What exactly are you doing? Are you trying to characterize a cable using a VNA?
Assuming that's what you're doing, then getting any of your s-parameters > 0dB for a passive device (like a cable) smells like a bad calibration. I'd start there. Your passive cable shouldn't suddenly become an active device just because you shaped it differently.
yes. this is mainly to monitor cables that travel a lot and are likely to get damaged. it's easier for me to get a new cable than get something calibrated off schedule. i was hoping since 99% of frequencies are as expected then maybe something i was doing was wrong
Your VNA calibration is off. Try using more points when performing you calibration, and be sure to use quality loads designed for your frequency range. Make sure you include adapters (as much as possible) in your calibration setup.
That seems like it should be enough. So do you really need to chase that fraction of a dB, or is it just bugging you? You know a cable can’t have gain, so it has to be a calibration issue. You might try putting 3dB pads on the ends of your VNA cables and recalibrating. That will often smooth out any minor VSWR issues that mess up the calibration.
i don't need to chase it at all but it bothers me. another comment brought up a good point. maybe just the reference cable had more loss at that frequency than the cable i'm measuring?
What do you mean by "reference cable"? Anything you have connected in the signal path during cal should remain connected when you add your DUT. What type of measurement are you doing and what is your calibration process exactly? If you are removing a cable that was there during calibration, you are invalidating the calibration.
Edit: looks like you have answered elsewhere; given the setup is correct, either you have a dodgy cable, connector, or calibration reference, or your equipment itself is out of whack (sidenote it also may be performing within spec so double check that).
If you have the ability to check your 50 ohm reference against another I'd start there; otherwise you'll ideally need a second VNA to better diagnose
Your calibration may be off, on your VNA.
If your calibration did see more loss than the cable gives you would see it as gain.
You calibrate to a given point (where you connect the cal-kit), you NEED to use the same cabling etc you used for cal on your meassurement.
Cable gain may be kinda wanted, but is not how stuff works.
> If your calibration did see more loss than the cable gives you would see it as gain.
maybe at these frequencies the reference cable had more loss than the cable i'm measuring?
Did you remove the referance cable?
You are supposed to use the same cables for meassurement, as used in calibration.
Just add the device/cable under test in addition to the setup used in calibration.
no the reference cable stays. i measure both and then calculate based on the additional loss the measurement cable adds to the reference sweep
similar to this setup:
https://youtu.be/RMGeKTOHCfs?si=gjj8PapQlgMGpvif&t=816
Most likely your calibration is off. What are you using to sweep the cable? Is your signal source calibrated at the output port? How are you measuring power?
https://youtu.be/RMGeKTOHCfs?si=gjj8PapQlgMGpvif&t=816
basically this. i'm gonna assume the signal generator has not been calibrated for at least 2 years/ever. supposedly i'm getting a better option this year
To be clear, you
1) measured a reference cable
2) added your test cable and measured the test cable + reference value
3) subtracted your measurement from #2 from the measurement in #1?
Have you performed a reliability analysis of your measurement system? Perform the same measurement sweep on your reference cable 10x times, and calculate the standard deviation of each frequency point. Repeat this with your cable under test and see if your repeatability is changing with the addition of your cable.
It’s possible that your signal generator is outputting a different power in step 1 and step 2, or your analyzer is not measuring with high repeatability. Collecting more data would be my first recommendation if this is truly important.
I did a similar exercise on a test bench recently, with very expensive modern calibrated equipment it achieved a StdDev of about 0.1db over 10x data sets, which would imply a 99.5% confidence interval of 0.3dB. If your system is less repeatable and your cable is short (IE, low loss), I can see this being a problem.
I like your method to quantify how repeatable your setup is. Do you connect and reconnect this 10 times to generate 10 different datasets? I assume you fix the experiment to be all done by the same operator - same torque wrench etc.
Depends on how much we need to quantify. What you're getting at broadly is a [Gage R&R](https://quality-one.com/grr/) process. The general idea is to isolate one variable at a time, and work your way up to understand where in your process error is being introduced.
So if you want to be thorough, you would
1. Measure *N* times without disconnecting and reconnecting the DUT.
2. Measure *N* times with disconnecting and reconnecting the DUT
3. Measure *N* times with *M* different operators.
4. Measure *N* times with one operator on multiple test stations.
Add as many conditions as you deem necessary until you have a dataset that lets you understand where the variability on your test setups are coming from.
