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Maybe the misunderstanding is in the fact that the reflectioncoefficient has another phaseangle than the phaseangle between current and voltage on the load. These are two different things. The relation is in the already quoted formula.


73,

Arie PA3A

Gregg-

I concur with your understanding 100%.

Reflection phase with an open is zero degrees and is 180 degrees with a short. Reactive loads (inductive or capacitive) will have phase angles somewhere between 0 and 180 dependent on the reactance at a particular frequency which depends on reactance (Zl +jX or - jX).

See the last 2 sentences in the book section on Purely reactive loads that was cited.

Ray WB6TPU
Senior Signal Integrity Staff Engineer
(Retired)

Where Roger and I have differed in the past is with regards to the concept of the S11 phase as measured by the nanoVNA.

While Roger has firmly stated in previous conversations that there is no relationship between the S11 phase and the phase change between voltage and current observed in reactive circuits, I still hold to my original thought that the voltage vs current phase of a reactive device is in fact related to S11 phase. It simply must be!

As the reactive component causes a change in phase of the voltage and current, so to does it cause a change in the S11 phase as measured by the nanoVNA.

Please see “Purely Reactive Load” in this reference:

(Ellingson)/03%3A_Transmission_Lines/3.12%3A_Voltage_Reflection_Coefficient

(Sorry, the link is not fully recognized, so one will need to cut and paste the link manually into a browser. I have included a screen shot with this post.)

The formula under the “Purely Reactive Load” heading for the voltage reflection coefficient clearly includes reactance. Where a reactive component is present, there will also be a change in phase between voltage and current at any given frequency.

Thus, I maintain that S11 phase is in fact related to voltage vs current phase.

Please feel free to prove me wrong.

--
VE6WGM

Forgot to my Signature

--
Cheers,
Ian
Melbourne, Australia

Hi Roger et al,

I am not about to admit to how many of those misconceptions I have fallen for .... publicly .... :)

I have been 'safe' in the analogue, low frequency, ideal component electronic world for awhile, but kept bumping into non-ideal component problems. I purchased my nanoVNA primarily as a challenge to that environment. It, and this Group, have been great teachers. Thanks.

Re: your Freq Phase S11 graph ... can you elaborate please? ....
* The Commentary refers to a misconception around S Parameters and Impedance phase ... is the distinction here Reflection co-efficient Gamma phase => Incident and Reflected voltages .. versus .. Impedance phase => Voltage and Current .... ???
* Can you describe the 'drivers' of the 3 key points to this graph ie 0 deg, -145deg, and +/-180deg?? 360deg rotation at self resonance??

I am unsure as to how many more 'nanoTraps' I have yet to climb out of .... :)

The use of a small piece of Teflon or other plastic between the contacts of the clothespin for doing the OPEN calibration may be a bit problematic.

It will increase the capacitance between the contacts which will probably lead to a less accurate calibration . I would suggest finding a way to hold the contacts open without the inclusion of the plastic.

For doing cals up to about 1Ghz for a nanoVNA it probably isn’t that big of a deal, but just thought I’d mention it. (I’m used to doing vna cals up to 30 or 40 GHz where it is a big deal)

Ray WB6TPU

Was just over reading Roger’s excellent thread on using a VNA to examine discrete components. I figured I should be clear with everyone that the method I presented below is not and was never intended to be used for higher frequency work. If you’re trying to determine device behaviour at higher frequencies, perhaps for an RF project you might be working on, the method of measuring inductors and capacitors that I presented in the video linked below is not suitable. The intent is to give people a simple way to quickly measure inductor and capacitor values at lower frequencies so as to simply discover or verify the basic component values. Using alligator clips at higher frequencies is not going to give very good results, so if you’re wanting to characterize a component at higher frequencies then you would be well advised to go take a peek at Roger’s thread here: /g/nanovna-users/topic/pitfalls_of_measuring/80744049?p=

--
VE6WGM

On Fri, Feb 19, 2021 at 12:44 AM, Adnan Yousaf wrote:

Dear All,
I want to use NanoVNASaver for continuously reading and storing S1.p data i.e.
automating my experiments for long duration tests (few hours). Since there is
no LabVIEW tool available for NanoVNA, I tried to use the Saver version v0.3.8
but with no success.
Would be great if someone has already a tip or solution in this context.

