measuring freq response

[2N3055]

More on the topic:

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

affordable but quite good stuff, and some reading...

https://www.sdr-kits.net/calibration-information-for-DG8SAQ-VNWA-3-3EC

and down the rabbit hole....

https://www.euramet.org/Media/news/I-CAL-GUI-012_Calibration_Guide_No._12.web.pdf

[metebalci]

Before I forget, I would like to thank you all for all the information you share, I learned a lot.

Meanwhile, I got a nanoVNA. I am still waiting for a few cables and adapters so I couldnt test much things yet, but I am already impressed with what I saw.

I mainly understand the point or difference about calibration kits. If I also understand correct, more expensive ones gives their LC characteristics in a polynomial model/fit (c0, c1, c2..) and their delay, loss etc. values. I saw a relatively cheap one (I think it was Rigol), it was saying all these values as 0, so I think this means it is an ideal thing, but naturally it is not, so that will cause some errors, and that is the difference. So I guess high quality calibration kit means to be close to ideal or maybe more correctly to be more close to its individually measured model, so the VNA can remove most of the error.

I am interested in stuff up to ~1 Ghz at the moment, so much easier and cheaper regarding to calibration, connectors and cables I guess.

Good you mentioned about PCB design. The actual reason I started questioning about these is I am working on some simple (opamp based at the moment) stuff but I would like to go as high frequency as possible (up to Ghz). Just by prototyping, manhattan style etc., what is the reasonable freq. I can go ? For example, with a high bw opamp (1Ghz+) in a voltage follower configuration, what freq. I can reach in a prototype ?

There is something else I wonder. Most of the simple VNAs or SAs with VNA features, can do s11 and s21 measurements, hence, if I am correct, they are called one path VNAs. The "big" VNAs are I think always two path, so they also measure s22 and s12. I understand this eliminates the need to reverse the DUT for measurements, but is there another benefit of this ?

Another question. I guess a differential amp has three ports, not two. So it cannot be measured with a two port VNA. Is there an approx. way or a workaround to accomplish this with two ports ?

[nctnico]

I'd create a PCB for anything over 100MHz. But before that I'd simulate the circuit first. It may take a few iterations to get an HF circuit right. For testing a differential amplifier you can either use a balun to convert single ended to differential or connect one leg of the amplifier to ground.

[Berni]

Yep proper VNA cal standards are individually measured at the factory for the true response to allow you to correct its imperfections. But unless you are chasing every last fraction of a dB it doesn't matter.

You can go into the 100s of MHz using dead bug prototyping if you carefully lay out your wires, keep them tiny and short and use a copper clad board as a ground plane right under it. Tho you would probably want to do that under a microscope since the wires need to be tiny and short and the ICs that work at those speeds also tend to be tiny to minimize parasitics. But yeah with RF don't expect to get it right on the first try, things often need a bit of tweaking to work well.

The difference between such a single path and true dual path VNA is simply that the dual path one has an extra RF coupler on the other port and a relay to switch the couplers of the ports around. All it does is save you from having to unplug your DUT and plug it in backwards to get the reverse measurement. It's mostly a convenience thing (and helps a bit with accuracy since you don't mess with your setup mid measurement) but then again you often don't need the reverse measurement anyway. Heck sometimes you don't even need the 2nd channel at all. If you are tuning an antenna then you just use the 1st channel to look at reflections, when no power is reflecting back into the port then the antenna is perfectly tuned.

There are also 4 port VNAs that can measure more complex devices, but you probably won't ever need one. As nctnico said you can get a balun to turn the single ended port of your VNA into a set of differential ports. Do the open/short/load calibration on the other side of the balun and the VNA will de-embed it for you. Tho doing the thru calibration is a bit sticky since the diff signal can't be fed back into the VNA, but you could terminate the negative output and just feed the positive output in to get a rough cal.

[metebalci]

More on the topic:

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

affordable but quite good stuff, and some reading...

https://www.sdr-kits.net/calibration-information-for-DG8SAQ-VNWA-3-3EC

and down the rabbit hole....

https://www.euramet.org/Media/news/I-CAL-GUI-012_Calibration_Guide_No._12.web.pdf

Thanks for the links, nice document@euramet.

[gf]

Do the open/short/load calibration on the other side of the balun and the VNA will de-embed it for you.

