Author Topic: Measuring low impedances with a VNA  (Read 5935 times)

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Offline precaud

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Measuring low impedances with a VNA
« on: November 10, 2017, 06:34:15 am »
I recently refurbed and calibrated an Anritsu MS420K network analyzer, with the intent to use it to measure impedance (milliOhm range) of pcb power distribution networks (PDN), in powered and passive states, as well as separate components. This is the basic setup, used/recommended by HP/Agilent, Ridley, and others:



I did this successfully in the late 90's with passive components (without the blocking-C) on a test jig using an HP 3570A and 3330B combo, with a Tek current probe in place of the 1 Ohm resistor.

Now I'd like to do this same measurement on a pcb, to see the VR output impedance. There will be no room for a current probe in most cases so the 1 Ohm R will be used.

Desired measurement range is 1m Ohm to 10 Ohm from 10Hz to 10MHz.

A major disadvantage of this configuration is that the isolation xfmr is basically working into a 1 Ohm load. Most of the available isolation xfmrs were not designed for this.

So I thought the place to start would be to examine the transformer freq response from a 50 Ohm source into a 1 Ohm load over the range of interest.

Attached is the FR of six candidates for the job, and a pic of the units. Some of these are recommended for measuring loop gain/phase in switching supplies.

All of these are dual-winding xfmrs except for the Jensen, which is a video isolation "humbucker". Other notes:

The Tokin common mode choke is the best of several such chokes I tested. It is maybe useful to 20kHz. Maybe.
Bandwidth-wise, the Jensen humbucker and the three North Hills units perform pretty much the same. Using them to 1MHz would be realistic; 10 may be pushing it under actual conditions.
Apart from the dip at 20kHz, the Ridley looks good. It measures flat well beyond the 10MHz target. Except for a ~3dB peak @ 22MHz it is usable to 30MHz.

Your comments are welcome, especially if you have experience with this technique (or similar).

  John
« Last Edit: November 11, 2017, 06:02:03 am by precaud »
 
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Offline Pitrsek

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Re: Measuring low impedances with a VNA
« Reply #1 on: November 10, 2017, 08:19:38 am »
I started with impedance measurements with S21 and vanilla 2 port VNA(no dedicated generator port), when we bought 3 port VNA, we bought current injector as well. So I never used the injecting transformer for impedance measurements(only for stability). The setup is for me more cumbersome compared to I/V. But I sure can understand that that's what you are equipped for. Colleague of mine experimented with it, I'll try to ask him about it.

Few hints for S21 and I/V measurements:
With S21 it's necessary to watch out for DC voltage levels!(even if you have AC coupled  VNA inputs, generator output is usually DC coupled). If you need to measure higher voltage than is allowable at ports of your VNA, you can add resistors in series with the ports. This will also shift your measurement window:
https://www.edn.com/electronics-blogs/impedance-measurement-rescues/4439326/Increase-range-in-2-port-impedance-measurements
For direct I/V measurements, take a look at AN 104 from LT and late Jim Williams - quite nice voltage controlled current load.
You can buy something similar from Picotest(that's what we use). With current injectors, you cannot obviously measure impedance with power off. 
When measuring low impedance at low frequency, there is a problem of braid induced error:
http://www.electrical-integrity.com/Paper_download_files/DC10_12-TH3_Novak-Mori-Resso_slides.pdf
Picotest is offering CM transformer to counter it(that we do not have), but you can roll your own. I Had a good experience with "slightly smaller than palm" sized EMC ferrite from Wurth. Also Vacuumschmelze is offering some cores with crazy high permeability. Depends on how low impedance and how low frequency you want to measure. If you want to go really low with frequency, you will need differential input. Differential probes works well, but most of them are 10x and will eat into your SNR. Use of short semi-rigid cable with DUT mounted as close as possible to port 1 also helps.
I usually provide two SMA per power plane on my boards for impedance measurements. SMA are 90deg. apart, so there is no collision between injector and voltage probe. Injector is mounted directly to PCB with adapter - no cable. Easy setup, great signal integrity. Just connect voltage probe, current injector, and you are set. 

For separate component measurements, I use S21(Z<1Ohm) shunt with CM transformer. Or dedicated fixture from VNA manufacturer(Omicron)
I would recommend reading up on papers from Istvan Novak, Steve Sandler (articles, videos on YT + live classes, I've been to one and it was really nice  :-+) and take a look at app notes from Omicron Labs (makers of our VNA). If you are interested, I can try to find PN of cores for CM transformer and snap a few pictures of the setup I use.

On a related note, it's possible to load your measured data into LT spice - both transfer functions, and impedance. So you can simulate with measured data.



