Cables, Connectors (N/SMA/BNC) & Adapters with Measured Results for Comparisons

[KungFuJosh]

The DUT cable I'm referring to is a cable connecting the VNA to the DUT, which in this case (the DUT) is also a cable. 😉

My question isn't about this VNA specifically, but about calibration in general on a two-port VNA. If you have two DUT cables to connect the DUT (in this case, a cable), and a high-quality pass-through adapter, it's pretty straight forward.

But if I want to test with a single adapter or single DUT cable, it becomes trickier to calibrate those things out. It will be easier once I order the other calibration adapters...but this does get a little ridiculous.

[Rubo]

Just for reference, this is what the above cable looks like when I do S21 measurement:

[Rubo]

I know what you mean 

If you want to measure a cable, I wouldn't use 2 cables for calibration.
I would calibrate with one cable for OSL & connect that same cable to the second port for a through.
That way your measurement plane will be at the connector of the second port, to which you connect DUT cable to test.
Idealy you would not use any gender adapters, but even if you do, they should not introduce too much loss (if they are from a reputable manufacturer).
And the adapter loss should be stated in the datasheet.

In my experience, N adapters have a loss of 0.1db to 0.4db depending on frequency (up to 3GHz).

[KungFuJosh]

I know what you mean 

If you want to measure a cable, I wouldn't use 2 cables for calibration.
I would calibrate with one cable for OSL & connect that same cable to the second port for a through.
That way your measurement plane will be at the connector of the second port, to which you connect DUT cable to test.
Idealy you would not use any gender adapters, but even if you do, they should not introduce too much loss (if they are from a reputable manufacturer).
And the adapter loss should be stated in the datasheet.

In my experience, N adapters have a loss of 0.1db to 0.4db depending on frequency (up to 3GHz).

Right, I want to set up the DUT cable on port one, but since the cables I'm testing are N to BNC, so is the DUT cable (but with female BNC), and that requires an adapter to connect to the 2nd N port. That means I need a male BNC calibration kit, right? I don't have that yet. I'm going to order some BNC loads to try, but I'm not going to spend a lot of money on this. Close enough is good enough. 😉

IME, the adapters add reflections and loss.

[KungFuJosh]

Just for reference, this is what the above cable looks like when I do S21 measurement:

Assuming any accuracy on my S21 test, that's double the attenuation compared to the Stealthflex 7 cable I tested. Can you adjust the scale to show more detail?

Thanks,
Josh

[Rubo]

That means I need a male BNC calibration kit, right?

You need OSL at the end of the cable on port 1 (the transmit port). So if it end with a male BNC, you would need female BNC OSL or vice versa.

IME, the adapters add reflections and loss.

Absolutely, especially between diferent types of connectors.

[KungFuJosh]

You need OSL at the end of the cable on port 1 (the transmit port). So if it end with a male BNC, you would need female BNC OSL or vice versa.

Exactly. The DUT cable is male N to female BNC, so I'll need male BNC OSL, but I'm going to get female also, cause who knows what stupid thing I'll do next.

The problem is, I'll want to make a nicer DUT cable after I do those things.

[Rubo]

Sure. The lowest I can go is 1db/div.
Frequency range is 100kHz to 3GHz

[KungFuJosh]

Sure. The lowest I can go is 1db/div.

Seriously? That kinda sucks. Shame on you, Agilent!

The nanoVNA App / device can both go to some fairly ridiculous scales.

[tautech]

Sure. The lowest I can go is 1db/div.

Seriously? That kinda sucks. Shame on you, Agilent!

The nanoVNA App / device can both go to some fairly ridiculous scales.

Yup, so can the SVA1032X you want however you should well know resolution does not equal accuracy.....for that we must consult datasheets. 

[KungFuJosh]

Yup, so can the SVA1032X you want however you should well know resolution does not equal accuracy.....for that we must consult datasheets. 

Thanks, Captain Obvious. 😉😉

Datasheets don't account for bad accuracy due to my dumbass screwing up the test setup and/or calibration. 🤣

[KungFuJosh]

It was suggested to me to calibrate both ports, and so I did. I used a different DUT cable, with an assumed higher quality N-BNC adapter. Attached are the results.

Is that a normal thing? Do people calibrate both ports on their VNAs? What's the process normally like if you do?

Never mind. Unless I'm wrong, that's only useful for VNAs that can do more than S11 and S21.

Thanks,
Josh

[Rubo]

Looks good 

You are correct, 2-port calibration is performed if both ports can transmit.

I'm with you on your previous question:

What are they showing under "Coax Loss" graph???

[KungFuJosh]

Two tests of my SDG6052X.

Chart 1 is using a nice HF5 cable.

Chart 2 is direct connection through a cheap adapter.

[KungFuJosh]

I'm exploring this again now that I have access to a real VNA. Both ports were OSL calibrated.

