider posting some details about the overall experiment. Maybe there is a better way to accomplish what your trying to do.https://scdn.rohde-schwarz.com/ur/pws/dl_downloads/dl_application/application_notes/1ez59/1EZ59_0E_RS-ZVx_Impedance-Matching.pdf

author: joeqsmith
author_rank: Super Contributor
timestamp: April 22, 2023, 11:06:08 pm
content: https://cdn.ttm.com/repository/products/wireless-xinger/balun-transformers/B0430J50100AHF/B0430J50100AHF_97_H.pdfAttached BALUN mounted on FR4. One one port shorted, the relative delay is 78 degrees. Open, 48 degrees, Load, 58 degrees.So about a 30 degree shift. At 900MHz, 3.1 ps per degree. So about a half inch difference in the cable, connectors, PCB would cause that kind of error.Keep in mind, that's a relative measurement.

author: msat
author_rank: Regular Contributor
timestamp: April 23, 2023, 06:07:30 am
content: Hey Joe,I was actually hoping you'd chime inQuote from: joeqsmith on April 22, 2023, 07:38:28 pmIf I can figure out the phase delay through one of the balanced ports, then I can do the same for the other and find the phase error between the two, but yeah, I'm not too concerned about that aspect. Specs list them at something like 180 degrees +/-5 and that seemed to be the case regardless of how they were loadedQuoteMy experiment will require that I know the phase delay of all the components in the setup. So I'll measure every part individually and then account for it in my final S21 phase measurements. Since the baluns I'm using are part of an assembly consisting of a PCB and SMA connectors, then I "only" need to know the end to end phase delay of the entire assembly, and not of the balun transformer itself. If the phase shift through the part depends on the way it is loaded (which appears to be the case), then yes, I want to know what the delay is based on the way it'll be used. As far as I'm concerned, that's the only thing that matters. So I guess figuring out a way to simulate the loading and measure that is the entire question at hand!(As an aside, I modified the nanoVNA firmware so that I can manually enter in a phase offset value which gets automatically added to the phase measurement - it wasn't necessary, but added a bit of convenience factor when reading the phase info. I also made it so when pause sweep is selected - which is now the default on power up - there is no output on "CH 0")QuoteMy experiment is performed by taking an end to end phase measurement, therefore the delay through all the components have to be quantified so they could effectively be nulled out from the final measurement. This is a pretty straight-forwards process for all the other components (amplifiers, filters, cables, attenuators), it's just these baluns that have me confused.QuoteI will read through the paper when I get a bit more time (I work long hour weekends). Thanks!QuoteQuick drawing attached. As you can see from the image, I'm using the nanoVNA-PRO 900000000X

author: msat
author_rank: Regular Contributor
timestamp: April 23, 2023, 06:22:54 am
content: Quote from: joeqsmith on April 22, 2023, 11:06:08 pmI didn't record my measurements, but I recall the difference being fairly substantial depending on how they were loaded, as you've seen with your own measurements. I'd like to better understand what the delay will be when the balanced side of my balun PCBs are attached to dipole halves

author: gf
author_rank: Super Contributor
timestamp: April 23, 2023, 07:36:47 am
content: Quote from: msat on April 23, 2023, 06:07:30 amIf you want to do it component by component, then you need to measure thefullS-parameters of each component because the components interact with each other. Measuring only the S21 phase response with 50 ohm source and 50 ohm load is not sufficient to calculate the phase response of the component with a different source and load.QuoteMeasuring the end-to-end phase response is no problem with a VNA. However, calculating delay from phase for a single frequency is ambiguous if the delay range of interest is larger than a single period of the signal. In order to unwrap the phase properly you rather need a frequency swep which starts at a low enough frequency. But the bandwidth of the DUT may prevent low frequencies from passing through, as I see a bandpass filter directly at the input.Quote from: msat on April 23, 2023, 06:22:54 amIt also depends on the environment of the antenna, since the antenna interacts with its environment (unless you can place the antenna in free space).And for a single antenna, you can't measure its "delay" anyway. Only an antenna pair forms a two-port network, where you can measure a delay between the two ports. I.e. you need to consider both antennas as a single 2-port component. So why don't you treat the baluns as if they were part of the antennas as well? Then you also don't need to deal with mixed-mode S-paramters for the balanced connection.Or why not even go one step further and ignore the individual components, and measure only end-to-end? Eventually this is also more accurate, since the measurement errors aggregate when you measure each component individually.

author: joeqsmith
author_rank: Super Contributor
timestamp: April 23, 2023, 02:11:42 pm
content: I was looking for the bigger picture of what you are attempting to do. Is your goal for this experiment to measure delays of the waves through the air and other media? I take it with the two amplifiers, these antennas are located a fair distance apart.