The NanoVNA app by One Of Eleven has this feature built in. It records .s2p files that contain the S11 data that you need. You go into continuous scan mode and then click the record button. It writes about 30 or 40 files per minute.

To use it you need to update your firmware. You can find the app and the necessary firmware here. I use the latest DiSlord version 1.0.45 instead and it works great. It is .rar compressed format ( like zip ) so you need a program like 7 Zip (free) to decompress it.



Roger

Manfred,

Thank you for your detailed and informative post. Lots of good information and tips on how to measure components. I have not seen the same problem with cables up to 30 cm but I will keep what you posted in mind. I agree with you about alligator clips being a poor test jig. It is so easy to construct a test jig that I never use them.

The NanoVNA MOD v3 software has some nice plotting features but I prefer the NanoVNA app by OneOfEleven that works with the 1.0.45 firmware by DiSlord. The firmware does 401 points on the -H4 instead of 101, faster data transmission and some nice new user features on the NanoVNA. The NanoVNA app is the best PC program I have used to date with nice scaling and plotting features, trace smoothing, calibration averaging and the ability to do firmware updates and much more.

That clothespin idea is really interesting for testing SMD parts. I am going to build a jig using this method. If you have one already built could you please post some pictures.?

You mentioned the serial and parallel impedance measurement of components. This is not familiar to some users and I will be doing a post later showing how this can be useful when measuring higher impedance parts.

Thanks for joining the discussion and sharing your experience.

Regards - Roger

On 2/19/21 5:44 AM, Larry Rothman wrote:

Adnan,
Do a search on the message forum for OCTAVE (similar to MATLAB)

There is a lot of info on using it with the instrumentation library - that may be useful to you.
...Larry

On Friday, February 19, 2021, 3:44:44 a.m. EST, Adnan Yousaf <adnan.yousaf@...> wrote:
Dear All,
I want to use NanoVNASaver for continuously reading and storing S1.p data i.e. automating my experiments for long duration tests (few hours). Since there is no LabVIEW tool available for NanoVNA, I tried to use the Saver version v0.3.8 but with no success.
Would be great if someone has already a tip or solution in this context.

Best Regards
Adnan

There is some python code in the NanoVNA repo that gives examples of how to access the NanoVNA, send commands, and retrieve data. It's somewhat simpler than NanoVNA_Saver, which is also Python.




For your application, you don't need the fancy graphics in NanoVNA-Saver.

Roger,

I think you basically said it all with this list of pitfalls:

- limitations of the NanoVNA hardware and software
- limitations of the test jig
- quality of calibration load and calibration method
- excess lead length
- misconceptions about the component under test
- insufficient technical knowledge

I can only add a little about my own experiences measuring components:

When trying to measure through20cm long SMA cables and connectors, I often got strange results. It seems that the NanoVNA's measuring range gets restricted when it has to compensate for the effects of those cables. I get much more reliable and credible results when using the test jig I published here, several days ago, which has only a very short piece of coax cable and large contact surfaces. So I would suggest to make and use any sort of test jig that has as short a connection as possible to the NanoVNA, so there are less parasitics to calibrate out.

About calibration: The best, of course, is having a perfect short and a perfect 50? load for calibration... But not having those, one can get closer to the truth about a component by adding the known (or estimated) imperfections of the test loads to the measured data. For example, for my basic simple test jig I use a shorting wire stuck to a clothespin, which is pretty good, having maybe 1 or 2 nH. But my 50? load is two leaded quarter-watt 100? resistors in parallel, and these obviously have some significant inductance. How much? Well... measuring any single resistor of the same size and type, with the same lead length, and of reasonable value, reports roughly 5nH of ESL. Now, it's logical that two resistors in parallel, reasonably separated, should have half as much inductance as a single one. So I conclude that a single quarter-watt resistor has around 10nH, and my 50? test load, having two of them in parallel, has 5nH. And that's why a single resistor measured in my jig reads 5nH. A poor calibration load, but entirely possible to compensate for!

