VNA 75 ohm - Page 1
Forum section: electronics / testgear

author: eurofox
author_rank: Unknown
timestamp: October 27, 2014, 08:35:44 am
content: I just scored an VNA with 75 ohm connections, it is an opportunity and bought it.I'm not involved in antennas, RF stuff ....My only interest is to test cables, spliters, components and home build filters that are not 50 ohm.I could find 75 ohm to 50 ohm Impedance Matching Pad, they got a loss of 5,7 dBhttp://www.pasternack.com/50-ohm-n-female-75-ohm-n-male-matching-pad-pe7083-p.aspxIn the case I use an Impedance Matching Pad should I use a 50 or 75 ohm calibration kit? The logic tell me to use a 50 ohm to be able to take into account the adapterSince I don't intend to use it on specific 50 ohm lines what is the motivation to move to 50 ohm instead of the 75 ohm?In my lab my standard for RF connectors is SMA, N 75 ohm connectors are quite exotic since they are not compatible with the N 50 ohmThanks in advance to share your advice and experienceeurofox

author: tggzzz
author_rank: Super Contributor
timestamp: October 27, 2014, 09:31:03 am
content: Quote from: eurofox on October 27, 2014, 08:35:44 amCaution: I haven't used a VNA in 20 years, so my memory is rusty.Connectors for N to SMA are commonplace, and I've used them as a "sacrificial connector" to protect the VNA from wear-and-tear when frequently re-connecting the circuit being measured.Whether or not to include the pad in the calibration will probably depend on how the calibration is done. If the VNA internally autocalibrates then itmightbe necessary to do it at 75ohms. Alternatively if the calibration is intended to exclude the effects of the cables then including the pads is reasonable. You'll have to check the manual, but I suspect the latter is the case. Either way you will want to do sanity checks with pads that are an approximation to calibrating the system.Either way, what would be the advantage of padding to 50ohm if the UUT isn't 50ohm? If you really want to pad, then pad to whatever you expect the circuit containing the UUT will be; that way you'll be testing the UUT in the "correct" context.Isn't a major point of a VNA to be able to measure the actual impedance and its deviation from 75ohms? If so, inserting a pad merely "distances" the VNA from the UUT, thereby making the measurement less accurate.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 27, 2014, 01:26:18 pm
content: I would suggest keeping the system 75 ohms until the UUT. If the UUT is any impedance, the VNA will show you what. It doesn't have to be exactly system impedance to see its behavior; though you won't be able to e.g. finely tune a bandpass filter with that mismatch.You could also make a un-un with a 4:5 turns ratio autoformer; careful construction on a nice beefy wideband ferrite bead should have little effect on the circuit, but do mind that it's there.Tim

author: G0HZU
author_rank: Super Contributor
timestamp: October 27, 2014, 04:28:20 pm
content: If it's a modern full 2 port VNA (S11 S21 S12 S22) with GPIB you could keep it all at 75R but I think you can create a "virtual 50R VNA" on a PC screen from the data taken from the 75R VNA.So for a 50R filter measured on the 75R VNA you would see the correct insertion loss and ripple (i.e correct S11 S21 S12 and S22) on the virtual VNA display on the PC as if it was a 50R VNA.But you would need to write a fair bit of code to create the PC display and do the sums to achieve all this. The screen refresh rate would depend on the limitations of the GPIB interface and how efficient your code was.

author: donmr
author_rank: Regular Contributor
timestamp: October 27, 2014, 08:25:00 pm
content: I recently learned (the hard way) that 75 ohm N connectors have a different sized center pin that 50 ohm ones.The 75 ohm pins are smaller. If you put a 75 plug into a 50 socket you get a intermittent connection. If you put a 50 plug into a 75 socket you damage it!Don

author: G0HZU
author_rank: Super Contributor
timestamp: October 27, 2014, 09:02:14 pm
content: That's partly why I'd want to keep it all at 75 ohm with 75 ohm connectors used everywhere and use a 75 ohm cal kit. If you wanted to test a commercial filter that had fixed 50 ohm N connectors on it then you would have to use something like the min loss pad which will have the correct N type connectors at each end to prevent damage.But I think the OP is looking to experiment with homebrew cables and splitters and filters. The filters and splitters could be built with 'safe' 75R connectors as long as the VNA has a port extension function to shift the reference plane to the PCB feed point of the homebrew (non 75R?) filter.The best advice I would usually offer would be to try to resell the 75R VNA and use the profit towards a 50R VNA. But it looks like eurofox specifically wants to use a 75R VNA.

