Measuring passive parts with VNA

[tszaboo]

Continuing the discussion from here:
https://www.eevblog.com/forum/testgear/12-ghz-active-probe-project/75/
So I ordered a test PCB to measure passive parts with a VNA. It's for 0402 components because that's what I use for antenna matching.

The reference plane for the measurements is the SMD pad of the 0402 component.
The measurement method is a though measurement, based on this article:
https://coppermountaintech.com/measurement-of-electronic-component-impedance-using-

The measurement method should yield the smallest errors. Reflection based measurements are possible on a second PCB.
I'm interested how well passive parts behave. I have some experience in antenna matching, mostly for 2.4 GHz, but I'm really not an RF guru. I had the idea a while back because some RF inductors were acting atrociously, doing all kinds of crazy things when I tried to match an antenna. And I don't have an expensive simulation tool, it's more a trial and error journey for me.
The tool to measure it is a Megiq 440e, which is a VNA that goes between 400MHz and 4 GHz.

[joeqsmith]

Have a look at Mario's board.  Note the short, open and thru.  Also note the reference plane for the match. 

https://www.mariohellmich.de/projects/trl-cal/trl-cal.html

[joeqsmith]

Some time ago I attempted to replicated an experiment where the VNA was used to measure the parasitic shunt capacitance of some resistor.    Not sure you will find anything of use there.

https://www.eevblog.com/forum/rf-microwave/shunt-capacitance-of-1206-smd-resistors-jeroen-belleman-december-2010/25/

[tszaboo]

Have a look at Mario's board.  Note the short, open and thru.  Also note the reference plane for the match. 

https://www.mariohellmich.de/projects/trl-cal/trl-cal.html

I'm not sure which difference are you trying to point out. The VNA I have at work only supports OSLT calibration, so I wouldn't be able to use that fixture. I've used an impedance controlled pool to make the board, and calculated the waveguide. I see that he is not using a clearance on the RF connector. A Würth appnote was specifically mentioning that there should be more space around the pin of these end launch connectors. Not a concrete guide, and I have no way to simulate it. I also don't recognize the connector he is using so maybe that has a different footprint.
The other thing I see is the via fencing. As I understand this is more for EMC compliance, and a waveguide will work just as good without it, at least for the frequencies in question. But I'm ready to be proven wrong.

[joeqsmith]

Added inductance and discontinuity. 

[rf-messkopf]

I'm not sure which difference are you trying to point out. The VNA I have at work only supports OSLT calibration, so I wouldn't be able to use that fixture.

I ran a quick test on a 22pF 0805 capacitor in the series fixture using TRL, TOSM and unknown thru (UOSM in R&S speak) calibration on that very board you are discussing. All calibrations only use the cal standards on that board and assume them to be ideal. As you can see, there is not a huge difference when you calculate the impedance magnitude from S21 data. TRL generally yields the smoothest traces with least ripple on this board, especially when you look at S21 data, and it surely is the most precise way to get the reference planes of the measurement directly at a particular point in a planar geometry.

Further, the advantage of TRL is that it does not need fully known standards. For example, the reflect only needs to have some not too small reflection coefficient, which is assumed to be the same at both ports. Also, the line only needs to have a precisely defined impedance (this determines the system impedance of 50 ohms), but it can be lossy and have an arbitrary propagation constant. Therefore you can make very precise measurements that way.

There will be some systematic error in the TRL cal because the line standard does not have exactly 50 ohms since the board was manufactured in a cheap pool process, without proper impedance control.

I've used an impedance controlled pool to make the board, and calculated the waveguide. I see that he is not using a clearance on the RF connector. A Würth appnote was specifically mentioning that there should be more space around the pin of these end launch connectors. Not a concrete guide, and I have no way to simulate it. I also don't recognize the connector he is using so maybe that has a different footprint.

The part No. of that connector is mentioned on my homepage. It is designed specifically for coplanar waveguides.

The other thing I see is the via fencing. As I understand this is more for EMC compliance, and a waveguide will work just as good without it, at least for the frequencies in question. But I'm ready to be proven wrong.

This is a grounded coplanar waveguide, hence the front side ground planes must be tied in some way to the rear ground plane. That's what the via fences are for.

Another option you could try is making a board with a footprint and two connectors like the one pictured in the attachment, then calibrate to the connectors (for example with TOSM/SOLT), and do a port extension to the SMD pads on the board (i.e., add the required offset length at each port).  You can check if your offset lengths are correct by temporarily soldering a short to ground at each SMD pad. This might already be precise enough below a couple of GHz.

[switchabl]

Another relatively simple approach that works well even at much higher frequencies is 2X-Thru de-embedding. It involves a standard coaxial calibration at the connectors and a single planar thru standard. The T-matrices of the left and right halfs of the fixture can be extracted from the thru measurement using time gating (under some reasonable assumptions) and they allow you to move the reference plane to the middle of the line. The connectors need to be far enough apart so that you can separate them properly in the time domain (this depends on the bandwidth of the VNA/fixture).