 

Offline precaud

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Re: Measuring low impedances with a VNA
« Reply #2 on: November 10, 2017, 01:15:33 pm »
I started with impedance measurements with S21 and vanilla 2 port VNA(no dedicated generator port), when we bought 3 port VNA, we bought current injector as well. So I never used the injecting transformer for impedance measurements(only for stability). The setup is for me more cumbersome compared to I/V. But I sure can understand that that's what you are equipped for. Colleague of mine experimented with it, I'll try to ask him about it.

Thanks Pitrsek, that would be great. BTW, I'm not married to this setup, I am open to other methods if they are effective.

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Few hints for S21 and I/V measurements:
With S21 it's necessary to watch out for DC voltage levels!(even if you have AC coupled  VNA inputs, generator output is usually DC coupled). If you need to measure higher voltage than is allowable at ports of your VNA, you can add resistors in series with the ports. This will also shift your measurement window:
https://www.edn.com/electronics-blogs/impedance-measurement-rescues/4439326/Increase-range-in-2-port-impedance-measurements

I will rarely be looking at anything with greater than 20VDC, but I'll look into it.

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For direct I/V measurements, take a look at AN 104 from LT and late Jim Williams - quite nice voltage controlled current load.
You can buy something similar from Picotest(that's what we use). With current injectors, you cannot obviously measure impedance with power off. 

Thanks for the AN reference. I was looking at the Picotest J2111A specs last night. A bit pricey...

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When measuring low impedance at low frequency, there is a problem of braid induced error:
http://www.electrical-integrity.com/Paper_download_files/DC10_12-TH3_Novak-Mori-Resso_slides.pdf 

Yes, that one I've been aware of. My previous setup avoided it, broke ground the loop, by the use of the current transformer, but it will be a factor now.

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Picotest is offering CM transformer to counter it(that we do not have), but you can roll your own. I Had a good experience with "slightly smaller than palm" sized EMC ferrite from Wurth. Also Vacuumschmelze is offering some cores with crazy high permeability. Depends on how low impedance and how low frequency you want to measure. If you want to go really low with frequency, you will need differential input. Differential probes works well, but most of them are 10x and will eat into your SNR. Use of short semi-rigid cable with DUT mounted as close as possible to port 1 also helps.

I guess it depends on what we define as "really low". My desired low limits are 10 Hz and 1 milliOhm.

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I usually provide two SMA per power plane on my boards for impedance measurements. SMA are 90deg. apart, so there is no collision between injector and voltage probe. Injector is mounted directly to PCB with adapter - no cable. Easy setup, great signal integrity. Just connect voltage probe, current injector, and you are set. 

That is smart. My applications will be to existing boards, mounting connectors won't always be possible. Sometimes I just have to solder the leads to the traces.

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For separate component measurements, I use S21(Z<1Ohm) shunt with CM transformer. Or dedicated fixture from VNA manufacturer(Omicron)
I would recommend reading up on papers from Istvan Novak, Steve Sandler (articles, videos on YT + live classes, I've been to one and it was really nice  :-+) and take a look at app notes from Omicron Labs (makers of our VNA). If you are interested, I can try to find PN of cores for CM transformer and snap a few pictures of the setup I use.

Thanks for the references, I have some homework to do  :)  Any pics of your setup would be great.

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On a related note, it's possible to load your measured data into LT spice - both transfer functions, and impedance. So you can simulate with measured data.

Yes, I do that with software I've written. Once I settle on the measurement setup, I'll modify it to accomodate.
« Last Edit: November 12, 2017, 04:27:53 am by precaud »
 

Offline precaud

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Re: Measuring low impedances with a VNA
« Reply #3 on: November 11, 2017, 06:05:02 am »
I just wanted to add, the purpose of this thread is not just to explore this one technique... If you're doing low-Z measurements with a VNA, pls share your setup.
 

Offline Pitrsek

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Re: Measuring low impedances with a VNA
« Reply #4 on: November 14, 2017, 07:16:29 pm »
Picotest injectors - yes, they are pricey. If you go for a bundle it gets a bit more reasonable. There is one "feature" of the injector that i did not manage to wrap my head around. The current sense port is "feedforward", ie. the signal is not about what is there, but about what should be there. Current sense output works even with disconnected load. I have on idea why did they designed it this way.  On the other side, it's nice for calibration

10Hz and 1mOhm - it's quite tough. Depending on dynamic range of your VNA, you might probably need to build a preamp. Ideally with differential input, to get rid of the braid error(If you feel like building something like that - let me know, I'd be glad to help - I could use it too, same goes for the current load). If I may ask, why do you need to go so low?  There is usually not much happening...

Apart from two sma(really nice if you can accommodate the space, I use a lot of edge mounted ones), I used diy probes from semirigid coax and some needles(as recommended in "right the first time" book) - it sucked.  I have a new version with spring loaded gold plated tip in works, I'll post pictures when it's done.