... if the calibration standards (having e.g. male or female SMA connectors) are directly insertable on the balanced side of the balun, and the DUT is directly insertable at the balanced side of the balun as well. Then we had a well-defined calibration plane at the balanced side of the balun. But this would require that the DUT has a single SMA connector (with the same gender as the calibration standards) for a differential signal, which is IMO unusual. My feeling is that thing are pretty straightforward when the DUT is directly insertable at the calibration planes established with the standards, but complications arise when insertablitiy is not granted, so that additionally required adapters/fixtures cannot be implicitly de-embedded by the calibration, but must be de-embedded explicitly then.

[metebalci]

So here is my first try and some questions. I am trying to measure s21 of wideband (2.4Ghz) opamp (THS4302) on its own eval board between 1Mhz and 2Ghz. I am not particularly interested in THS4302, just using it as an example.

First some questions regarding to nanoVNA (particularly V2.2). It comes with M SOL and a F-F T for calibration. For calibration, I set the stimulus, connected two SMA cables that comes with it to Port 1 and 2. The problem is because SOL are all M, I used F-F T first and then used SOL for calibration on Port 1, and then connected the other cable which is connected to Port 2, and calibrated for T. Because DUT has F SMA connectors, I removed F-F T. I guess there is no other way (?) to do this properly until I get an F SOL kit. So I am damaging the calibration (not sure to what extent?) and I think I can use electrical delay option but not sure how to do properly yet, so the issues below can be related to that maybe, at least partially. I also know/learned that I am not measuring the opamp directly like this, since there is no calibration options on the board, so also not sure how this affects the results.

I didnt realize before (but I didnt check many wideband opamps, maybe that is common in this category), the datasheet of THS4302 is pretty nice, it even has all measured s parameters. You can look at the datasheet (https://www.ti.com/lit/ds/symlink/ths4302.pdf) but quick summary and questions:

- the opamp has 14dB fixed gain, but I see ~9dB. what could be the reason ?
- the overall shape of logmag curve is I think not too bad, the opamp has 2.4G bandwidth and it looks like it is still within 2dB at 2Ghz.
- I dont get the phase curve, naturally it should not go down to -180, on the datasheet the arg s21 goes under -100 after 1.5Ghz. what could be the reason for this ?

[metebalci]

I'd create a PCB for anything over 100MHz. But before that I'd simulate the circuit first. It may take a few iterations to get an HF circuit right. For testing a differential amplifier you can either use a balun to convert single ended to differential or connect one leg of the amplifier to ground.

OK, so if I reach 100Mhz or so properly, I will be happy and if I cannot improve that I will move to PCB.

[metebalci]

The difference between such a single path and true dual path VNA is simply that the dual path one has an extra RF coupler on the other port and a relay to switch the couplers of the ports around. All it does is save you from having to unplug your DUT and plug it in backwards to get the reverse measurement. It's mostly a convenience thing (and helps a bit with accuracy since you don't mess with your setup mid measurement) but then again you often don't need the reverse measurement anyway. Heck sometimes you don't even need the 2nd channel at all. If you are tuning an antenna then you just use the 1st channel to look at reflections, when no power is reflecting back into the port then the antenna is perfectly tuned.

There are also 4 port VNAs that can measure more complex devices, but you probably won't ever need one. As nctnico said you can get a balun to turn the single ended port of your VNA into a set of differential ports. Do the open/short/load calibration on the other side of the balun and the VNA will de-embed it for you. Tho doing the thru calibration is a bit sticky since the diff signal can't be fed back into the VNA, but you could terminate the negative output and just feed the positive output in to get a rough cal.

I wish I could live only with s11, I arrived this point mostly because s21.

I did a first pass reading on some technical documents, but I am confused on something. So to measure s11 and s21, it is assumed a2=0, which means it is perfectly terminated. I think I understand then the dual operation is done in reverse path for s22 and s12 and then it is superposition to get s matrix. But I am only measuring forward path s11 and s21 and a2 is I guess not 0 (so b2 should be reflecting back from VNA?), and I guess a2 is not measured by one path devices (nanoVNA, Siglent SVA etc.). Is this somehow not important or eliminated by something or what am I missing ?

I understand roughly the model behind single port calibration with SOL, but didnt read much yet to understand how T is then used to calibrate s21 measurement. Maybe that is why I am confused for the topic above.

Thanks both for explaining the balun workaround.

[ogden]

- the opamp has 14dB fixed gain, but I see ~9dB. what could be the reason ?

Could be overload of VNA input that could be solved with 20dB attenuator after amp?

[metebalci]

Do the open/short/load calibration on the other side of the balun and the VNA will de-embed it for you.