With this jig I have been playing around over the last days, measuring resistors, capacitors, inductors, transformers (main inductance and leakage inductance), and crystals. It works very well.

I use the NanoVNA MOD v3 software, and find it most practical to use the series RLC display for most component testing. While named RLC, it actually gives RXZ curves, not RLC... But the L or C values are easily obtained by sliding the mouse over the graph. This allows easily seeing over what frequency range the capacitance or inductance stays pretty constant, and that's likely to be the range in which the nanoVNA is reading reasonably correct while at the same time the component value is still reasonably pure.

With relatively high impedance parts I switch into the parallel RLC display, which then makes more sense because the parasitics in high impedance parts tend to be dominated by parallel capacitance. It's important to understand what one is measuring, of course.

When I measure leaded parts, I include as much lead length in the measurement as I will need later when mounting the part on the board. Usually the shortest length possible. Since my test jig has wide connection areas, its own inductance is reasonably low, and the exact spot on the jig where each lead connects is rather uncritical.

A while ago I was comparing the performance of quarter-watt carbon film and metal film resistors, at frequencies into several hundred MHz. It works nicely, and is very useful to dispell long-standing myths, to select the best parts for a project, and even to find out whether a given part is actually usable for the intended application.

About alligators: They add quite a bit of inductance, which can be calibrated out to some extent, but limiting the NanoVNA's measuring range. What's worse is that the flying leads can end up in different positions between calibration and use. So indeed I would only use them in situations where 10 or 20nH uncertainty, and 1pF or so uncertainty, are acceptable. A fixed, solid test jig is always better.

I have become a fan of clothespins. For measuring SMDs, an easy test jig could be made from a single clothespin, with two copper foil (or PCB pieces) glued to it, and a short piece of coax cable with an SMA plug soldered to them. The short circuit calibration would be done simply with nothing in the clothespin, just a direct contact. A small 50? SMD resistor would be used as 50? calibration load. And a small piece of plastic, such as teflon or polyethylene, the same thickness as the length of the SMD to be tested, as an open calibrator, or simply doing the open calibration while holding the clothespin slightly open. This jig should allow pretty accurate testing of SMDs. Just the tiny inductance of the 50? calibrator would need to be compensated for after the measurement.

The obvious advantage of such a clothespin SMD testing jig, of course, is that the part can still be used after testing... I would prefer not using an SMD in a circuit after having soldered it to a test connector, and then desoldering it... And I hate wasting parts.

Adnan,
Do a search on the message forum for OCTAVE (similar to MATLAB)

There is a lot of info on using it with the instrumentation library - that may be useful to you.
...Larry

Dear All,
I want to use NanoVNASaver for continuously reading and storing S1.p data i.e. automating my experiments for long duration tests (few hours). Since there is no LabVIEW tool available for NanoVNA, I tried to use the Saver version v0.3.8 but with no success.
Would be great if someone has already a tip or solution in this context.

Best Regards
Adnan

This is very good. Thank you for sharing!

--
VE6WGM

Hi Mike

Here is a brief textual overview of what I shared in the video..

To measure an inductor or capacitor, use a shunt configuration (basically attach the component to your nanoVNA using “alligator leads” so as to be able to make S11 measurements... hahaha! Oh yeah, I said that! Sorry RF Engineers.) Measure the capacitive or inductive reactance at a frequency where the reactance of the component is equal to the characteristic impedance of the system you are using to measure the inductor or capacitor (most likely 50 ohms). This will correlate to a phase angle of 90 degrees as measured by the VNA (positive for an inductor, and negative for a capacitor).