I've attached pics of my setup - short semi rigid line +  common mode transformer. Of course it could be improved, ie. directly from coax to bnc(no adapters) and semi flexible coax for the CM transforem(+box). PN of the core is W518-03. Measured are 4x0.1R 0603 in parallel. I'll try to get something smaller, to show you the braid error with this setup. Actually this is quite a bit better than old setup with WE emc ferite core.

 
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Offline Pitrsek

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Re: Measuring low impedances with a VNA
« Reply #5 on: November 14, 2017, 07:45:07 pm »
Managed to find 10mOhm resistor(LVK12R010FER). Below 10Hz you can see braid error kicking in. Unfortunately I do not have anything smaller at hand. With worse CM transformer, the transition would just happen higher in the frequency domain.
 

Offline precaud

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Re: Measuring low impedances with a VNA
« Reply #6 on: November 15, 2017, 08:25:06 am »
Thanks for the details and pics, Pitr. That CM xfmr is a monster!

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Picotest injectors - The current sense port is "feedforward", ie. the signal is not about what is there, but about what should be there. Current sense output works even with disconnected load. I have no idea why did they designed it this way.

I'm puzzled by this.

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10Hz and 1mOhm - it's quite tough. Depending on dynamic range of your VNA, you might probably need to build a preamp. Ideally with differential input, to get rid of the braid error

Yes, preamps with diff inputs, a source that can comfortably drive a few watts into 1 Ohm at 30MHz... they're on the list of things to build some day.

Dynamic range with the Anritsu is better than 90dB with 30Hz IF BW, and even better than that at lower BW's. It has isolated inputs, though "how much" they float off common is not specified. Perhaps I should determine that, both inter-channel and channel-to-source-out. I have read several ways to deal with the braid error issue. Diff inputs are one. Isolation xfmrs are another. Keysight does in their VNA's by lifting the input grounds by 30 Ohms, shown starting on p. 19 of this PDF. It's pretty effective:

literature.cdn.keysight.com/litweb/pdf/5990-5902EN.pdf

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If I may ask, why do you need to go so low?  There is usually not much happening...

Some Vregs dip even below 1mOhm in places. To accurately do simulations combined with measured data, the data needs to be accurate...

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I've attached pics of my setup - short semi rigid line +  common mode transformer.
Very interesting.This looks good for SMD parts. I see in the circuit setup, there is no connection to the R(ef) input of the Bode 100. Where does it get its reference? Do you make a "thru" sweep first? Or open/short/load sweeps which the instrument stores and applies to each measurement?

I am going to have to read further and experiment with this "shunt-thru" setup.

I can see, it is going to be a challenge to do on-board probing that is good to 10MHz. Unfortunately I won't be able to solder SMA's to the necessary test points most of the time. Ideal would be to clip onto the test points.

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Managed to find 10mOhm resistor(LVK12R010FER). Below 10Hz you can see braid error kicking in. Unfortunately I do not have anything smaller at hand. With worse CM transformer, the transition would just happen higher in the frequency domain.
Yes, it is an interesting result. Is the behavior above 100kHz entire the DUT, or is some of it residual L in the setup? I would expect a device that small to not have so much.

Thanks again for sharing your setup and results.
 

Offline Pitrsek

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Re: Measuring low impedances with a VNA
« Reply #7 on: November 15, 2017, 07:53:28 pm »
Floating inputs are nice  :). The Anritsu looks nice. Thx for the link to Agilent VNA - an interesting solution.
Unfortunately yes, some regulators have ultra low output impedance. In my experience, you can usually see the lowes Z out of regulator before the braid error kicks in(so you have your R an L for model). Anyway, with your floating inputs, it's a moot point anyway :). Take a look at some presentations from Steve Sandler about flat output impedance and why it is good idea. Might be interesting for you.

Ref channel is connected internally. Bode is quite flexible in this regard(see attachment). For S21 shunt, I calibrate for trough only. It should be ok for Bode 100 bandwidth - info from Mr. Sandler - i forget to ask why(so many information in one presentation... ). For higher bandwidth, full calibration is recommended.

If SMA is too big, you can solder coax directly to capacitor Pads - capacitor should be removed, if you keep it, the ripple current flowing trough vias will impair your measurements. There's an article about it from Istvan Novak somewhere on the internet.

Or you can make impedance probes - piece of semi-rigid coax(first one I built were from semiflex, not a good idea, too soft/bendable), for a ground connect you can use a needle, or better gold plated spring loaded pin - the problem is that center connector is still quite soft copper, but it works.
On the left it's a new design - proper semi-rigid calbe + center conductor replaced with Ingun test needle (50mil series, with housing, tip is gold plated blade type, changable).It's work in progress. The probe is quite short, as the center conductor is replaced with test needle housing(after pulling out the center conductor, the hole needs to be increased slightly to accommodate needle housing). Both contact spring loaded. It's a still work in progress. Or alternatively, you can take a look at Ingun, they do offer a HF test probes, which could be "misused". I will probably try them in future.