... if the calibration standards (having e.g. male or female SMA connectors) are directly insertable on the balanced side of the balun, and the DUT is directly insertable at the balanced side of the balun as well. Then we had a well-defined calibration plane at the balanced side of the balun. But this would require that the DUT has a single SMA connector (with the same gender as the calibration standards) for a differential signal, which is IMO unusual. My feeling is that thing are pretty straightforward when the DUT is directly insertable at the calibration planes established with the standards, but complications arise when insertablitiy is not granted, so that additionally required adapters/fixtures cannot be implicitly de-embedded by the calibration, but must be de-embedded explicitly then.

For this reason, I was planning to use M on one side, and F on the other side for the boards I was planning to prototype (both single ended not differential).

[metebalci]

First some questions regarding to nanoVNA (particularly V2.2). It comes with M SOL and a F-F T for calibration. For calibration, I set the stimulus, connected two SMA cables that comes with it to Port 1 and 2. The problem is because SOL are all M, I used F-F T first and then used SOL for calibration on Port 1, and then connected the other cable which is connected to Port 2, and calibrated for T. Because DUT has F SMA connectors, I removed F-F T. I guess there is no other way (?) to do this properly until I get an F SOL kit. So I am damaging the calibration (not sure to what extent?) and I think I can use electrical delay option but not sure how to do properly yet, so the issues below can be related to that maybe, at least partially. I also know/learned that I am not measuring the opamp directly like this, since there is no calibration options on the board, so also not sure how this affects the results.

I just tried better cables (DC-4Ghz) and better T (DC-18Ghz), the 180 deg shift point moved like 100Mhz to right but everything looks same.

[tautech]

- the opamp has 14dB fixed gain, but I see ~9dB. what could be the reason ?

Could be overload of VNA input that could be solved with 20dB attenuator after amp?

Nah, scaling is wrong...2dB/div and a 9.75dB measurement  and no mention of the stimulus setting. 
Setup needs more attention I suspect.

[metebalci]

- the opamp has 14dB fixed gain, but I see ~9dB. what could be the reason ?

Could be overload of VNA input that could be solved with 20dB attenuator after amp?

Nah, scaling is wrong...2dB/div and a 9.75dB measurement  and no mention of the stimulus setting. 
Setup needs more attention I suspect.

I also thought it could be overload but I couldnt find the input output specs yet, I dont have an attenuator to try at the moment unfortunately. I found ADF4350 tw power setting, and decreased it (set=0, I think it means -4dBm then). The result is attached. I see it starts from 14dB now, but it falls rapidly. The phase response did not change.

[Kleinstein]

The 14 dB gain from the THS4302 datasheet should be without thermination at the output. The 100 Ohms shown as load in the circuit should be 50 Ohms series termination (that should be in the real circuit) and 50 Ohms load / cable impedance. This would give about 6 db loss.

For cal set being all female: the reference plane for the 1 st port (excitation) is the male cable end. The reference plane for the input side would be the female side, either directly at the instrument, or with female-female adapter and cable.

[tautech]

I see it starts from 14dB now, but it falls rapidly. The phase response did not change.

Take a step back from the instrument to fully comprehend what you see...most of it is staring right at you.
Your sweep is for 1 -2 GHz and the trace Ref levels are at 0 dB with a 2dB/div setting. Plonk a few markers down at various frequencies to get the full picture.

Of course the phase response hasn't changed as it's the performance result of the DUT.

[metebalci]

The 14 dB gain from the THS4302 datasheet should be without thermination at the output. The 100 Ohms shown as load in the circuit should be 50 Ohms series termination (that should be in the real circuit) and 50 Ohms load / cable impedance. This would give about 6 db loss.

For cal set being all female: the reference plane for the 1 st port (excitation) is the male cable end. The reference plane for the input side would be the female side, either directly at the instrument, or with female-female adapter and cable.

There is an S-Parameter table on the datasheet (p6, I am running it also at Vs=5V), measured on the same board, it says S21 is ~14dB up to 1Ghz, then slowly decreases to ~12dB at 2 Ghz.

Not sure if I understand the cal part. Maybe I said it wrong, the cal set is male, that is why I need to use f-f thru to be able to use them at the end of cable (male) connected to Port 1. Then I remove f-f, and connect both cables to DUT (has female connectors like VNA).

[ogden]

14dB is quite a gain. Even using pro VNA's you have to know what you are doing when measure active circuits. I am not sure those toy VNA's are up-to task in this case. Just my two cents.

[metebalci]

I see it starts from 14dB now, but it falls rapidly. The phase response did not change.