author: eurofox
author_rank: Unknown
timestamp: October 27, 2014, 09:51:16 pm
content: QuoteI certainly don't want a 75 ohm version, my aim is to have a usual 50 ohm version, I bough it because it is good deal, HP8713B with options, cosmetic quite nice, screen very bright and no burns, up to 3Ghz and better than 100dB dynamic range.I mainly want it for testing "custom build stuff" that is not 50 nor 75 ohms and could of course use min loss pad to use on 50 ohm gear, it seems to be even possible to change it to 50 ohm according some messages on HP yahooI was hesitating to buy a USB VNA when I just got this option and jump on it.There is one problem with the instrument is the function key but I suppose that this could be fixed, some key are working.Or it is the contact behind the gummy key and I know how to repair it, I did already on other instruments or it is the decoding.I will try to find an old IBM PC keyboard that can be used as well and functions work on the PC keyboard.If it is a decoding issue it should not be hard to fix it.I put a wire between the 2 ports since I don't have 75 ohm N connectors, they are dangerously different than the 50 ohm versionsI check power, loss and other function and seems to work.

author: G0HZU
author_rank: Super Contributor
timestamp: October 27, 2014, 11:43:46 pm
content: I'm familiar with the HP8714B (got one here) but not the 8713B. I'm assuming it's basically very similar to my 8714 in terms of it being a T/R VNA so this means it isn't a full 2 port VNA. So I guess that rules out the remote/virtual 50R VNA possibility via GPIB.You could still do it in slow time by taking each s parameter measurement via GPIB (and swapping the fixure around as required) and then make up a full two port file in a PC but this is a slow manual process. I do this myself with my 8714 and it takes several minutes to create a full 2 port file with this old VNA. I also use precision inline attenuators for the S12 and S21 measurements to manage the mismatch uncertainty better.If you don't really want a 75R VNA then my advice would be to sell it on once you sort out any minor issues with the keys etc. There is limited demand for 75R VNAs but then again there is limited supply of them so you may get a good price for it if you are patient

author: eurofox
author_rank: Unknown
timestamp: October 28, 2014, 12:12:51 am
content: QuoteOn ebay US HP8713B go between 3-6k$ including 75 ohm version

author: eurofox
author_rank: Unknown
timestamp: October 28, 2014, 11:45:30 am
content: Keyboard is now 100% operationalInstrument fully working and tested on the limit of my VNA understanding and knowledge.Now I will update the firmware to the latest version, just issue with all my computers on 64 bitsI will have to install a virtual old OS needed to create a floppy with upgrade softwareVery nice toy, with this I can very easy measure loss, reflectionsUnfortunately I don't have 75 ohm connectors and I use a breadboard wire wire to connectNow I have option .... selling it for a good price and buy a USB VNA that will never go to 3Ghz or keep and buy some connector stuff and adapters to 50 ohm

author: eurofox
author_rank: Unknown
timestamp: October 28, 2014, 07:56:42 pm
content: Pictures of the unit

author: G0HZU
author_rank: Super Contributor
timestamp: October 28, 2014, 08:35:27 pm
content: Note: You may find that it's only a scalar network analyser and not a VNA. Some of this range are VNAs and some are scalar...