author: rf-messkopf
author_rank: Regular Contributor
timestamp: April 23, 2023, 05:53:21 pm
content: Quote from: msat on April 21, 2023, 08:03:32 pmTo fully characterize a linear DUT with port 1 single-ended and port 2 symmetrical, the usual way to go is mixed mode S-parameters. These include \$S_{\rm 21ds}\$, which describes transmission from single ended to differential mode (i.e., insertion loss and phase for that mode), and \$S_{\rm 21cs}\$, which is transmission from singe-ended to common mode (aka common mode rejection, which is a measure for the DUT phase error).You don't necessarily need a symmetrical test set or baluns to measure mixed mode S-parameters. If you consider your DUT as a three port device (with port 1 the single ended port, and ports 2 and 3 the two conductors of the symmetrical port), you can convert from the full \$3\times3\$ S-matrix to mixed mode S-parameters. Seeherefor example. Also,this application noteincludes an easy introduction.If you don't have a 3-port VNA to measure the full \$3\times3\$ S-matrix, you would have to make several connections with a 2-port VNA, paste the data together, and then convert to mixed mode. Also, you can do de-embedding to the DUT's SMD pads in the usual way before converting to mixed mode.Needless to say that this is not a true differential or common mode measurement, and assumes the DUT to be linear. You can get real differential measurements in upmarket VNAs as a hardware option; Rohde & Schwarz calls that "true differential mode".

author: msat
author_rank: Regular Contributor
timestamp: April 23, 2023, 08:45:50 pm
content: I appreciate all the responses!Quote from: gf on April 23, 2023, 07:36:47 amThis wouldn't have dawned on me, but I think I can see how that would be the case just given the effect I've observed on the balun measurements when terminating the 3rd port in different ways.QuoteSince most of my other experiment parameters will be known, ambiguities that arise from a phase delay greater than 180 or less than -180 degrees isn't an issue.QuoteI will try to place both antennas in free space as much as practical as well as separate them enough to minimize near-field effects.Last night I was thinking along the lines of what you suggested: end-to-end baseline measurement, perhaps touching the bases of the TX dipole halves with the bases of their corresponding RX dipole halves. Though if I do indeed use amplifiers in my setup, then I'll need to do so with attenuators at certain points during that measurement, and hope that they add negligible uncertainties due to their interactions with the rest of the circuit as you mentioned could be an issue earlier.

author: msat
author_rank: Regular Contributor
timestamp: April 23, 2023, 08:55:01 pm
content: Quote from: joeqsmith on April 23, 2023, 02:11:42 pmThis whole thing goes back a couple of years, when I was trying to figure out a way to test a hypothesis regarding the so-called "3 polarizer paradox" and I suppose the interaction of EM radiation with matter in general. I posted some questions on this forum a while back, but since I have to leave for work shortly, I don't have time to look them up.But specifically to your question: The antennas will be spaced apart far enough to minimize near-field effects, and that's it. I don't know whether I'll actually need both amplifiers or even any amplifiers at all (haven't done any calcs), but I suspect that I might at least need an LNA on the RX side.

author: msat
author_rank: Regular Contributor
timestamp: April 23, 2023, 08:57:51 pm
content: Quote from: rf-messkopf on April 23, 2023, 05:53:21 pmThanks for all the info! I will look at these papers when I get some more time. Hopefully I'm at least smart enough to understand some of it

author: joeqsmith
author_rank: Super Contributor
timestamp: April 25, 2023, 01:15:28 pm
content: Quote from: msat on April 23, 2023, 08:55:01 pmShown are two horns. They are clocked 0 or 90 degrees. There is a polarizer that is clocked at 0,45 or 90.VNA wasn't calibrated as we are really just looking for a relative change. Brown and black show the two horns in phase, with and without the polarizer also in phase. Red is with the two horns 90 degrees out of phase with the polarizer at 90 degrees. Orange showing the two horns 90 degrees out of phase and the polarizer at 45 degrees.As you can see, this experiment wasn't anything fancy. Just some PCB material to make the horns and coaxial transitions, some wire and wood blocks for the polarizer.

author: msat
author_rank: Regular Contributor
timestamp: April 25, 2023, 11:08:42 pm
content: Having seen a similar setup as yours (tx & rx horns and a polarizer grating) with demo on some university's website is actually what led me to buying some cheap RF electronics & waveguide hardware for experimenting. This led to my interest in potentially 3D printing horns (since they're among the most expensive components on the used market) so I was quite happy when I saw you making your latest videos covering so many aspects of this subject (and I learned a lot! Thanks!! btw, I found it fascinating how the waveguide cal kits work!)I've gone over the papers linked in this thread along with some others and I'm still confused, though about other things. While I believe that with the info I now have, I could measure my balun assemblies themselves, which, to be fair, is what I asked for, that itself is of limited utility as I will have limited knowledge of how their behavior changes when attached to their antennas, which I can't individually test afaik. I think taking an end-to-end measurement and using that as a baseline as gf suggested will be the way to go. And if I can't make any sense of the results of the experiment, then I'll look into this further.Lastly, I'm gonna keep that impedance matching technique in my back pocket

author: joeqsmith
author_rank: Super Contributor
timestamp: April 27, 2023, 11:12:03 am
content: Is your goal then to run similar experiments with dipoles?There are a few YT university physics channels I follow. I watched some good videos where they ran a similar demonstration using light bulbs for the receiver. The visual effects are an effective way to show what is going on.If you get something going, consider posting about it.