Measuring low impedances with a VNA

[precaud]

Well the last set of plots was too good to be true (an axial-lead R025 measuring flat Z to 2MHz ? Nope!). After correcting the phase inversion in the thru sweep routine, the curves look more realistic now. The first set of plots shows 25, 50, and 100 mOhm axial R's with short compensation. Above 100mOhm the resistance starts to swamp the reactance mess, and the curve becomes increasingly flat and extended.

The second set of curves shows three 10mOhm current sense resistors with different "leads"; a radial, an axial, and an SMD.The inductance of the axial package is clearly evident. Most interesting to me is that the radial-lead one was nearly as low-inductance as the SMD.

So as it sits, this setup is usable to 200kHz from 10mOhm up. 2MHz from 100mOhm up. If everything but the xfmr, clip leads, and Rsense was moved to a pcb, and the plugs/jacks were replaced by a low-impedance switch, it would no doubt extend higher in freq.

EDIT: I have added a pic of the three 10mOhm R's used in the second plot. In the plot, the SMD and radial parts measured quite similarly. And in the pic, we can see that their terminal spacing is pretty close. So as Pitrsek (and others) have said, the spacing sets the baseline inductive profile.

[precaud]

Going back to the shunt-thru method... I made a "fixture" to test SMD parts by cutting off the barrel of one end of a BNC tee flush with the center receptacle, so SMD's can be soldered directly across. See first pic. I have plenty of these things laying around doing nothing... finally putting them to good use 

Second pic shows the Z/phase in the MS420K using the shunt-thru method, axial vs SMD R025's. Much improved for the SMD. There is still some residual inductance in there, though; the Z rises too far and too fast. And the phase curve sits a bit above 0º even down at 1kHz. Makes me think it might be a setup error. Or perhaps short correction needs to be used. Thoughts?

Pitrsek, when you do your "through" measurement, is it simply the test setup without a DUT? That's how I did mine.

I'm 3/4 done making the rigid coax probe. Just gotta find a test point to use for the ground. Sad to say, I threw away a perfect specimen a few weeks ago. Sigh...

[R_G_B_]

I was looking at this method:inserting a resistor between each VNA port to increase dynamic range "TEE" configuration.

I have A VNA 8714  and anritzu network analyzer, 10H 30Mhz network analyzer,   and analogue discovery with the updated software that now supports impedance analysis. I was following this thread and I would like to see how to implement this and how each of these compare when measuring small ESR resistance.

The Omicron looks good its signal source like the analogue discovery is not permanently connected to the channel 1  which makes it versatile.

see the following:

https://www.omicron-lab.com/fileadmin/assets/Trainings_Events/Webinar_2014/141119_Webinar_Impedance.pdf

https://www.edn.com/electronics-blogs/impedance-measurement-rescues/4458562/Accurately-measure-ceramic-capacitors-by-extending-VNA-range

https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&cad=rja&uact=8&ved=0ahUKEwiI__Ch_O3XAhVMKsAKHe80DRsQFghBMAQ&url=http%3A%2F%2Fwww.picotest.com.tw%2FDownload_File%2FINJ_um%2FApplication%2520Article%2FMeasuring%2520Output%2520Impedance%2520with%2520the%2520J2130A%2520and%2520Bode%2520100.pdf&usg=AOvVaw1_LB8_cPMuYVKD1YSgxJMU

 

 

[precaud]

Hi R_G_B,

Feel free to share your process and results, I look forward to it.

I have chosen not to explore the "adding series resistors to shunt-thru" approach because it adds measurement range (and accuracy) where I don't need it; at the low-frequency end. If you look at the charts in the EDN article you linked to, adding series resistors adds nothing to the measurement accuracy above 1MHz. The improvement it makes is all entirely below 100Hz. I don't need to see the entire impedance range of a 1nF capacitor from 1Hz to 50MHz. I have defined my range of interest as 10Hz to 10MHz, 1mOhm to 10 Ohms. Even this range is not easy...

Cheers.