Actually the 10mOhm resistor is surprisingly good in this regard. The sharp rise of the blue trace above 100khz is phase. I would say that impedance starts to rise a decade later, around 1M.  It ain't easy to have a flat and low impedance to MHz range. The inductance of package will bite you in the ***. For example 0.314nH will exhibit cca 0.010R imedance at 50Mhz, since the resistor is 10mOhm on it's own, it needs to be way smaller not to impact resistor impedance significantly. For comparison, mounting inductance for various packages:
http://www.sigcon.com/Pubs/news/6_09.htm

Glad to help ;)

 
 

Offline precaud

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Re: Measuring low impedances with a VNA
« Reply #8 on: November 16, 2017, 01:19:18 pm »
Unfortunately yes, some regulators have ultra low output impedance. In my experience, you can usually see the lowes Z out of regulator before the braid error kicks in(so you have your R an L for model).

So, far my experiences agrees, lowest Z has always been above 100Hz.

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Take a look at some presentations from Steve Sandler about flat output impedance and why it is good idea. Might be interesting for you.

Thanks. I've watched some of his presentations and he definitely knows his stuff. I am already convinced of the need.

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If SMA is too big, you can solder coax directly to capacitor Pads - capacitor should be removed, if you keep it, the ripple current flowing trough vias will impair your measurements. There's an article about it from Istvan Novak somewhere on the internet.

I've either been clipping or soldering to the test points, which quite often coincide with the output capacitor(s). Exact location sort of depends on where the regs tie into the ground plane (assuming one exists!). I'll have to look up that article, thanks.

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Or you can make impedance probes - piece of semi-rigid coax(first one I built were from semiflex, not a good idea, too soft/bendable), for a ground connect you can use a needle, or better gold plated spring loaded pin - the problem is that center connector is still quite soft copper, but it works.
On the left it's a new design - proper semi-rigid calbe + center conductor replaced with Ingun test needle (50mil series, with housing, tip is gold plated blade type, changable).It's work in progress. The probe is quite short, as the center conductor is replaced with test needle housing(after pulling out the center conductor, the hole needs to be increased slightly to accommodate needle housing). Both contact spring loaded. It's a still work in progress.

Very nice. This appears worth experimenting with. I bet you never guessed that one of your required engineering skills would be to have a "steady hand" to hold the probe during the Z sweep!  :)

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Or alternatively, you can take a look at Ingun, they do offer a HF test probes, which could be "misused". I will probably try them in future.

I've never heard of them, will have a look , thanks.

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Actually the 10mOhm resistor is surprisingly good in this regard. The sharp rise of the blue trace above 100khz is phase. I would say that impedance starts to rise a decade later, around 1M.

Wow, so at that low impedance, the L component becomes visible already at 100kHz.  Hmmm...

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It ain't easy to have a flat and low impedance to MHz range. The inductance of package will bite you in the ***.

So I am seeing...

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For example 0.314nH will exhibit cca 0.010R imedance at 50Mhz, since the resistor is 10mOhm on it's own, it needs to be way smaller not to impact resistor impedance significantly. For comparison, mounting inductance for various packages:
http://www.sigcon.com/Pubs/news/6_09.htm 

Interesting. Beside the trace width, the direct correlation is between the via spacing and the inductance.

Does the Bode100 box have open/short/load compensation routines available? Back in the 90's, my component Z measurements with current probe didn't work very well until I added those routines to the program. The short and load, in particular. At low Z's, we can pretty much forget about the open condition.
 
Thanks for bringing all these stinky details to my attention.  :)   This is going to be quite a challenge, indeed.
 

Offline Pitrsek

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Re: Measuring low impedances with a VNA
« Reply #9 on: November 20, 2017, 07:13:08 am »
Yes, Bode supports open,short,load calibration. I like Bode 100, it's a handy piece of kit, and techsupport from manufacturer is top notch :-+
Stinky details, that's a rather fitting description... It reminded me about switchable probes, it's ultra easy to accidentally switch probe attenuation, when trying to connect it to a pcb that was not really prepared for probing. Bought dedicated 1x probes - saves the hassle with checking probe settings, provides better bandwidth than switchable probe, and as a bonus, none is interested in measly 1x 40MHz probe....  so  nobody is "borrowing" them
Good luck and let us know how your measurements turned out. 
 

Offline rx8pilot

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Re: Measuring low impedances with a VNA
« Reply #10 on: November 20, 2017, 08:13:47 am »
Joining the thread.....
Factory400 - the worlds smallest factory. http://www.youtube.com/c/Factory400
 

Offline precaud

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Re: Measuring low impedances with a VNA
« Reply #11 on: November 24, 2017, 01:49:51 pm »
Thx for the link to Agilent VNA - an interesting solution.