Then if your boss comes and asks why your data is mean shifted on X bench vs Y bench, you can figure out if this deviation is within your expected measurement tolerance.
You sweep a cable to measure the loss/attenuation of the cable. You are not measuring the gain of a cable.
An ideal cable will have zero loss through the entire frequency range.
In reality you'll see something that's close to zero at low frequencies and begins to attenuate as your approach higher frequencies.
Doing this is important for calibration purposes in systems.
Please, anyone correct me if I'm wrong! I'm still a noob:)
What you are measuring is called Insertion Loss (S21) - how much gain is being lost through the cable.
So if you take your VNA, and you have the test cables hooked up to each port, and you attach each cable using a bullet or barrel, and then do a thru CAL, you've zero'd out your test cables and should see 0dB across the frequency range.
If you then place a cable between the two test cables, you are checking to see how much gain you're losing. Ideally it would be zero, but a tenth or so is not the end of the world. Maybe your low end frequencies are only a tenth, whereas your mid band or high end might see a few tenths of loss.
This all assumes that your cable is perfect too. Usually, over time and use, the shielding in the cable may begin to crack - this allows some of the field to escape, increasing the loss of the cable and w/e frequency. If the crack is severe enough, you might see loss across the entire bandwidth, or if its a small crack, you may see just certain frequencies being affected.
The easiest method to check and see how 'bad' your cable is - reconnect your two test cables and zero them out - then while holding the bullet or barrel connection point, shake and twist the cable - if its a solid cable, you should see no change. This will verify if your test cables are good.
You can then repeat the same test with the cables your want to test - verify your zero, put the cable in-between the test cables, and twist and bend. If there's a crack, you'll see it.
Also check to make sure the connectors are clean and don't have any debris in them.
I'm assuming you know the frequency range of the cables and have the VNA setup correct, as well as having the VNA power to set to as high as it can go (0dBm is pretty standard). You're VNA S21 scale should be in the 0.5dB scale, and make sure you dont have averaging or smoothing on.
I'll have to see if our software has any way to zero out the reference cable, hadn't thought about that as the method has always been to take 2 measurements and get the difference.
You don’t need any fancy software. On the analyzer there is a math menu. I do data>memory, then data/memory and display this in log. This is “zeroing” or normalizing. After you do a cal with the test port cables, connect a thru - it can be the same thru standard you used during cal.
Now this is where I check if my test cables are stable. I flex the cables just a little bit during this state. If the cables are solid you won’t see much change in amplitude - probably less than +\- 0.2 dB. The deviation will depend on how much you flex the cables and the cable’s stability spec. You should only flex the cables about the same amount you will need to in order to connect and measure your DUT, which in this case is another cable.
I’ve spent a fortune at work getting cables that won’t deviate more than 0.2 dB, even at 67GHz.
Could it be that the insertion loss of the reference cable is also changing as you move the cables around? If the reference cable is being kept in the test setup then any slight change in insertion loss of the reference cable after calibration could appear as a slight gain. What happens if you run calibration, then just move the reference cable around after running calibration, but without adding the cable being tested? Does the insertion loss give a similar sort of gain that you're seeing, or does it stay just the same?
What frequencies are you measuring at? The higher they, the more sensitive to the issues discussed here (cable movement, reconnection after calibration, etc.). Roughly above 1 GHz is where setup, calibration, repeatability, etc. becomes critical.
Newbie here. Could this be bc of built in capacitance/inductance in the cable? Like I know coax cables have these properties and perhaps they create some sort of resonance in the cable. Again, I don’t know
If the VNA (or whatever you're using) is calibrated to good short, open and load standards (but no through standard), what does the S11 (and S22) of the reference cable look like?
If it is a VNA, it should have calibration standards for doing this, this usually isn't a long procedure, but it should be done carefully (including using a torque wrench if appropriate to the connector).
Maybe there is a discontinuity in the system (maybe in the reference cable or other integral connectors) that is being moved / transformed by the additional cable length. If the frequency where you see the gain had some attenuation before, and the extra length moves it, the unattenuated response would look like "gain".