author: Howardlong
author_rank: Super Contributor
timestamp: October 28, 2014, 08:39:45 pm
content: The screen looks really crisp, unlike my 8753a which is getting rather tired now :-(On my 8753a I have a 75 ohm t/r test set, and I use a minimum loss pad on the test set port to keep everything at 50 ohm beyond that, including and calibrations. The MLP also serves as a connector saver, I think I've only unscrewed it three times in ten plus years, and as others have said, if you leave it like that it saves you from operator error plugging in a 50 ohm connector into a 75 ohm socket, you don't want to do that.On the 8753a with a t/r test set, the thru port is 50 ohm, so I only need one MLP.On the 8713b it looks like both ports are 75 ohm so you'd need two. Each MLP will degrade your sensitivity by 5 or 6dB, which isn't a big deal (it's not bothered me) for one but possibly two you may start to notice a bit more.Regarding saving connectors, I generally leave my VNA and test set plugged in with cables permanently unless on the rare occasion it needs moving, like to fix that pesky display!

author: Howardlong
author_rank: Super Contributor
timestamp: October 28, 2014, 08:40:46 pm
content: Quote from: G0HZU on October 28, 2014, 08:35:27 pmPretty sure the 8713 is scalar.

author: KJDS
author_rank: Super Contributor
timestamp: October 28, 2014, 10:55:01 pm
content: Quote from: Howardlong on October 28, 2014, 08:40:46 pmThe 8713 is a VNA that only allows the scalar information to be displayed on screen, but phase info is available via GPIB

author: G0HZU
author_rank: Super Contributor
timestamp: October 28, 2014, 11:51:00 pm
content: QuoteHow odd...I did wonder why/how it had the "fault location" listed as a possible option because this would require phase information. So could the analyser be uprated to display vector measurements with a firmware change? Another EEVblog hack?

author: eurofox
author_rank: Unknown
timestamp: October 29, 2014, 12:06:04 am
content: Thanks guy's for your helpFor me the best is selling it and buy an USB VNA, it will take less space and results will be similar.

author: Howardlong
author_rank: Super Contributor
timestamp: October 29, 2014, 11:30:12 am
content: Quote from: KJDS on October 28, 2014, 10:55:01 pmVery interesting, that actually adds quite a bit to its engineering value. Not sure a hack would ever become available for the firmware though, I for one haven't programmed a 68000 for 20 years.Is there host software that takes advantage of the GPIB data?I wonder if other HP scalar analysers do this too, particularly those going up a bit higher in frequency.For example, the Anritsu SiteMaster I have does return loss on its own screen, but if needs be I download the data via the RS232 and with its own PC software I can get Smith charts out of it quite easily.

author: ZS6GST
author_rank: Contributor
timestamp: August 05, 2016, 09:16:56 am
content: I have a 50 Ohm HP8713C. It has a facility allowing a 75 Ohm calibration procedure. This does not change the impedance of its ports but corrects for correct test results The only consideration is that there will be a ~ 6 dB reduction in dynamic range. There are however operating means that will in many cases result in achieving adequate dynamic range. If I was you I wud therefore keep the analyser73 de Pine, ZS6GST

author: andy2000
author_rank: Frequent Contributor
timestamp: August 06, 2016, 06:50:57 am
content: It's relatively simple to enable all options and turn it into an 8714 with phase display. I was able to turn my 8711C into an 8712C with all the options enabled.https://www.eevblog.com/forum/testgear/enable-those-extra-features-on-your-hp-8714c-network-analyzer/https://translate.google.com/translate?hl=en&sl=cs&u=http://minolta1.cninet.cz/HP8712C/&prev=search

author: willhelmx
author_rank: Newbie
timestamp: April 03, 2017, 09:51:11 pm
content: Hi guy´sDoes anyone of you have a 8711C, i would like to do some changes with itBest regards/ Andy

author: Bud
author_rank: Super Contributor
timestamp: April 03, 2017, 10:11:53 pm
content: @op see if you can find an option in the setup to change system impedance. You can use a 50 ohm cal kit then and work in 50 ohm space. The problem with connectors remain though.

author: ZS6GST
author_rank: Contributor
timestamp: November 26, 2017, 01:20:51 pm
content: Externally, this analyzer is a scalar machine. But there lurks a full vector analyzer inside it. A hack to change it into a full blown vector machine is floating around the internet.