There are actually several different approaches described in that article. It covers most of the bases in pretty good detail. I read thru it again today, and while the the shunt-thru is good for component testing, it may not be the best choice for measuring regulator Z. Because the terminal impedances are so low (50 Ohms), the DC-blocking C's have to be huge (and bipolar) 'lytics. And that brings yet another bag of worms to the table, with capacitor self-L becoming a factor at quite low freq's (<20kHz) in big 'lytics...

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On the left it's a new design - proper semi-rigid calbe + center conductor replaced with Ingun test needle (50mil series, with housing, tip is gold plated blade type, changable).It's work in progress. The probe is quite short, as the center conductor is replaced with test needle housing(after pulling out the center conductor, the hole needs to be increased slightly to accommodate needle housing). Both contact spring loaded. It's a still work in progress.

I like the idea, but I'm thinking that contact resistance would be variable with that sort of setup, which would be fatal for millOhm-range measurements. Are you seeing that?

I'm also wondering about the jig made with rigid coax for testing SMT parts. As Johnson shows, "parasitic inductance" is basically related to electric length, i.e. spacing between conductors, vias, or test leads. Since that measurement setup has a fixed distance between the leads, they can't be brought together for an accurate Short compensation measurement. So short will include that inherent inductance, and it will be erroneously removed from every measurement by the post-processing math.

Do you have a chart of "useful Z measurement range vs frequency" for the Bode 100? Here's one for an AP Instruments AP200 VNA. This chart is from an application guide for the instrument.

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  It ain't easy to have a flat and low impedance to MHz range. The inductance of package will bite you in the ***.

That is SO true...
« Last Edit: November 24, 2017, 02:09:42 pm by precaud »
 

Offline precaud

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Re: Measuring low impedances with a VNA
« Reply #12 on: November 24, 2017, 06:09:37 pm »
I made a 50 Ohm splitter today and did first experiments with the shunt-thru method using the MS240. See first attached image for how everything is connected. Because I used short test clips between the DUT and the inputs, and axial-leaded current sense resistors, I knew that above 100kHz it would not be correct. I consider this a "test of concept" anyway, and also very much wanted to see if the MS420's isolated inputs do in fact eliminate the braid loop error at low freqs.

Second image shows 10Hz-10kHz sweeps of 1 Ohm, 100mOhm, 10mOhm, and short DUT's. (Formulas for finding the impedance value:  Zdut= 25 x S21 / (1 - S21), where S21= Vt / Vr. Convert dB to voltage ratio by 10dB/20). The Z's calculate out correctly, so the method works. And there is no sign of braid loop error, so the isolated inputs do the trick.

There are a couple problems, however. First, the minimum IF bandwidth is 3Hz, so at 10Hz it is still tracing out the shape of the IF filter at low impedances. You can see this in the 10mOhm and short plots. This results in a minimum usable freq at 10mOhm of about 20Hz, and at 1mOhm it would be about 30Hz. That's not bad, but not what I was hoping for. And second, in order to keep sweep times at low freqs reasonable, the MS420 has a mode (which I used) where it automatically adjusts the resolution bandwidth to the frequency as it sweeps (at 1-3-10... intervals above 100Hz). Wider bandwidths allow faster sweep times but result in a higher noise floor. In the 10mOhm and short sweeps, you can see the noise rise above 1kHz and again at 3kHz. The workaround is to use a fixed narrow IF bandwidth for the entire sweep. The penalty for doing so very is looooong sweep times. Using a 3Hz filter, it would take several minutes per sweep.

So right off the bat we see the MS420 is not bad but not the ideal instrument for my goal of 10Hz to 10MHz and 1mOhm to 10 Ohms. At low freqs my HP 3577A would be even worse, with its 10Hz minimum IF BW and non-isolated inputs. For measurement down to 10Hz, a 1Hz IF filter BW is needed.

Another workaround would be to use the VNA for 100Hz and above, and use an FFT-based dynamic signal analyzer for the low freqs.
« Last Edit: November 25, 2017, 12:44:46 am by precaud »
 

Offline Pitrsek

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Re: Measuring low impedances with a VNA
« Reply #13 on: November 24, 2017, 06:30:27 pm »
Yes, the 25R(50R of generator and 2 port are in parallel) of loading is limiting the S21 usefulness. Yet, if you go for DC block on port 2, the cap need not to be ultra large, and it can be calibrated out. Another trick is to increase your system impedance with series resistor:
https://www.edn.com/electronics-blogs/impedance-measurement-rescues/4442173/1/Fix-poor-capacitor--inductor--and-DC-DC-impedance-measurements
This will decrease your DUT loading, but will shift your measurement window as well(in many cases, this might not be a problem at all)
I use the increased system impedance technique for measuring inductors and chip-beads - otherwise one would need to combine S21 and S11 data to have accurate measurements trough whole impedance range. The S21 measurement for regulators got one really nice advantage, you can measure just passives/caps, then turn the psu on, measure it again, and observe the difference....

