impedance analyzer extension cable max lenght?  - Page 1
Forum section: electronics / rf-microwave

author: coppercone2
author_rank: Super Contributor
timestamp: October 09, 2023, 01:39:59 am
content: So previously my 4191A was having calibration problems with a longer BNC cable (with adapters and standards) but since I did some of the maintence on it with the trimmers, it calibrated OK on a 9 foot cable.If I were to say put a 50 foot cable on it, and calibrate it on the ends, would that work?I would just try it but I don't have such a long cable handy.Kinda wondering what the limit is on this sort of thing? I read people saying something about if the calibration plane is extended too far it complains.Also curious for VNA. What starts to happen? I know with a VNA you can do much more, because they have long cables for em. Assuming you can get the power up.. is there anything else that starts to happen? Say if you went really long.

author: E Kafeman
author_rank: Regular Contributor
timestamp: October 09, 2023, 08:00:19 am
content: Everyone doing S21 measurement in an anechoic chamber need to run full chamber length. In my case, one cable is running outside chamber to a horn antenna mounted in rear wall and one cable to turntable inside chamber.In total 2x10 meters or somewhat longer. At antenna in rear wall is mounted a polarization shifter and a LNA to the horn antenna.The LNA is adjusted adding minor gain at 1 GHz and increasing gain up to 10 GHz to compensate for cable loss, making loss relative linear over usable frequency range so that dynamic range not is reduced.It had been better not having any cable losses but this is a problem most RF labs have.Have a similar problem for my outdoor measurement range but here is cable length 12 and 2 meters, in total 14 meters and no LNA is used.Measuring S11 using longer cables is not a problem as long as system dynamic range is good enough. SOL calibration do not cause any problems.If signal is attenuated to much will it be visible noise for weaker signals or signal will be lost below system noise level.It is reflection signal that is weakest and first will be lost and by that reduce usable measurement amplitude range and by that for example limit how bad VSWR ratio that is possible to measure.

author: coppercone2
author_rank: Super Contributor
timestamp: October 09, 2023, 08:32:40 am
content: hmm so you need a leveling amp?how about for the impedance analyzer?

author: jonpaul
author_rank: Super Contributor
timestamp: October 09, 2023, 09:45:18 am
content: Most LCR/Z analyz WILL NOT work with long cables.The HP 4175A, 4277, etc have a 0.5 or 1 m legnth selection.Use short cables or genuine HP/Yokogawa test leads, do a full OC SC Zo CAlj

author: ahbushnell
author_rank: Frequent Contributor
timestamp: October 09, 2023, 06:06:34 pm
content: What frequency range are you interested in?

author: coppercone2
author_rank: Super Contributor
timestamp: October 09, 2023, 09:01:39 pm
content: the ranges are 500mhz to 22ghz for the vna and 1-1000 megahertz for the IA.The 4191A did calibrate with something like 9 foot cable. But I need to make some different loads to compare with and without the cable

author: coppercone2
author_rank: Super Contributor
timestamp: October 10, 2023, 01:02:03 am
content: Quote from: jonpaul on October 09, 2023, 09:45:18 amwhat measurement load do you recommend for highlighting the error brought on by cables?I want to just solder a R/C/L into the end of a BNC connector and connect it to a cable that was previously calibrated with a 50 ohm load and compare it to the same load connected only to re calibrated instrument without long cable.Does any particular load type error show up more then others?

author: coppercone2
author_rank: Super Contributor
timestamp: October 10, 2023, 03:42:13 am
content: i.e. for clarity something like soldering 35 ohm resistor in parallel with ceramic capacitor, or a series RC circuit, or a LC, LRC, etc.I want to see how error develops on a long cable. Lots of possibilities so I am wondering if there is theory to back up which one will show up good.

author: Bud
author_rank: Super Contributor
timestamp: October 10, 2023, 04:24:59 am
content: With long cables and/or higher frequencies the cable's phase instability will have effect on readings. You bend the cable or simply move it - that will cause phase error. There is a reason why professional VNA test cables are short, thick and hugely expensive. It can be noticed in many youtube videos the engineers have DUT cables taped to the bench or clamped in a vise to prevent movement. There is also phase drift with temperature.