[Pitrsek]

Still there, just not enough time to chime in. I'll be definitely back when time permits with some comparative measurements.
For coax s21 fixture, I do only trough calibration. That I can get by because the fixture inductance is so low. For the T setup, I would definitely consider performing short/load calibration as well. Even if you file down the T, there still a decent loop that will not get calibrated out with trough calibration, so you basically add this loop to you DUT.
Actually I have DIY SMA "calibrators" with cal. plane just at the back of the connector(you can use spare SMA connector and solder dut at pins). I'll try it and let you know.

[precaud]

Still there, just not enough time to chime in. I'll be definitely back when time permits with some comparative measurements.

OK, we'll wait. 

For coax s21 fixture, I do only trough calibration. That I can get by because the fixture inductance is so low.

Interesting. Is that for the coax probe, too?

For the T setup, I would definitely consider performing short/load calibration as well. Even if you file down the T, there still a decent loop that will not get calibrated out with trough calibration, so you basically add this loop to you DUT.

OK here are the same two 25mOhm resistors as measured last time, plotted with and without short compensation. It's quite an improvement. You can see by the phase curve that it is actually over-compensated a bit, the phase is in capacitive territory. That suggests to me that we may be up against the limit for this technique. It appears that using BNC T as a Z test fixture is OK up to 1MHz. For many things, this will be quite good enough.

The unanswered question, of course, is: what should the curve be for a single R025 of this size? Your four paralleled 0.1 Ohm R's gave nearly flat Z and phase all the way to 10MHz in your coax setup (remarkable, really). How much worse is just one .025 supposed to be?

It would be interesting to see the front end of your Omicron analyzer. I bet they have taken great care to maintain the 50 Ohm characteristic impedance right into the FET buffer.

BTW, I was told some time ago that HP/Agilent's 4-terminal standards had four paralleled SMD R's soldered across two copper plates for the lower values.

I found a ground pin late this afternoon, need to check if it takes solder. If so, I should have time to complete the coax probe tomorrow, unless work interferes 

Actually I have DIY SMA "calibrators" with cal. plane just at the back of the connector(you can use spare SMA connector and solder dut at pins). I'll try it and let you know.

Thanks. Yes, SMA has smaller loop and less inductance and will always be better than BNC in that department.

[R_G_B_]

I had a go with the analogue discovery.
 
I looked at both the impedance analyser and the network analyser
Function.  The impedance analyser sweeps up to 25Mhz from approximately 0Hz.

 The VNA has an option to sweep up to 50Mhz
And you can use an external signal source.

On the VNA signal source is good up to 5Mhz strange enough the output remains flat up to around 25Mhz. Maybe they are using compensation on the output amplitude as the output phase tracks the fall off in input  amplitude.
I had a difficult time getting the impedance analyser to work because the way it had to be calibrated using resistor values from 10, 100, 1K, 1M to calibrate for open and short compensation.

The resister I measure was a 0.05ohm current sense resistor. The noise you see is from the test setup and would not calibrate out.

[precaud]

The range being plotted is too wide to see what's going on. With a bottom range of 1mOhm, try a top range of 1 Ohm. Then we can see how it's handling the 50mOhm R.

Also, I would suggest get it working correctly at low frequencies (< 1MHz) first, and then open the measurement up to higher freqs.

What resistor value dis you end up using for the cal? I would think the lower one is better for 50mOhm DUT.

[R_G_B_]

I was comparing the analogue discovery with the ESR 70 capacitor meter which uses a test frequency of 100khz. The resistor I used for the cal of the analogue discovery was 10 ohm.
The ESR 70  measured the resistance of of the sense resistor as
0.02 ohms @ 100Khz. It's resolution is 0.01ohms.

[precaud]

I finished the rigid coax probe last night (see attached pics). A BNC female on the connector end. While soldering the ground pin on, the insulating material started oozing out. Ooops. I changed my approach and only worked one spot at a time instead of heating the whole thing up. It looks funky but turned out OK. To connect to the system, put a tee on the end, one lead goes to the source out, the other to the T input. Simple.

To test it, I soldered a 25mOhm SMD R onto pcb pads and cut the traces to them to isolate it. That way I can compare it to the 25mOhm R measurements made previously.