Yes, contact resistance will be variable, but a) its gold plated blade pin, b) the sensitivity of S21 to contact resistance is quite low
The contact resistance is THE reason to use S21 instead of S11 for impedance measurements. For S11, contact resistance is in series with DUT and you are not able to tell it apart from dut. With S21, the contact impedance is in series with ports. So you have 50ohm + mOhm of contact impedance, then dut to ground, and parallel to DUT is another contact impedance in series with 50ohms of port 2.
see "Why S11 VNA Measurements Don’t Work for PDN Measurements" and "Accuracy Improvements of PDN
Impedance Measurements in the Low to Middle Frequency Range "

For the rigid coax jig, I only use trough calibration. You are correct about the package inductance. In some cases, it gets calibrated out.
I've seen this even in some books, I need to ask the book author(Sandler) why he did it this way. 

Not for the S21 measurements. Only for use of their component fixtures(b-wic and b-smc).

I've inquired about price of Ingun HF probes, I'll let you know when I hear back.

 
 

Offline precaud

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Re: Measuring low impedances with a VNA
« Reply #14 on: November 25, 2017, 01:25:02 am »
Yes, the 25R(50R of generator and 2 port are in parallel) of loading is limiting the S21 usefulness. Yet, if you go for DC block on port 2, the cap need not to be ultra large, and it can be calibrated out. Another trick is to increase your system impedance with series resistor:
https://www.edn.com/electronics-blogs/impedance-measurement-rescues/4442173/1/Fix-poor-capacitor--inductor--and-DC-DC-impedance-measurements
This will decrease your DUT loading, but will shift your measurement window as well(in many cases, this might not be a problem at all)

Yes, but that addresses a different problem. It shifts the measurement window up, extends S21 usefulness into higher impedances, at the expense of lower. I can see how it would be useful for your beads and inductors at high freqs, though.

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The S21 measurement for regulators got one really nice advantage, you can measure just passives/caps, then turn the psu on, measure it again, and observe the difference....

Yes... very convenient!

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Yes, contact resistance will be variable, but a) its gold plated blade pin, b) the sensitivity of S21 to contact resistance is quite low
The contact resistance is THE reason to use S21 instead of S11 for impedance measurements. For S11, contact resistance is in series with DUT and you are not able to tell it apart from dut.

Good points, I forgot about that.

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For the rigid coax jig, I only use trough calibration. You are correct about the package inductance. In some cases, it gets calibrated out.
I've seen this even in some books, I need to ask the book author(Sandler) why he did it this way.

I saw a video where the residual L was being manually "tuned out" by making successive sweeps and tweaking the L constant for the software until it looked good. I'm not fond of this approach. It's probably OK for medium impedances. I've seen it used in several instruments, even 4-wire Z meters (like HP 4192A). The problem is, the residual L is not pure and so it varies with freq. HP abandoned that approach in their next generation of Z meters. A better approach is to take correction data at numerous freqs across the whole range, and interpolate between them. I'm pretty sure your Bode 100 box does that. The AP VNA's use 15 points.

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I've inquired about price of Ingun HF probes, I'll let you know when I hear back.

No doubt there will be some sticker shock involved   ;D
 

Offline Pitrsek

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Re: Measuring low impedances with a VNA
« Reply #15 on: November 26, 2017, 08:59:12 am »
Yes, but that addresses a different problem. It shifts the measurement window up, extends S21 usefulness into higher impedances, at the expense of lower. I can see how it would be useful for your beads and inductors at high freqs, though.
Yes, indeed. The think is that in many cases, this will not be a problem. Usually when the DUT loading is the problem(low power LDO, under Amp), the Z out of LDO is quite high anyway, so there is no harm in shifting measurement window up and crating voltage divider for VNA prots in the process. The same goes for higher voltage regulators, usually the Zout is not really low so, there is no harm in adding series resistors. I'm not saying that there are no 15V 3mOhm power supplies, not at all. I'd just like to point out that in some cases(quite a few in my experience), simple addition of the resistor might be very helpful  :).

For low frequency measurements a good sound-card  could be suprisingly potent, off course bandwidth is quite limited.
I used it for Bode measurements before we bought Bode, it could be used for impedance measurements with current load.

Are you interested in stability measurements as well?


 

Offline precaud

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Re: Measuring low impedances with a VNA
« Reply #16 on: November 27, 2017, 02:17:08 am »
For low frequency measurements a good sound-card  could be suprisingly potent, off course bandwidth is quite limited.
I used it for Bode measurements before we bought Bode, it could be used for impedance measurements with current load.