author: jonpaul
author_rank: Super Contributor
timestamp: October 10, 2023, 04:25:19 am
content: Cu which analyzer? Settings, freq etc? cable eg RG58/U, RG174?A 50 Ohm Zo cable has only atten IF Zgen=Zlod=Zo.At low F eg 1..10 kHz effect is lump of cable eg capacitance (see cat spec for particular cable) ~ 50 pF/ft + inductance~ 2 nH /ft.At 100k..10 MHz, use transmission line theory and Smith chart to determine Z cable + load over freq.We use only the mfg (HP/Yokogawa) leads and test fixtures. Legnth 0.5 or 1 M.See HP book on Impedance Measurements.Bon ChanceJon

author: coppercone2
author_rank: Super Contributor
timestamp: October 10, 2023, 04:33:02 am
content: its rg400 cable with the 4191AAny frequency settings. I just wanna see how it degrades. I have a few coax of different length I made up with RG400.I have a BNC open (rubber gasket), closed (gold shorting pin) and 50ohm load. The idea is to perform cal at just the adapter, then test the unknown at some suspected frequency, then put on the cable, calibrate at end of cable, and attach unknown again to see.I am wondering if different impedance parameters will display greater error with cables. I want to see the deviation from the unknown DUT tested right at the calibrated port vs the DUT tested at a supposedly calibrated end of a long cable.the unknown DUT would just be a SMD part soldered into a coaxial connector. I wonder what DUT would show the biggest error to highlight the importance of cable quality/length.

author: jonpaul
author_rank: Super Contributor
timestamp: October 10, 2023, 04:37:13 am
content: RG400 costly , for ADS-B, unneeded, and stiff, we have no experience. See mfg specs.All our stock is RG/58/U, RG59/U RG174/URG174/U is very thin and flexible and higher C/ft so better for your tests.Jon

author: coppercone2
author_rank: Super Contributor
timestamp: October 10, 2023, 04:38:52 am
content: Well I already have it. I guess I am just gonna try some random stuff like a 10pF capacitor or something..Like If I measure at the port and get 10pF at 250MHz, then measure at the end of a cable (recalibrate) and get 15pF, then I would know that there is 50% error because of the cable even thought it calibrated with the cable. Like the limitations of the machine.It is irritating to me that you CAN calibrate the machine at the end of a cable, but then there will be error, even though it shows that your 50 ohm load is reading 50 ohms.

author: coppercone2
author_rank: Super Contributor
timestamp: October 10, 2023, 05:30:45 am
content: ok I did a little experiment I soldered a small ceramic disk leaded capacitor (y5u/bad/cheap ceramic) in between the leads of a BNC isolated chassis mount connector.When I did the cal and measured it at the instrument port, it read something like 16pF and 1.6ohm ESR at 250MHz. Goes down a little when you decrease frequency.I attached a 9 or maybe even 12 foot BNC rg400 cable and did the calibration at the end of the cable with the same standards. When the 'unknown DUT' was connected at 250MHz is was reading inductance (negative)... where without the cable it was 16 pFIt was reading gibberish even at 25MHz (10 times lower frequency, I think it read 50pF).So it does appear that this instrument is really unhappy with cables, even at low frequencies! the calibration will mislead youso I confirmed, it really does not work! it looks like it might work because the calibration goes through and it reads the standard correctly, but it does not work! the output is total baloneyLater I might try a 6 foot and a 3 foot cable RG400 with the same experiment.

author: coppercone2
author_rank: Super Contributor
timestamp: October 10, 2023, 05:45:05 am
content: Quote from: Bud on October 10, 2023, 04:24:59 amwell in the case of a IA it appears to be so god damn sensitive that its unusable at basically any frequency above dc. On a VNA you make it sound 'manageable' but on the IA its just not compatible.I also have the VNA with the TR sets (0.01MHz - 300MHz for VNA#1 and the other one is 500MHz - 18GHz). I might try those with cables later on the same capacitor DUT to see if fares any better then the IA.I actually have rather long VNA cables, I think 20 feet. I bought them but never really used it before. Forgot what brand. I guess I expected the IA to work like the VNA seemed to, but it did not. But I don't know if I tried an unknown DUT on the VNA in a comparitive test to try to determine error. I figured it was not an issue since the cables exist. Because with VNA cables I just was informed that you need to keep them still between calibration and tests, but with the IA it just seems to do some completely whack stuff.