Measuring passive parts with VNA

[G0HZU]

My old Agilent VNA has FS (fixture simulator) mode that can accept an s2p file based model of the test fixture at each port. This is usually better than using a basic port extension to model the fixture.

Often, it is possible to measure or model the test fixture as a two port network and so this VNA can correct for the test fixture using the s2p file data.

[rfclown]

I often use an SMA connector with the center pin clipped short. A 0402 isn't quite big enough to bridge the gap between center pin and ground, so I blob solder on the ground side to make the connection. I grabbed a few things out of a box to show in the picture. The larger inductor was probably something I wanted to measure. The resistors are actually a cheap homemade mismatch load that I use for testing. I think the resistors are 0603.

Do a single port OSL calibration on VNA. Measure SMA connector with NO component. Dial out electrical length of connector so that you see an open. Solder component on SMA connector and measure.

[tszaboo]

https://www.vishay.com/docs/31006/tnpw.pdf This is an example datasheet of a general purpose resistor where the RF performance is mentioned. So it's aligns with jonpaul's app note, the low value resistors are slightly inductive, the high value ones are slightly capacitive (Figure 1-9). Honestly, I was expecting it to be the opposite, high value resistors have a lot of "wigle-wigle yeah" in them which is inductive. I wonder if each series has a "golden value" that has high bandwidth and good RF parameters.

[LM21]

Are you really telling me that there is a difference between various 22pF capacitor brands. I'll probably use something larger than 0402 but still.

[coppercone2]

propery rf caps are quite expensive! I am not sure what you mean. That you think there is no difference between various brands of the same type of ceramic capacitor?

[tszaboo]

Are you really telling me that there is a difference between various 22pF capacitor brands. I'll probably use something larger than 0402 but still.

Absolutely, even at 1-2GHz. Imagine one capacitor with one pair of plate and one with two pairs at double the distance. They are going to have different inductance. The ceramics used matters as well.

[coppercone2]

the reason why it sometimes works is because the ESR of the capacitor is low. Even if it becomes inductive, the ESR is so short that even with the effect of inductance its still a pretty good short circuit. but the question is for how long and how consistently. Thats why random unspecified capacitors might work in RF even if they are past their SRF.

The ESR represents the lowest possible impedance for the capacitor, which is at SRF.

For instance this graph.


Does not look like the slope before and after the SRF is the same or even really a slope! it shows you how you can possibly get screwed making assumptions about capacitor performance from just ideal equations that relate to SRF

[LM21]

Are you really telling me that there is a difference between various 22pF capacitor brands. I'll probably use something larger than 0402 but still.

Absolutely, even at 1-2GHz. Imagine one capacitor with one pair of plate and one with two pairs at double the distance. They are going to have different inductance. The ceramics used matters as well.

One worry more. I  have known this before kind of, but didn't think of it  much. And my interest is mainly below 1GHz.

[joeqsmith]

Still, even below 1G, materials used can become very important.   High Q circuits for example. 

[ProphetM]

My old Agilent VNA has FS (fixture simulator) mode that can accept an s2p file based model of the test fixture at each port. This is usually better than using a basic port extension to model the fixture.

Often, it is possible to measure or model the test fixture as a two port network and so this VNA can correct for the test fixture using the s2p file data.

G0HZU,
Please return to the uk vintage forum?

[tszaboo]

50 Ohm thin film resistor. The previous ones were thick film.
TE Connectivity RP73PF1E49R9BTDF
It measures as 86Ohm (40+50), + 16 J at 4GHz so you don't want this as your cal resistors either.
Has similar inductive behavior as the thick film resistors.

[tszaboo]

Now, something else.
DUT:Würth Electronic WE-MK Multilayer ceramic inductor 15nH part number 7447840115
Has self resonant frequency of 2.3GHz minimum

I measured the SRF at 3.32GHz. It's very nicely on the 1.0 circle.

[tszaboo]

DUT:Würth Electronic WE-MK Multilayer ceramic inductor 1.5nH part number 7447840015
Has self resonant frequency of 6GHz minimum, which is beyond what I can measure with this VNA.

[tszaboo]

DUT: 2.2pF +/- 0.5pF NP0 Capacitor WE 885012005051
This is their jelly bean 0402 capacitor.

[tszaboo]

DUT: 2.3pF +/- 0.05pF NP0 Capacitor WE 885392005009
This is their RF range 0402 capacitor.
I'm not sure if I can tell the difference with this test setup. The tolerance is of course a big differentiator, otherwise I see very similar curves.
Someone more knowledgeable than me, can maybe tell me what sort of difference should I be looking at?

[coppercone2]

i think you might need to measure 50 of each from different batches  to see the difference

[tszaboo]

I'm taking the VNA for anther spin.
Aliexpress cheap attenuator board.
The phase is out, because the electrical length is different than the calibration (that happened on the connector)

What I noticed is, these don't have their SMA connector soldered on the back side of the board. It's also not a nice SMA, it's a bit "crounchy" to connect.
The electrical length, compared to the cal kit is 233ps, that has been de-embedded.
I changed the graphs. I think it was a mistake to just go with the default settings for the previous measurements, while I could display an impedance vs frequency graph.
"0dB" through:

[tszaboo]

The -10dB attenuator.
Made with 0603 resistors. 2x 63.5 Ohm to GND, 33.9 + 29.5 in series.
It measures between -10.15 and -9.71dB.

[tszaboo]

The -20dB attenuator. It measures between -18.5 and -19.5. The electrical length is different (strange) at 213ps.
0603 resistors, 52 to GND, 46 in series, 32.2 to GND, 49 in series, 54 to GND.
I took these measurements in circuit, so these are not the actual resistor values, but I don't feel like solving a bunch of math right now. I't more interesting that it doesn't seem symmetrical, or the used components have a bunch of variability. Maybe they are 5% resistors?

[tszaboo]

The -30dB attenuator. Electrical length is 195ps. Why does it get shorter?
Gain is between 29.6 dB and -30.75dB

[szoftveres]

I wonder what would happen if this board was designed with symmetrical "mode" termination - i.e. the coplanar wg has TE waves on both sides; on this particular board, TE waves hits a resistor one side but go freely on the other.

Would be great to see a design with double value shunt resistors and having soldered two on each side (essentially in parallel), and do an apple to apple comparison with this board, particularly at high frequencies.

[tszaboo]

For me it's unclear what kind of layer stackup they have on the board. I would rather design one with the option to place resistors on both side and work from that. I'm not sure how it would differ, or if it would be better in practice. The track width is about the same as the pad size for this resistor, so the track would need to change thickness, or a non-symmetrical placement would be necessary. Or some wierd CPW dimensions that are hard to launch from the end connector with a thick trace. So maybe on two layers?