Here's the StealthFlex7 N to BNC cable again. Still using a crap adapter and no through cal.

This looks more believable, right?

Thanks,
Josh

[KungFuJosh]

This is a short blue cable that came with a nanoVNA. This seems really good, right?

[rf-messkopf]

This is a short blue cable that came with a nanoVNA. This seems really good, right?

I'd be more concerned about the rather large S21 ripple. This can mean that there are are discontinuities in the cable (reflections at the cable-connector transitions), or that the calibration is not very precise (the effective source and load port match is off, hence the VNA sees discontinuities with respect to the cable).

Attached is the S21 and S11 of a Huber+Suhner Sucoflex 106 (blue traces) and a Sucoflex 104 (red traces) microwave cable. The 106 is almost double the length of the 104, as you can see from the S11 ripple. The load in the cal kit is well below -40 dB return loss when measured against an known-good reference kit, so these ripple are within the dynamic range of the measurement. Overall, this is a good result for a high-quality microwave cable.

[tautech]

This is a short blue cable that came with a nanoVNA. This seems really good, right?

I'd be more concerned about the rather large S21 ripple.

No need to be when Josh has just discovered what happens when you don't do a full SOLT Cal and then do a through measurement.

I'm exploring this again now that I have access to a real VNA. Both ports were OSL calibrated.

When you shift scaling to mdB these things start showing up. 

[rf-messkopf]

For comparison a random cheap SMA cable of unknown origin. The shape of the S21 trace depends heavily on the bending, and is not even monotonic with frequency. The matching at the cable end is not too bad (|S11| remains below -20 dB), but it has a loss as big as 3.5 dB at certain frequencies. So either a very lossy dielectric, or the cable radiates at these frequencies because the shielding is crappy.

Btw.: The calibration was a two-port unknown thru (UOSM), with the cal planes in the adapters on the VNA ports.

[rf-messkopf]

When you shift scaling to mdB these things start showing up. 

But they require an explanation if one wants to measure precisely in this area. Which is perfectly feasible with the appropriate means. 

[KungFuJosh]

I guess I need to use more cables/adapters so I can do the through cal also. Fiiiiiiiiiiiiiiine. But it's a pain in the ass  because I don't have any really good DUT cables. We'll see what happens I guess.

[tautech]

When you shift scaling to mdB these things start showing up. 

But they require an explanation if one wants to measure precisely in this area. Which is perfectly feasible with the appropriate means. 

100%
But a S21 is a through measurement so to do it accurately a through needs be part of the VNA Cal.

I guess I need to use more cables/adapters so I can do the through cal also. Fiiiiiiiiiiiiiiine. But it's a pain in the ass  because I don't have any really good DUT cables. We'll see what happens I guess.

It's only when you open the rabbit hole of mdB measurements you see these things.
Although not an RF expert it didn't take me long to work out what cabling was to be the most useful and which Cal kit (M or F) to get so to Cal at the DUT.

Your new to you SNA has some pretty neat features for Port extensions and such to enhance accuracy further and remove discontinuities', reflections and such.
Mine lives with member hendorog whom is away for a few days but I'll point him here for some comments to give you a better understanding of this VNA's capabilities.
Check out his Cal kits:
https://vnalab.net/

[rf-messkopf]

When you shift scaling to mdB these things start showing up. 

But they require an explanation if one wants to measure precisely in this area. Which is perfectly feasible with the appropriate means. 

100%
But a S21 is a through measurement so to do it accurately a through needs be part of the VNA Cal.

It wasn't immediately clear to me that he hadn't performed a full two-port calibration. 

I guess I need to use more cables/adapters so I can do the through cal also. Fiiiiiiiiiiiiiiine. But it's a pain in the ass  because I don't have any really good DUT cables. We'll see what happens I guess.

I just looked at the datasheet of the SNA5000A series VNA and they seem to be able to perform unknown-thru calibrations. So assuming your cable is reciprocal (which it is), you can use the cable under test itself to do the thru step of your calibration.

[KungFuJosh]

Mine lives with member hendorog whom is away for a few days but I'll point him here for some comments to give you a better understanding of this VNA's capabilities.
Check out his Cal kits:
https://vnalab.net/

I've looked at his site before a few times, but I've never seen N stuff there. I know you prefer SMA, but I want N and SMA cal kits. I'd like a Siglent F504TS to magically appear on my bench.

[KungFuJosh]

It wasn't immediately clear to me that he hadn't performed a full two-port calibration. 

I may have mentioned it once or twice. I usually know when I'm doing something stupid.

I just looked at the datasheet of the SNA5000A series VNA and they seem to be able to perform unknown-thru calibrations. So assuming your cable is reciprocal (which it is), you can use the cable under test itself to do the thru step of your calibration.

That's crazy. How does that work? Do I use the cable for the through, and then just run S21, or do I need to use one of those fancy features I haven't properly tried yet?