I agree. Most of my work is within the audio bandwidth, and I have good tools for it. HP 3567A (a PC-based 3562A with more power and 16 bit DACs and ADCs), and HP 4276A, a highly underappreciated Z meter and the most capable unit ever made within its freq range.

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Are you interested in stability measurements as well?

Not at this point.
 

Offline Pitrsek

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Re: Measuring low impedances with a VNA
« Reply #17 on: November 28, 2017, 09:12:39 pm »
The Ingum probes are not cheap(who would have thought ;D), but I'm yet to find a solution that is cheaper(picotest and sequid are way more expensive). The price is cca 170eur for one. One of the type was cheaper, 140eur, but it had a wider spacing. Not cheap, but compared to others, actually not that bad either. I'll probably try to finish the spring loaded DIY probes...

 
 

Offline precaud

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Re: Measuring low impedances with a VNA
« Reply #18 on: November 29, 2017, 03:36:46 am »
Are you looking at what they call "dipole" test probes, in the RF section?
 

Offline Pitrsek

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Re: Measuring low impedances with a VNA
« Reply #19 on: November 29, 2017, 04:11:47 am »
Yes, I was interested in following:
HFS-810 204 051 A 02 V1-AS3 - cheaper one
HFS-810 201 051 A 06
HFS-810 358 080 A 02 V2-00S
HFS-810 201 051 A 29 V2
HFS-810 307 100 A 02 V2-36S





 

Offline precaud

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Re: Measuring low impedances with a VNA
« Reply #20 on: November 29, 2017, 02:14:19 pm »
Well they're beautiful. 2GHz is way beyond anything I'd need.

I have some thinking to do about my objective. Given that the spacing between the test points (and, therefore the probe terminal spacing) determines the highest useful frequency (before inductance dominates), my 10MHz target is looking unrealistic for what I want to do with it. Most of my measurements will be made to existing boards, with mostly thruhole parts. In most cases I can't control the location or spacing between the test points. Most of the time I will have to use either clip (or solder) leads to the test points.

To see the impact of short leads, here's a plot of shunt-thru using the MS420K from 1kHz to 10MHz, of two "DUT's", one a short using 21-ga wire, the other an axial-leaded 25mOhm current sense R, with different test setups. One setup uses short (3") clip leads for the DUT. The other setup, the DUT is soldered directly into one end of a BNC tee, which maintains the 50 Ohm environment. As you would guess, the smoother curves are the ones using the tee.

For either setup, 1mOhm is out of the question. It's right at the noise floor. If I drop the IF bandwidth another step and slow the sweep, it might clean it up a little. 10mOhm would be usable, but only to 100kHz max.

So just like the low-freq test a few posts back, 1mOhm is right on the bleeding edge, and imposes frequency limits at both ends. Perhaps it's time to take a look at the transformer I/V method to compare its results to the shunt-thru.


If I'm limited to 10mOhm and 100kHz, I don't need a VNA for that. An HP 3562A will work just fine, and much faster.

Hmmm...

PS - The Ingun website is the slowest I've encountered in years. Is it that way in EU too?
« Last Edit: November 30, 2017, 01:10:12 am by precaud »
 

Offline R_G_B_

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Re: Measuring low impedances with a VNA
« Reply #21 on: November 30, 2017, 07:11:39 am »
I'm not sure if there are people who are aware that digilent have released an update to their analogue discovery waveforms 215 that now includes an impedance analysis tool.

R_G_B
 

Offline Pitrsek

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Re: Measuring low impedances with a VNA
« Reply #22 on: November 30, 2017, 07:45:26 am »
Could you please post pics of your test setup?
I might be able to replicate, so we can compare results. What is representative spacing on your boards?
I would say that the inductive rise is not about limit of the test setup, but more of a DUT limit. Axial-leaded and low inductance don't go well in one sentence. Off course, it's connected. If the DUT is big, it requires big spacing, it's inductive(regardless of test probe spacing). The thing is, if it's THT board, there are no mOhms at 10MHz anyway.... If the DUT was small, but your setup forced use of unnecessary big spacing, then we could talk about exces inductance of the test setup.
Depending on noise performance of the analyzer, simple opamp based low noise pre-amp might help with the dynamic range.
Have you tried the semi-rigid test setup with small SMD resistor? To see where is the limit is?
Soldering coaxial pigtails to vacant capacitor pads is ok an works surprisingly well.
Ingun page is ok in EU.
 

Offline precaud

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Re: Measuring low impedances with a VNA
« Reply #23 on: November 30, 2017, 02:29:43 pm »
Could you please post pics of your test setup?

Sure, see attached pics. 50 Ohm output goes thru 50 Ohm divider. One side goes into the Ref channel termed 50 Ohms, the other to the Tee and into the Test channel termed 50 Ohms. The short is just a wire soldered to the shell. The .025 Ohm current sense R is indeed axial, so some lead length is involved, as you can see.

