Electronics > RF, Microwave, Ham Radio
VNA for cable characterization
Pinörkel:
Hello,
I am new to RF measurements and would like to perform some high precision cable impedance measurements to characterize some short(0.5m to 2m) 50Ω BNC cables of different types in the MHz and low GHz range.
I read that cable impedance measurements of the required accuracy can be done with a TDR sampler like the Tektronix 7S12 with a respective pulse generator and sampling head. A setup like this should be able to measure the impedance along the cable with an accuracy of +/- 0.1Ω or +/- 1 mρ and a time resolution in the sub nano second range. It should also be possible to do frequency dependent impedance measurements with a suitable milli-ohms-meter like the R&S URV5 with a suitable insertion unit. Unfortunately, both vintage setups are extremely difficult to acquire and I do not have the required bench space and operating knowledge for a suitable sampling oscilloscope. So, I tried to dig into the topic in order to maybe find some kind of measurement setup that could somehow fulfill my needs and stumbled onto vector network analyzers.
It seems that high quality VNAs, like most kind of test equipment, can get ridiculously expensive, but there are also affordable solutions, like the liteVNA 64, which might or might not be sufficient, for what I want to do. However, until now I could not figure out what measurement precision can be expected from a device like the liteVNA 64 with respect to cable impedance and TDR measurements. This is where I hope I can get some answers here.
I already found the very feature packed Solver64 software from forum member joeqsmith, who has put tremendous amounts of work into providing countless features like TDR measurements for the inexpensive VNAs. I read that the performance of these devices depends heavily on the usage of high quality calibration standards, which might easily cost more than ten times the price of a budget VNA. So my question would be: does anyone here know what TDR measurement performance in terms of impedance deviation can be achieved with a liteVNA 64 or similar device, combined with some affordable calibrations standards? Or am I mistaken and a VNA is not the right tool for this task? Any help in this matter would be appreciated.
jwet:
I saw that you got no responses so I'll give some guidance to get a conversation started perhaps. I'm not a renowned expert at all but there are some on this board.
I think a standard VNA, cheap or fancy is not a great high precision tool for your application. The problem is that since VNA's depend on reflections and the reflections will be very small (loads close to 50 ohms) , relative errors will be large. The dynamic range of a decent VNA is 80+ db, cheap VNA's are about 70 dB. Flatness of a good VNA is .05 db which will turn into fraction of an ohm.
There are RF impedance measurement devices that use an V/I technique. One is the HP4396B that I'm familiar with. It looks and acts like a VNA but in impedance measurement mode, it uses the I/V technique. HP has an impedance measurement handbook that is pretty good and goes through their solutions and limits of accuracy. Good standards are expensive and will be required to get accurate absolute results.
Good Luck.
Pinörkel:
Thank you for the kind and helpful reply. So, it might be as I already feared: my best bet is to lurk on ebay and wait a few years for a chance to get some vintage TDR equipment. Unfortunately, this stuff is much harder to get here ein Germany than in the US. In any case, I will have a look at the HP impedance handbook you mentioned. Until now my main literature source about VNAs was The VNA Applications Handbook from Bonaguide and Jarvis.
--- Quote from: jwet on July 06, 2024, 05:09:59 pm ---I saw that you got no responses so I'll give some guidance to get a conversation started perhaps. I'm not a renowned expert at all but there are some on this board.
I think a standard VNA, cheap or fancy is not a great high precision tool for your application. The problem is that since VNA's depend on reflections and the reflections will be very small (loads close to 50 ohms) , relative errors will be large. The dynamic range of a decent VNA is 80+ db, cheap VNA's are about 70 dB. Flatness of a good VNA is .05 db which will turn into fraction of an ohm.
There are RF impedance measurement devices that use an V/I technique. One is the HP4396B that I'm familiar with. It looks and acts like a VNA but in impedance measurement mode, it uses the I/V technique. HP has an impedance measurement handbook that is pretty good and goes through their solutions and limits of accuracy. Good standards are expensive and will be required to get accurate absolute results.
Good Luck.
--- End quote ---
joeqsmith:
You never state your requirements, what your goals are, or the reasons behind them. Reading your post, I get the impression that you feel that 50 vs 50.1 is a big deal for your application, like the quote below taken from the linked article. I want to understand why?