Most modern VNAs should have this as an option (Keysight calls this "automatic fixture removal", R&S has something similar). But you could always do it offline:
https://scikit-rf.readthedocs.io/en/latest/examples/networktheory/IEEEP370%20Deembedding.html
https://www.amphenol-cs.com/software

[G0HZU]

Here's a quick measurement of a cheapo 22pF 0805 capacitor vs a decent 22pF 0805 cap made by PPI. The PPI cap should have much lower ESR and a lower SRF and the VNA plot below shows this.

A good way to test your fixture quality would be to see if you can get fairly sensible data for the ESR of the cheapo 0805 22pF cap up to a few GHz. Then compare against a decent 22pF cap to see if you can measure the difference in ESR between the two caps.

You can see that there is a clear improvement with the PPI part. The VNA predicts the ESR is only about 0.11R at 2GHz.  In reality, it's probably a bit higher than 0.11R at 2GHz. The VNA is really being stretched here and there will be a lot of uncertainty in the measurement, but there is a clear difference in ESR between the two 22pF caps.

Note that this was done with a really old homebrew SMA cal kit. it's probably about 20 years old. I think the cal kit is getting tired now, I keep meaning to make a new 50R load as the VSWR above 1.5GHz has definitely degraded slightly over the years.

[joeqsmith]

Comparing various caps with my LRC meter, datasheets and low cost VNA.   Used a few fixed resistors as a sanity check.  The test runs from about 4min in to about 20min.    Data was also taken with homemade standards and the ideal model.   



Of course for these really low number, the ground loop is a problem.   Even with the common mode transformer, the NanoVNA's noise floor limits the measurement.

[tszaboo]

The part No. of that connector is mentioned on my homepage. It is designed specifically for coplanar waveguides.

Thought so. People knowing RF usually don't make simple mistakes like that.

Added inductance and discontinuity. 

The RF signal in theory goes between the ground and the trace. So it doesn't in theory make that much difference between via fenced waveguide or regular waveguide.

Here's a quick measurement of a cheapo 22pF 0805 capacitor vs a decent 22pF 0805 cap made by PPI. The PPI cap should have much lower ESR and a lower SRF and the VNA plot below shows this.

A good way to test your fixture quality would be to see if you can get fairly sensible data for the ESR of the cheapo 0805 22pF cap up to a few GHz. Then compare against a decent 22pF cap to see if you can measure the difference in ESR between the two caps.
You can see that there is a clear improvement with the PPI part. The VNA predicts the ESR is only about 0.11R at 2GHz.  In reality, it's probably a bit higher than 0.11R at 2GHz. The VNA is really being stretched here and there will be a lot of uncertainty in the measurement, but there is a clear difference in ESR between the two 22pF caps.

Note that this was done with a really old homebrew SMA cal kit. it's probably about 20 years old. I think the cal kit is getting tired now, I keep meaning to make a new 50R load as the VSWR above 1.5GHz has definitely degraded slightly over the years.

I'm interested in measuring resistors. So I plan to get some regular 50 Ohm and some ultra high end RF resistors, and we will see.

[nctnico]

The part No. of that connector is mentioned on my homepage. It is designed specifically for coplanar waveguides.

Thought so. People knowing RF usually don't make simple mistakes like that.

Added inductance and discontinuity. 

The RF signal in theory goes between the ground and the trace. So it doesn't in theory make that much difference between via fenced waveguide or regular waveguide.

But the problem is that without good connection between the grounds, the inductance starts to play a role. If you are simulating this in a program like Sonnet, you'll see the HF current to start flowing around the edges of the 'ground' on the top (signal) layer if you don't have enough vias to stitch all the grounds together to form a single ground (at least to a few GHz).

[rf-messkopf]

Thought so. People knowing RF usually don't make simple mistakes like that.

Just a hobbyist here.

Added inductance and discontinuity. 

The RF signal in theory goes between the ground and the trace. So it doesn't in theory make that much difference between via fenced waveguide or regular waveguide.

But the problem is that without good connection between the grounds, the inductance starts to play a role. If you are simulating this in a program like Sonnet, you'll see the HF current to start flowing around the edges of the 'ground' on the top (signal) layer if you don't have enough vias to stitch all the grounds together to form a single ground (at least to a few GHz).

In fact, on my board, the front side ground plane even would be cut in two halves without a rear ground plane and the via fences since there are traces that run from board edge to board edge. Also, the rear ground plane adds some shielding.

On the other hand, on my TRL board the launcher to trace transition is not super critical since it is behind the calibration plane, and hence eliminated by calibration. The assumption is, however, that all launcher to trace transitions are identical for the cal standards and the DUT. This is also true for TOSM/SOLT. The biggest problem with TOSM the above measurement is that the cal standards were uncharacterized and assumed to be ideal. This is a problem especially for the open since there is always fringing capacitance.

But when you calibrate to the connectors and use port extension to the DUT fixture SMD pads, then you do not eliminate the launcher to trace transition, and you should be careful to avoid added inductances and discontinuities.

[tszaboo]

So I got the boards, got the connectors, and I'm still waiting for the ultra high bandwidth RF resistors to make the calibration possible against the pedestrian cheap 50 Ohm resistors.