Take a step back from the instrument to fully comprehend what you see...most of it is staring right at you.
Your sweep is for 1 -2 GHz and the trace Ref levels are at 0 dB with a 2dB/div setting. Plonk a few markers down at various frequencies to get the full picture.

Of course the phase response hasn't changed as it's the performance result of the DUT.

Not sure I understand what you mean. The sweep is from 1MHz (not G) to 1GHz.  I dont think I need more markers, it is pretty clear, it starts around 14dB, which is aligned with datasheet, but falls much more rapidly (I think not surprising if the phase response measured is correct but that is the problem). I think the performance of DUT is pretty much clear from the datasheet, obviously there is something I either do wrong or I cannot do using nanoVNA or with cables or with calibration etc.

[metebalci]

14dB is quite a gain. Even using pro VNA's you have to know what you are doing when measure active circuits. I am not sure those toy VNA's are up-to task in this case. Just my two cents.

I should get an attenuator and re-try 

[metebalci]

14dB is quite a gain. Even using pro VNA's you have to know what you are doing when measure active circuits. I am not sure those toy VNA's are up-to task in this case. Just my two cents.

Side note, ADF4350 output level was +5 dBm by default (I think), and I decreased it to -4 dBm already (I think), the manual is not very helpful unfortunately. I need to search a bit more info.

[metebalci]

14dB is quite a gain. Even using pro VNA's you have to know what you are doing when measure active circuits. I am not sure those toy VNA's are up-to task in this case. Just my two cents.

Side note, ADF4350 output level was +5 dBm by default (I think), and I decreased it to -4 dBm already (I think), the manual is not very helpful unfortunately. I need to search a bit more info.

I didnt check the schematic but according to block diagram (of nanoVNA v2) there is a AD8342 mixer, and its max RF input level is 12 dBm. So if I didnt damage it, with -4dBm tx output, I think it should be OK.

[gf]

Note that any constant-time delay is equivalent to a phase shift which dependes linearly on frequency.
The measured S21 phase is only correct between the established calibration planes. But you did remove the f/f adapter after calibration.
Without de-embedding the removed f/f adapter (at least via electrical delay) you won't measure the correct S21 phase of the DUT, but there will be a (frequency-dependent) offset.
Another question is whether the specified phase is meant to be between between the SMA connectors of the board, or rather between input and output of the IC.
In the latter case you would also need to de-embed the on-board fixtures, from the SMA connectors to the amp.

EDIT: Btw, if you specify an electrical delay, the NanoVNA firmare applies it to both, S11 and S21. You cannot specify separate delays for port1 and port2.

[metebalci]

Note that any constant-time delay is equivalent to a phase shift which dependes linearly on frequency.
The measured S21 phase is only correct between the established calibration planes. But you did remove the f/f adapter after calibration.
Without de-embedding the removed f/f adapter (at least via electrical delay) you won't measure the correct S21 phase of the DUT, but there will be a (frequency-dependent) offset.
Another question is whether the specified phase is meant to be between between the SMA connectors of the board, or rather between input and output of the IC.
In the latter case you would also need to de-embed the on-board fixtures, from the SMA connectors to the amp.

EDIT: Btw, if you specify an electrical delay, the NanoVNA firmare applies it to both, S11 and S21. You cannot specify separate delays for port1 and port2.

Great, that is the answer I was looking for, thanks a lot. I set electrical delay to 165ps (I just heard somewhere f-f delay of T that comes with it is 165ps, but I used different f-f while calibrating, anyway), and the result is attached.

I am aware about the other issue, obviously I am measuring the board at best not the IC. It would be pretty blind at this point, but the datasheet has measurements on the same board (*), so I was thinking I should be able to achieve something similar at least in a proper setup.

* Just confused the part number I realized when double checking the datasheet, I have 4303 (not 4302), it has even more 20dB gain. The weird thing is s21(Ang) numbers on the datasheet, did they just wrote the freq instead of actual value or the value is almost same as the frequency ?!

[tautech]

I see it starts from 14dB now, but it falls rapidly. The phase response did not change.

Take a step back from the instrument to fully comprehend what you see...most of it is staring right at you.
Your sweep is for 1 -2 GHz and the trace Ref levels are at 0 dB with a 2dB/div setting. Plonk a few markers down at various frequencies to get the full picture.

Of course the phase response hasn't changed as it's the performance result of the DUT.

Not sure I understand what you mean. The sweep is from 1MHz (not G) to 1GHz.

Sorry 

Still I have difficulty looking at the 2dB/div and the marker measurement not agreeing with the scaling.
Even you last pic shows this as wrong yet markers are somewhere in the right paddock that you'd expect.