The measurement generally had the correct profile (relatively flat to 1MHz and inductive rise from there), but was way off (about 7mOhm too low), and with lots of spikey noise, especially at 1MHz and above. (I accidentally overwrote the file so I don't have a plot to show.) At any rate, the results are unacceptable, for three reasons: 1. I didn't get a good short across the test points, so the short measurement was too high, which in turn made the short compensation inaccurate. 2. I may have made the probe too long. (that is correctable, of course). 3. It is frankly a pain in the arse to use. Holding a long probe perfectly still with steady pressure on the pcb pads for the 23-second-long sweep is no fun and not easy. In a couple dozen tries I'm not sure if I got one entirely right.

So I will not pursue this probe any further at this time. A special shorting bar will have to be made, something that clamps on to the pins at the end and makes good contact. I would prefer having a probe that clamps or clips on so I don't have to be a hero and hold it perfectly still. So for now, I'll go back and see if I can make the BNC T fixture work better.

[nctnico]

For this kind of testing I like to solder a piece of coax to a board. Soldering an SMA onto the board is also an option.

[Pitrsek]

You need two probes. I'm sorry I did not mentioned it, I've never seen used the way you used it - it did not occurred to me to mention. You have two probes, one per channel. And the power plane is your "T splitter". I'll try to find a picture tomorrow.  If one is not fond of two probes(not enough free hands to do anything else), there is S21 probe from picotest which can be used as source of inspiration. Basically two probes side by side, with connected ends - ie. the T as at your probe tip. You trade a bit of inductance for more easier use.   

[precaud]

You need two probes. I'm sorry I did not mentioned it,

Now he tells me...     

You have two probes, one per channel. And the power plane is your "T splitter".

Understood. But hmmmm... last week I experimented using longer cables with the BNC tee, and they made no difference to the measurement. So this is a surprise. But at this point it doesn't matter, I am not excited about holding a probe on a pcb to make a measurement...

[Pitrsek]

Now he tells me...     

Yeah, I felt kinda stupid when I realized it...  Hey, I have a beer ready as an apology, if you are ever decide to visit Prague 

[precaud]

No worries, I was just having a little fun with it. I appreciated your help, period.

So I'm back working with the BNC-Tee-as-test-fixture. Referring to the plot in post #30, it bothered me that the phase had gone capacitive after the short compensation routine, while the impedance curve was clearly inductive. Obviously this can not be the case. I know the short comp math is good. It should leave whatever residual inductance is in the DUT. So what could cause this problem? Well, if the "ground reference" itself had exaggerated (more than actual) inductance at high freqs, the compensation math would not work as expected.

So I looked at the Tee with the ground wire, a single solid 20 gauge wire (see 1st pic). That could be the cause right there. So I soldered in three more wires to put the inductances in parallel (see 2nd pic), and ran the measurement again. (3rd pic). Voila. The original single-wire ground reference had enough inductance to throw the measurement off significantly. Compared to the plot in post #30 above, the impedance has been cut in half at 10MHz and the phase is now properly inductive and accurate, with 45º phase shift at the freq where the impedance is twice that of the level region, as it should be.

I'm becoming more impressed with this shunt-thru method. It's usable to 2MHz now. And so simple.

CORRECTION: I think I was pessimistic yesterday when saying this was giving good measurement to 2MHz. After looking at some graphs of Z measurements of single SMD current sense resistors of 30mOhm and below, they all have inductive rise to them starting a bit above above 1MHz. It's simply a function of the package dimensions (in this case, 1/4"). So I am more inclined to say that this technique is giving good info to 10MHz right now.

It would be nice to have a set of "reference" curves so one knows what Z/phase distribution to expect for a given lead/terminal spacing.

[R_G_B_]

I don't know if this will be helpful or interesting but i though i would post

[precaud]

Bob Pease was quite a character and always interesting.

[Pitrsek]

Hi,
I was finally able to run some more test. I started with my first semirigid fixture (Y.jpg), as you can see there is a severe braid error. Then I moved on to me new very short semirigid fixture. I've tested it with and without the mammoth CM coil. Final step was a setup similar to percaud's , but with SMA and CM transformer. One go with only trough calibration. The last one is with open,short, 50 and trough. As you can see, part of the mounting inductance got calibrated out, as well as some of braid error. To be honest the T setup turned out better than I expected.   