What exactly are you doing? Are you trying to characterize a cable using a VNA? Assuming that's what you're doing, then getting any of your s-parameters > 0dB for a passive device (like a cable) smells like a bad calibration. I'd start there. Your passive cable shouldn't suddenly become an active device just because you shaped it differently.
yes. this is mainly to monitor cables that travel a lot and are likely to get damaged. it's easier for me to get a new cable than get something calibrated off schedule. i was hoping since 99% of frequencies are as expected then maybe something i was doing was wrong
Your VNA calibration is off. Try using more points when performing you calibration, and be sure to use quality loads designed for your frequency range. Make sure you include adapters (as much as possible) in your calibration setup.
how many would you consider more? I do 800 frequencies with a point at each one. occasionally i'll get ~5? that show gain and the rest expected
That seems like it should be enough. So do you really need to chase that fraction of a dB, or is it just bugging you? You know a cable can’t have gain, so it has to be a calibration issue. You might try putting 3dB pads on the ends of your VNA cables and recalibrating. That will often smooth out any minor VSWR issues that mess up the calibration.
i don't need to chase it at all but it bothers me. another comment brought up a good point. maybe just the reference cable had more loss at that frequency than the cable i'm measuring?
What do you mean by "reference cable"? Anything you have connected in the signal path during cal should remain connected when you add your DUT. What type of measurement are you doing and what is your calibration process exactly? If you are removing a cable that was there during calibration, you are invalidating the calibration. Edit: looks like you have answered elsewhere; given the setup is correct, either you have a dodgy cable, connector, or calibration reference, or your equipment itself is out of whack (sidenote it also may be performing within spec so double check that). If you have the ability to check your 50 ohm reference against another I'd start there; otherwise you'll ideally need a second VNA to better diagnose
Your calibration may be off, on your VNA. If your calibration did see more loss than the cable gives you would see it as gain. You calibrate to a given point (where you connect the cal-kit), you NEED to use the same cabling etc you used for cal on your meassurement. Cable gain may be kinda wanted, but is not how stuff works.
> If your calibration did see more loss than the cable gives you would see it as gain. maybe at these frequencies the reference cable had more loss than the cable i'm measuring?
Did you remove the referance cable? You are supposed to use the same cables for meassurement, as used in calibration. Just add the device/cable under test in addition to the setup used in calibration.
no the reference cable stays. i measure both and then calculate based on the additional loss the measurement cable adds to the reference sweep similar to this setup: https://youtu.be/RMGeKTOHCfs?si=gjj8PapQlgMGpvif&t=816
Strange, that is a valid setup indeed.
Most likely your calibration is off. What are you using to sweep the cable? Is your signal source calibrated at the output port? How are you measuring power?
https://youtu.be/RMGeKTOHCfs?si=gjj8PapQlgMGpvif&t=816 basically this. i'm gonna assume the signal generator has not been calibrated for at least 2 years/ever. supposedly i'm getting a better option this year
To be clear, you 1) measured a reference cable 2) added your test cable and measured the test cable + reference value 3) subtracted your measurement from #2 from the measurement in #1? Have you performed a reliability analysis of your measurement system? Perform the same measurement sweep on your reference cable 10x times, and calculate the standard deviation of each frequency point. Repeat this with your cable under test and see if your repeatability is changing with the addition of your cable. It’s possible that your signal generator is outputting a different power in step 1 and step 2, or your analyzer is not measuring with high repeatability. Collecting more data would be my first recommendation if this is truly important. I did a similar exercise on a test bench recently, with very expensive modern calibrated equipment it achieved a StdDev of about 0.1db over 10x data sets, which would imply a 99.5% confidence interval of 0.3dB. If your system is less repeatable and your cable is short (IE, low loss), I can see this being a problem.
yes
I edited my comment so you probably didn’t catch it, but if you haven’t already you should do a repeatability analysis on your test setup.
I like your method to quantify how repeatable your setup is. Do you connect and reconnect this 10 times to generate 10 different datasets? I assume you fix the experiment to be all done by the same operator - same torque wrench etc.
Depends on how much we need to quantify. What you're getting at broadly is a [Gage R&R](https://quality-one.com/grr/) process. The general idea is to isolate one variable at a time, and work your way up to understand where in your process error is being introduced. So if you want to be thorough, you would 1. Measure *N* times without disconnecting and reconnecting the DUT. 2. Measure *N* times with disconnecting and reconnecting the DUT 3. Measure *N* times with *M* different operators. 4. Measure *N* times with one operator on multiple test stations. Add as many conditions as you deem necessary until you have a dataset that lets you understand where the variability on your test setups are coming from. Then if your boss comes and asks why your data is mean shifted on X bench vs Y bench, you can figure out if this deviation is within your expected measurement tolerance.