I plan on cutting the shell flush with the center conductor. It will shorten the path length and I can then solder smaller parts to it.

To test the effect of cable length, I inserted a 16" extension BNC to each cable in turn, and sweeping the .025 Ohm R (to 10MHz) to see if there was any change. Nothing. So one nice thing about the shunt-thru method is, it is relatively insensitive to cable length (to 10MHz, anyway) . Very conducive to use with probes.

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I might be able to replicate, so we can compare results. What is representative spacing on your boards?

I'd say 5 cm would be typical.

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I would say that the inductive rise is not about limit of the test setup, but more of a DUT limit.

You may be right. I'm not sure yet. (Trust but verify...) The inputs on the MS420 go through several relays and RC trims before reaching the FET buffer. I remember comparing Z measurements with it to the HP 3570A years ago and I got better results with the HP. The 3570A input goes right into the FET. Very clean signal path. I attributed the better results to the input stage being much simpler. But I could be wrong.

Back in the day, Anritsu sold an impedance probe for the MS420, called the MA413A. It had a xfmr in the probe body right behind the tip. Rated impedance range was 1Ohm to 1MegOhm. So it seems they too got messy results at lower impedances, and so they just spec'ed it above the mess  :)

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Axial-leaded and low inductance don't go well in one sentence. Off course, it's connected. If the DUT is big, it requires big spacing, it's inductive(regardless of test probe spacing).

Yes, that's essentially saying the same thing I said last post. The spacing (of probe, test points, DUT) basically sets the upper freq limit.

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The thing is, if it's THT board, there are no mOhms at 10MHz anyway....

Hmmm... really? Even with a good ground plane and distributed capacitance? I guess it's fair to say, that is why I want to do this measurement - to see what you already know  :)

The other response I would make is: and therein lies the opportunity.

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If the DUT was small, but your setup forced use of unnecessary big spacing, then we could talk about exces inductance of the test setup.

Yes, that is not the case. I know to avoid that...

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Depending on noise performance of the analyzer, simple opamp based low noise pre-amp might help with the dynamic range.

I was thinking that yesterday. There's a 20-40dB level drop to the T channel (depending on the impedance). A 10MHz 20dB preamp could be put together.

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Have you tried the semi-rigid test setup with small SMD resistor? To see where is the limit is?

I don't have any yet. But I'll be parting out a Tek 7000-series mainframe in the next couple days, it has an analog delay line made from coiled rigid coax and then I'll have a large supply of it  :)

Do you have SMA on the back end of your DIY probe? I'll probably stay with BNC. It's convenient.

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Soldering coaxial pigtails to vacant capacitor pads is ok an works surprisingly well.

Makes sense. I'll be doing that sometimes, for sure.

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Ingun page is ok in EU.

Ah, ok. They might have someone in their basement hosting it for the US  :)

Thanks for your input on this!
« Last Edit: December 01, 2017, 01:36:34 am by precaud »
 

Offline precaud

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Re: Measuring low impedances with a VNA
« Reply #24 on: December 01, 2017, 04:46:52 pm »
Today I made a setup for the injection transformer method, using one of the North Hills xfmrs.The setup is "leggy" (the leads are long) and some are unshielded, but again this is proof-of-concept and will be refined if it works. The setup for this method is in the first post of this thread. With DC blocking C of two back-to-back 3300uF lytics, it is ready to be used on a Vreg.

This is definitely a larger and more cumbersome setup than the shunt-thru. But to be fair, the latter didn't have DC blocking C's yet. And two are required.

This method must be used with short or short/load compensation in post-processing. Otherwise it is wildly inaccurate.

I have read that, for best accuracy, it is important to minimize the distance between the 1R current sense resistor and the DUT, so I attached it to the back of the grabber.

The reason for the banana plugs/jacks is that the test channel lead needs to be moved. It connects to the Vr test point for the thru sweep (to calibrate the analyzer and test jig). Then it is moved to the DUT for short compensation, and for the actual measurement.

And that in itself creates a problem. The thru sweep connects to the other side of the xfmr, so it is always 180º out of phase to the measured sweeps. That is why the phase is 180º in the attached graph. The fix is: I'll have to read the thru-sweep into the computer, flip the phase, and then shove it back into the analyzer.

The last attachment is Z/phase sweeps of two axial 3W current sense R's, .025 and .050 Ohms using this setup. As you can see, despite its messiness, it is actually better-behaved than the shunt-thru chart (see a couple posts earlier). The R050 is actually worse at high freq's, because it is further away from the short condition which is used to correct the data. To pull it into line, I'd need to add load compensation too. Very doable.

So shunt-thru has some competition!  :)
« Last Edit: December 02, 2017, 02:19:50 am by precaud »
 


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