This strategy places the impedance being measured within the nanoVNA’s abilities to measure accurately, and ensures that the measurement is performed well below the self resonant frequency of the component. You’ll get correct results so as to be able to label an unknown inductor or to verify the value of a capacitor.

Where the use of such ‘crude’ test fixture will break down is if you happen to be measuring components that require a relatively high frequency in order for the reactance to be 50 ohms... in that case, the high frequency unfriendly alligator leads will need to be replaced with a proper test fixture.

--
VE6WGM

Hello Roger,

Thank you very much for the professional description.

I think alligator leads are not useful in VNA measurements above 5 MHz.

73, Rudi DL5FA

References were given as convenient clickable links in the description below the YouTube video.

--
VE6WGM

I thought it might be interesting to start a post dealing with how to accurately measure components like inductors, capacitors and resistors on a NanoVNA. As an RF engineer I have made mistakes over the years using trial and error methods. By reading technical books/articles and posts by from others skilled in this area I learned about some of the pitfalls one can easily make and how to avoid them. So I hope some of you will jump in and share your tips and knowledge.

It is easy to make a mistake or draw a false conclusion about a component's characteristics when using a VNA. This can be due to many factors including the following:

- limitations of the NanoVNA hardware and software
- limitations of the test jig
- quality of calibration load and calibration method
- excess lead length
- misconceptions about the component under test
- insufficient technical knowledge

Rather than discuss each of these individually it might be more interesting to do some tests on actual components and point out the pitfalls and things to watch out for when making these measurements.

Here is a test I did on a 47 pF SMD capacitor using a NanoVNA-H4 with the DisLord 1.0.45 firmware. The PC software used was the NanoVNA app by OneOfEleven. Both of these individuals have done an excellent job developing this software and thanks for sharing it with the user community.

The test fixture used was a female SMA connector with a small header row attached. Cal loads were made using male pins and a SMD 49.9 ohm cal load. The idea originally came from a post by Owen Duffy on his blog. A annotated photo of the setup is attached.

A sweep from .05 to 900 MHz. was done and the reactance plotted (graph attached). A graph converting the reactance to "apparent capacitance" is also shown. A pitfall made by those new to VNA's is to assume this is the actual capacitance vs. frequency of the part. This is not the case because a physical capacitor also has some inductance associated with it as shown in the simplified capacitor model. This results in positive inductive reactance adding to the negative capacitive reactance and a faster rise in total reactance with frequency than just the capacitor alone. In the graph below note the rapid reactance increase with frequency as we get close to the self-resonant frequency (SRF) of the capacitor.

Now one might think it is reasonable to measure at a very low frequency in order to get the actual capacitance but this is not possible because the capacitance reactance is so high at low frequencies that it cannot be actually measured by the VNA. By looking at the markers you can see that we can only start making accurate measurements around 2 MHz. for this part. Another instrument in my lab a DE-5000 can make these measurements at low frequencies (below 100 kHz.) and the part measures close to 47 pF in this range. The frequency at which accurate measurements can be made will be based on the capacitance of the device under test. Larger capacitors can be measured at a lower frequency and smaller ones like 10 pF at a higher frequency.

If we wish to determine the frequency of self resonance we can look for the frequency where the reactance is zero. This is when the capacitive reactance and inductive reactance are equal and opposite in sign and result in zero at the SRF. This is also the frequency where the S11 phase angle abruptly changes from - degrees to + degrees for this example. For those familiar with Smith charts this is a data point on the horizontal line of the Smith chart.

Roger

Can the menu buttons/text be made larger? My eyes are getting old :-)

The TinySA's menu system is much larger and easier to see.

Thanks!

Gregg,

I really appreciate your videos but I am old school and like stuff in writing. Would you have this procedure documented? Nothing fancy but just a short synopsis of
theory
settings
calibration
inductance measurement
shunt configuration accuracy reference
capacitance measurement

Thanks,

Mike N2MS