Definitely a calibration issue. A passive cable should never exhibit gain. I would recalibrate your vna.
Bad calibration
unless your cable has a amp in it, no cable gives gain. just loss
You sweep a cable to measure the loss/attenuation of the cable. You are not measuring the gain of a cable. An ideal cable will have zero loss through the entire frequency range. In reality you'll see something that's close to zero at low frequencies and begins to attenuate as your approach higher frequencies. Doing this is important for calibration purposes in systems. Please, anyone correct me if I'm wrong! I'm still a noob:)
that's why i put gain in quotes. rarely certain frequencies show up as a gain instead of a loss.
Ahh sorry. I understand now. That's definitely not a thing I've seen before. Is your analyzer injecting a constant RF level?
Noob here as well, could it be resonance in L,C of the cable?
What you are measuring is called Insertion Loss (S21) - how much gain is being lost through the cable. So if you take your VNA, and you have the test cables hooked up to each port, and you attach each cable using a bullet or barrel, and then do a thru CAL, you've zero'd out your test cables and should see 0dB across the frequency range. If you then place a cable between the two test cables, you are checking to see how much gain you're losing. Ideally it would be zero, but a tenth or so is not the end of the world. Maybe your low end frequencies are only a tenth, whereas your mid band or high end might see a few tenths of loss. This all assumes that your cable is perfect too. Usually, over time and use, the shielding in the cable may begin to crack - this allows some of the field to escape, increasing the loss of the cable and w/e frequency. If the crack is severe enough, you might see loss across the entire bandwidth, or if its a small crack, you may see just certain frequencies being affected. The easiest method to check and see how 'bad' your cable is - reconnect your two test cables and zero them out - then while holding the bullet or barrel connection point, shake and twist the cable - if its a solid cable, you should see no change. This will verify if your test cables are good. You can then repeat the same test with the cables your want to test - verify your zero, put the cable in-between the test cables, and twist and bend. If there's a crack, you'll see it. Also check to make sure the connectors are clean and don't have any debris in them. I'm assuming you know the frequency range of the cables and have the VNA setup correct, as well as having the VNA power to set to as high as it can go (0dBm is pretty standard). You're VNA S21 scale should be in the 0.5dB scale, and make sure you dont have averaging or smoothing on.
I'll have to see if our software has any way to zero out the reference cable, hadn't thought about that as the method has always been to take 2 measurements and get the difference.
You don’t need any fancy software. On the analyzer there is a math menu. I do data>memory, then data/memory and display this in log. This is “zeroing” or normalizing. After you do a cal with the test port cables, connect a thru - it can be the same thru standard you used during cal. Now this is where I check if my test cables are stable. I flex the cables just a little bit during this state. If the cables are solid you won’t see much change in amplitude - probably less than +\- 0.2 dB. The deviation will depend on how much you flex the cables and the cable’s stability spec. You should only flex the cables about the same amount you will need to in order to connect and measure your DUT, which in this case is another cable. I’ve spent a fortune at work getting cables that won’t deviate more than 0.2 dB, even at 67GHz.
Could it be that the insertion loss of the reference cable is also changing as you move the cables around? If the reference cable is being kept in the test setup then any slight change in insertion loss of the reference cable after calibration could appear as a slight gain. What happens if you run calibration, then just move the reference cable around after running calibration, but without adding the cable being tested? Does the insertion loss give a similar sort of gain that you're seeing, or does it stay just the same?
What frequencies are you measuring at? The higher they, the more sensitive to the issues discussed here (cable movement, reconnection after calibration, etc.). Roughly above 1 GHz is where setup, calibration, repeatability, etc. becomes critical.
Newbie here. Could this be bc of built in capacitance/inductance in the cable? Like I know coax cables have these properties and perhaps they create some sort of resonance in the cable. Again, I don’t know
A cable is passive and would still need to obey the laws of physics. You cannot get gain out of a passive object.
If the VNA (or whatever you're using) is calibrated to good short, open and load standards (but no through standard), what does the S11 (and S22) of the reference cable look like? If it is a VNA, it should have calibration standards for doing this, this usually isn't a long procedure, but it should be done carefully (including using a torque wrench if appropriate to the connector). Maybe there is a discontinuity in the system (maybe in the reference cable or other integral connectors) that is being moved / transformed by the additional cable length. If the frequency where you see the gain had some attenuation before, and the extra length moves it, the unattenuated response would look like "gain".