[tszaboo]

Added inductance and discontinuity.

A Würth appnote was specifically mentioning that there should be more space around the pin of these end launch connectors.

But yeah, maybe the coax transition is not optimal. Good thing than that it get's calibrated out then.

[tszaboo]

So I finally got some Vishay CH0402-50R resistors as calibration resistor, and the plan is that I'll compare them to some plain old thick film resitors this weekend to start with.

[tszaboo]

Note to self: measurement error happens if you don't tighten the SMA connectors correctly!
Also please modify footprint of the RF connector its PITA to solder it correctly.


DUT: Samplekits.de (no longer active AFAIK) generic resistor, unknown brand. 51Ohm.
Sweep: 360 steps (overkill) between 400MHz and 4000MHz, 0dBm.

[tszaboo]

DUT: Vishay CRCW 49.9 Ohm CRCW040249R9KFED
This is the resistor I ordered, and intended to use for VNA calibration in the future
At 4GHz the S11 measures 70 + 31J
I'm actually doubting this second board is assembled correctly. I'll put it under the microscope next week.
The measurement is repeated on a 3rd assembled board, and it's pretty much the same.

[tszaboo]

DUT: Samplekits 1.1KOhm resistor, because I wanted a ~950 Ohm but grabbed the wrong one.
I've also added the S22 result zoomed in.
400MHz: 1129 + 1.6J
4000MHz 91 +310J 

[coppercone2]

can you try the same dimension short circuit by soldering something like precisely filed copper?

or buy a 0402 short circuit. keysight sells little blocks, not for soldering. i wonder if you can buy em cheaper

probobly easier to buy a larger resistor and try to duplicate it with a short circuit, 0402 is hard to just manufacture. Maybe it can be painfully lapped with tweezers under a microscope to make something generally similar out of a bit of wire or something

[tszaboo]

can you try the same dimension short circuit by soldering something like precisely filed copper?

or buy a 0402 short circuit. keysight sells little blocks, not for soldering. i wonder if you can buy em cheaper

probobly easier to buy a larger resistor and try to duplicate it with a short circuit, 0402 is hard to just manufacture. Maybe it can be painfully lapped with tweezers under a microscope to make something generally similar out of a bit of wire or something

Good idea. A perfect piece of copper wouldn't be much different from the "Trough" calibration, that measures as 50Ohm exact, since it was used for the calibration.
For now I measured a 0 Ohm resistor, Vishay CRCW series. It Shows as much as 50Ohm + 0.4nH on the S22. I need to look at those calculations again, if this means that the 0 Ohm resistor is actually 0.4nH or not.

[tszaboo]

So I tried to solder a piece of copper into the resistor's place, and this is what I'm getting:
Measures as 44+4J Ohm, or 44 Ohm + 0.2nH. A 0Ohm resistor is clearly better choice.
Also note to self: When assembling another test fixture, sanity check it with a 0 Ohm resistor. Also, solder down all pins of the SMA connector properly.

[jonpaul]

See vintage HP app note 77-3

https://www.hpmemoryproject.org/an/pdf/an_77-3.pdf

Modern imp  meas handbook 4th ed

https://www.pearl-hifi.com/06_Lit_Archive/15_Mfrs_Publications/20_HP_Agilent/HP_App_Notes-Handbooks/Z_Measurement_H-book_4th_Ed.pdf

We use HP4195A and test fix 41952

Good 100k..500 MHz.

Jon

[coppercone2]

the proper kits come with copper and also dielectric blanks for every component size that you measure.

I believe but I am unsure that the machine does some kind of regular calibration (say with a APC-7 OSL) and then a special short circuit and open circuit calibration on the fixture, without the 50 ohm load.

I think they have 50 ohm standard fixture calibration resistors for some sizes, but not all. I think they extrapolate some measurements from some components based on open and shorts in the fixture without a exact replica load.

[G0HZU]

Here's a VNA measurement of a SMD 27nH inductor in an 0603 package. The measurement covers up to 6GHz.
The plot below shows Rs, Ls as well as Rp, Lp and inductor Q vs frequency. I used the fixture correction feature of the (Agilent) VNA to set the reference plane right at the pads of the little SMD inductor. The VNA can model (and therefore cancel out) the fixture connections using two port s-parameter data and this gives really good results over a wide frequency range.

The SRF of the inductor is just over 3GHz and the Q peaks at about 75 and this is similar to the manufacturer's data.

[tszaboo]

Here's a VNA measurement of a SMD 27nH inductor in an 0603 package. The measurement covers up to 6GHz.
The plot below shows Rs, Ls as well as Rp, Lp and inductor Q vs frequency. I used the fixture correction feature of the (Agilent) VNA to set the reference plane right at the pads of the little SMD inductor. The VNA can model (and therefore cancel out) the fixture connections using two port s-parameter data and this gives really good results over a wide frequency range.

The SRF of the inductor is just over 3GHz and the Q peaks at about 75 and this is similar to the manufacturer's data.

So the VNA has the model of the test fixture in it? And you don't have to calibrate it, just enter the type number?