[Pitrsek]

I've used same coax cable for both runs with CM and without CM choke. I did not have a shorter one on hand for the without variant. Very short semi rigid would be off course preferable.  More pics + bonus/teaser pic for next time.
I'll be comparing test probes vs sma cables. Stay tuned 

[precaud]

Excellent tests, Pitrsek. Your small fixture is giving very good results, and with only a through cal. Do you think it is the "ground integrity" of the semi-rigid coax that makes the difference?

And of course the SMA Tee with compensations looks excellent. (At 10mOhm, I think you can skip the Open cal!)

With the Tee setup, when you do Through and Open/Short/Load sweeps. what is the difference (if any) for the Open and Through sweeps? Aren't they the same setup?

On the Anritsu, I run the Through with the fixture in place, but no DUT, which is the same as an Open. So to run it again as an Open would be redundant.

I look forward to your "probes vs Godzilla SMA cables" head-to-head.

The great value I see in using things like the Tee (vs soldering parts into a dedicated fixture) is efficiency. It enables you to solder the DUT in place, set it aside to cool down before testing it, and go on testing other devices while you wait. The question that remains for me is: how often will I use a setup that requires soldering the DUT in place? That is why I want some kind of drop-in fixture for my BNC Tee setup that will accept thru-hole radial parts without soldering.

[Pitrsek]

Small fixture - biggest thing going in favor is really low mounting inductance. You are usually able to solder SMD pat to it in a way that you do not add any(or very little) inductance. Short semi-rigid coax also helps with the braid error, but the biggest killer of braid error is the CM toroid.

You are absolutely correct, the the open cal could be skipped, and at that point open=trough, same setup. Did no occurred to me at the time.
If I was going to measure 0603 and smaller capacitors, I'd definitely go for soldering dut to the small fixture. I would say that key is the ratio of your fixture inductance to your DUT inductance. If your DUT is inductive as hell(IMHO all THT stuff compared to 0603), you don't really care for fixture inductance anyway.
For regular part testing, I use component fixtures that we have for Bode. I take out the small fixture only when I need to measure really low impedance/low inductance stuff - yes, soldering DUT is PITA.

Actually you can easily built fixture that you are dreaming about .
Small board, SMA on opposite edges, 50Ohm trace going in between. Mill-max pin receptacles for THT parts pins, in 50mil grid. See the picture.
I used mill-max receptacles for transformer when I was developing my last SMPS - great stuff, you design tight layout from the start and mess with transformer design all day long. No soldering necessary(a tip from Mr.Ridley). You can use multiple columns of the receptacles for different lead diameter. Also you can add series resistors if you'd like to shift the measurement range. And you could probably experiment with CM choke on the output as well, as I do not believe that for low MHz range you need CM choke with coaxial winding. This way you will be measuring device + mounting inductance. Depending on your use, it might be useful to do it on 4L board, to have board inductance as low as possible(ground plane in first inner layer). if you use same size resistor for calibration, you will calibrate out the DUT inductance and you will need to add it later based on the DUT geometry. Or you can provide 4 grounding millmax pins around the center(pattern of 5 on a dice) and create shorting plate. Do you have access to machinist shop? 

 

[precaud]

Small fixture - biggest thing going in favor is really low mounting inductance. You are usually able to solder SMD pat to it in a way that you do not add any(or very little) inductance. Short semi-rigid coax also helps with the braid error, but the biggest killer of braid error is the CM toroid.

I am fortunate (so far) to not have to worry about braid error...

If I was going to measure 0603 and smaller capacitors, I'd definitely go for soldering dut to the small fixture. I would say that key is the ratio of your fixture inductance to your DUT inductance.

That makes sense.

If your DUT is inductive as hell(IMHO all THT stuff compared to 0603), you don't really care for fixture inductance anyway.