--- Quote ---Perhaps now maybe the “50 Ohm” cable idea makes some sense and you are now a “50 ohm” systems zealot. You now strive for “perfect 50 ohms” in all your cabling, connections and devices. You have become so unreasonable that you insist that all systems be EXACTLY 50 ohms. Well now you are in trouble.
--- End quote ---
https://www.dsinstruments.com/support/understanding-characteristic-impedance-vswr-reflection-coefficient/
Pinörkel:
Thank you for the link and for pointing out that I did not provide enough information for the question to be answered. I tried not to overload the question with irrelevant information, to prevent readers from thinking: Too long didn't read. Thankfully, the situation is not as bad as in your quote. I have mainly two goals with this:
The first and less critical goal is that I want to be able to verify the quality of BNC cables I buy or make. In the past, I have bought cables from sources I thought I could trust, only to discover later, that the connectors were of terrible electrical quality or the cables had abysmal frequency dependent return losses. Also some cables may go bad with time unnoticed, e.g. foam dielectric coax cables which have been bend too much. This made me believe several times that certain measurement gear was bad, when in fact it was only a matter of bad cables. So being able to ballpark my cables would be nice to find out, which cables to throw away and which not to use for precision measurements.
The second goal is that I have repaired some vintage calibration gear. For a correct calibration of the gear and the devices that will be calibrated with it later on, there is the requirement of using a precision 50Ω +/- 1% cable with a length of 36 inches. These cables are then paired with the calibration device in order to later be able to make precise statements about the calibration accuracy of devices calibrated with this combination. Unfortunately, the original cables used for this e.g. the Tektronix 012-0482-00 are near to being unobtanium nowadays and I know from people that worked with these cables on a daily basis back in the days, that they had a tendency to go out of spec after a while. So, original vintage cables cannot be trusted without testing and a way is needed to validate them.
Due to the bad availability of original cables I tried to get information on the precise specifications of the cables to see, if maybe there was a way of making those using parts that are available today. According to my current findings, the validation of a cable should at least include verifying that the impedance stays within 50Ω +/- 1% below 1GHz and that the VSWR below 3GHz is less than 1.3. A few years ago, Dennis Tillman did an in-depth TDR evaluation of an original cable (link) to reverse engineer its specifications using a Tektronix 7S12. This evaluation suggests that an impedance measurement accuracy of at least +/- 0.1Ω or +/- 1 mρ is possible and also allows a precise look at what is going on in the used BNC plugs (given a high quality 50Ω termination is used). So, 0.1Ω will certainly be no big deal for my application as long as the 1% spec is met, but I had the impression that this resolution may be the minimum required to see IF the 1% spec is met.
Based on this, I managed to identify several combinations of available parts(BNC plugs and cables of different types) that could be suitable for creating respective cables. However, the the quality of the cables not only depends on the parts, but also on the manufacturing process. The latter has some potential to be screwed up by me. So, just buying several combinations of parts and making different types of cables would be useless without a way to measure and compare the cables. At the moment, I do not have sufficient gear or knowledge to reliably take the required measurements. So, I am now trying to find out what I need to know to take the required measurements and if there is a way to acquire the respective measurement gear. After all, just owning measurement gear usually does not mean that you know how to use it correctly. So, time for some research. At the end, if possible at all, it would be nice if I could come up with a recipe, that can reliably be used by others to create calibration cables of this type, optimally for a reasonable price.
--- Quote from: joeqsmith on July 07, 2024, 03:26:42 pm ---You never state your requirements, what your goals are, or the reasons behind them. Reading your post, I get the impression that you feel that 50 vs 50.1 is a big deal for your application, like the quote below taken from the linked article. I want to understand why?
--- Quote ---Perhaps now maybe the “50 Ohm” cable idea makes some sense and you are now a “50 ohm” systems zealot. You now strive for “perfect 50 ohms” in all your cabling, connections and devices. You have become so unreasonable that you insist that all systems be EXACTLY 50 ohms. Well now you are in trouble.
--- End quote ---
https://www.dsinstruments.com/support/understanding-characteristic-impedance-vswr-reflection-coefficient/
--- End quote ---
Navigation
[0] Message Index
[#] Next page
Go to full version