I'm not so sure about that. The work I am preparing for involves modifications to existing boards, populated by a mix of SMD and THT parts. Cleaning up old messes, you might say. So I need to be able to characterize these THT parts in freq ranges where we know they do not behave well.

For regular part testing, I use component fixtures that we have for Bode. I take out the small fixture only when I need to measure really low impedance/low inductance stuff

I am hiding my jealousy  Can you post a close-up pic of the fixture?

Actually you can easily built fixture that you are dreaming about .
Small board, SMA on opposite edges, 50Ohm trace going in between. Mill-max pin receptacles for THT parts pins, in 50mil grid. See the picture.

Yes, I have some of the Mill-Max and thought of something like that, but I decided that the variable contact resistance of those connectors was not suitable for low-impedance stuff. And that's what I am concentrated on right now.

Above 1 Ohm is easy. You can do it without special fixtures, even. (Have you seen the YT video about Z measurement, soldering a radial 1nF cap with an axial 47 Ohm R and sweeping Z all the way to 65MHz? It looks so easy!) Low Z @ high freq is tough!

I used mill-max receptacles for transformer when I was developing my last SMPS - great stuff, you design tight layout from the start and mess with transformer design all day long. No soldering necessary(a tip from Mr.Ridley).

Yes, for those impedances, it's a viable setup.

And you could probably experiment with CM choke on the output as well, as I do not believe that for low MHz range you need CM choke with coaxial winding.

Fortunately I can skip the CM choke business. At least with the Anritsu...

Do you have access to machinist shop?

Only in my dreams 

Just for the helluvit, last night I put together a fixture based on the "typical" way of measuring impedance; measuring voltage on either side of a series resistor (see below). Not useful for SMT at all, but good for radial parts. I didn't take pics or save a plot (will do so next time and post), but the results were a bit worse than the injection xfmr method. Even with short signal paths and careful grounding, there is excessive inductance in the setup. Using 1 Ohm reference R, measuring the axial 25mOhm R used previous, without short compensation, it was accurate to maybe 30kHz. Short comp extended it to 100kHz or so. Not very useful.

Before I abandon it, I am going to try moving the reference R into the ground leg and measure the current there. It's a lower-Z node so the L should be lower and usable bandwidth higher.

[precaud]

Here are some measurements using the configuration shown in the previous post. The series R is 10 Ohms. I first tried it with a 1 Ohm R, but the results were not good at low or high frequencies, the 1 Ohm being too close in value to the impedance being measured. The test jig is set up for measuring radial-lead components, such as large electrolytic caps, with push-post terminals spaced at 8mm (5/16").

1st pic shows the setup, with a 20mOhm radial current sense resistor mounted.
Second one shows the 20mOhm R with and without short compensation. As you can see, it primarily cleans up some resonances above 1MHz, leaving a smooth inductive rise.
Third one shows three electrolytic capacitors, with the 20mOhm R for context. The ESR of the capacitors is accurately shown in their impedance at around 100kHz where the phase crosses zero.

The rising impedance slope above 100kHz is mostly due to the fixture spacing plus the length of the push posts. It does not calibrate out with the short compensation. Also of interest is that the 1000uF lytic actually measures lower than the 20mOhm resistor above 500kHz. The capacitor has a lower impedance up there than the resistor does!

So with its high self-inductance, this fixture is limited to 100kHz at 20mOhm, 40-50kHz at 10mOhm, and 1MHz at 100mOhm. Fairly similar to the "injection transformer" method. I think this configuration can be improved a little bit, with lower source impedance (not a 50 Ohm output) and shortening the posts. But I don't expect it to bring it anywhere near as good as the shunt-thru results.

Next is to move the current-sensing R into the ground leg.

[bson]

Isn't Z(f) unbiased in your fixture, and isn't this problematic with a polarized device like an electrolytic cap?

[precaud]

Isn't Z(f) unbiased in your fixture

Yes

and isn't this problematic with a polarized device like an electrolytic cap?

Not in my experience. I experimented with this in the mid-90's using an HP 4274A, and saw no change in impedance of a 'lytic between biased and unbiased states. That was up to 100kHz. Is it different at, say, 1MHz and above? That I can't answer. But I rather doubt it.