Constructing test jig for an impedance analyser - Page 1
Forum section: electronics / projects

author: AndyC_772
author_rank: Super Contributor
timestamp: February 08, 2015, 09:43:11 pm
content: Hi all,My latest score off Ebay is an HP 4395A spectrum / network / impedance analyser. Like a lot of s/h kit, it doesn't come with accessories.I need it to perform network and impedance analysis around the low (single digit) MHz region, so I'm not too bothered about accuracy up at the hundreds of MHz. What I do want, though, is good repeatability at the bottom end.The equivalent circuit of the impedance analysis jig is given in the instrument's manual, and it's a predictably simple (probably over-simplified, if truth be told) arrangement of resistors. Since I don't need the high frequency performance, and genuine HP or Agilent jigs are stunningly expensive even by their standards, I plan to build my own.Does anyone have experience of doing this that they'd be able to share please? I'm looking for practical advice in terms of what types of connectors you've used, construction methods, and any tips you can share or problems you've encountered.I'm thinking to build the circuit into a die cast box with panel mounted BNC connectors, and to use N-to-BNC cables or adapters to connect it to the analyser. Inside the box, ideally I'd be using a solid copper clad board, though as I don't have an etch tank or CNC mill it could get ugly. Or I could just get a PCB professionally fabricated, but I need it quickly. Or maybe I'm over-engineering this, and some neat soldering of coax cables is all that's required.All tips gratefully received, thanks.

author: joeqsmith
author_rank: Super Contributor
timestamp: February 09, 2015, 12:56:51 am
content: Nice score!What parts did you get with it?What exactly are you wanting to measure with it?

author: G0HZU
author_rank: Super Contributor
timestamp: February 09, 2015, 05:12:01 pm
content: You may have already seen this online pdf here:http://www.hparchive.com/Manuals/HP-35676-SCHEMATIC.pdfIf not then it could be useful as it shows a schematic and internal layout...This is the classic resistive signal divider circuit used in many old school HP analysers. At work we used to have half a dozen HP4396A/B units kicking around but they fell out of favour some time ago as they only go to 1800MHz. I think most of them are long gone/scrapped. I think we may still have an old HP4195A somewhere. But all the couplers/dividers for these are kept in a box on a shelf. I had a look in it and found loads of 87512A versions (presumably these were used with the HP4396A/B) and also a 35676-66301 and a 41952A.Which one are you looking to copy? I could do some measurements on them if that helps? I can't take them apart though as they are sealed units and I wouldn't want to risk damaging them as they are company property.

author: AndyC_772
author_rank: Super Contributor
timestamp: February 09, 2015, 10:32:23 pm
content: Hi,That's a handy link, thanks.My most immediate requirement is something that can be used as a functional substitute for a 43961A impedance test jig.I like the "only up to 1800 MHz" comment! This unit only goes up to 500 MHz, and that's about two orders of magnitude faster than I need. Hence, not too worried about whether or not I get a box with an HP sticker on the front!

author: G0HZU
author_rank: Super Contributor
timestamp: February 10, 2015, 01:17:44 am
content: It's years since I've used a HP4396A/B but I think all of ours came with the external HP87512A T/R test box + cal kit.This allowed a basic SOL calibration and so impedance could be measured reasonably accurately over a reasonable range of impedances with this basic test setup. it wouldn't be very accurate at very high impedance or very low impedance though. I can't be certain but I think it contains a resistor network as per the 35676-66301 unit in the previous link. But I'd imagine that the layout and components would be more suited to operation up to 1800MHz on the 87512A. I've not seen inside one though...This would be much easier to replicate than the 43961A. I thought you had to buy the 43961A from the factory with an option (fitted to the 4395A) to enable its functionality? Does your analyser already have the option fitted that supports the 43961A?

author: AndyC_772
author_rank: Super Contributor
timestamp: February 10, 2015, 03:07:08 pm
content: Yes, the 4395A has option 010 fitted which enables the impedance analysis feature. The 43961A is the specified fixture to use in this case.

author: AndyC_772
author_rank: Super Contributor
timestamp: February 12, 2015, 10:07:35 pm
content: My box of BNC connectors, resistors and stuff arrived today, so I went to start putting together something with the equivalent circuit that's in the manual... and...The analyser works by measuring the V and I characteristics simultaneously to calculate the impedance of the device under test. No problem, except that the current measuring input on the analyser is 50R to ground (it's one of three identical inputs on the analyser), and the equivalent circuit shows that same 50R connected between two points neither of which is ground. So I can't just build the circuit out of wires and resistors after all.I may have used a naughty word at this point!Another HP document from the time elaborates on the technique used by the 4395A, and shows a current transformer as part of the circuit. I was hoping to avoid this as I don't need the ability to apply a DC bias to the DUT, but it looks like I may have no choice.There's a very thorough article on RF current transformers here:http://www.g3ynh.info/zdocs/bridges/Xformers/part_1.html... which is over my head in many respects, but does give some recommended parts to use and a clear photo which I can work from. I don't know the correct turns ratio, but that's probably something I can infer with a little trial and error.

author: free_electron
author_rank: Super Contributor
timestamp: February 13, 2015, 12:16:34 am
content: Quote from: AndyC_772 on February 09, 2015, 10:32:23 pmthe service manual for that jig does exist. it even shows internal construction , but it is more of a meccano than electronics....you don;t need to worry too much about the construction as the calibration routines for open short and 50 happen at the plane of the sexless connector.i would ditch the apc-7 altogether. yes it has superior performance but only when combine with a 4396 ( which is identical to the 95 but goes up to 1.8 GHz )i used a 4395 and the 43961 all the time to do impedance plots of capacitors and power planes in a board.search for the pdf fie. it does exist. if you can't find it let me know. i have it somewhere.the advantage of the 4395 is that it can has internal bridges so it can do forward reverse and some clever math to shift the plane all the way up into the 961athere really is not a lot in that box.

author: AndyC_772
author_rank: Super Contributor
timestamp: February 13, 2015, 08:48:43 am
content: I was excited when I found the service manual, but as you say, it's really more about how the thing physically fits together than anything else. There's no detail of the electrical circuitry, just a drawing which shows what I presume to be various rigid coax lines disappearing into hidden places that could contain anything.The only electrical measurement described is the resistance at the DC input, but 2k +/- 200R seems awfully vague for such an otherwise precisely made bit of kit... and it's not consistent with any of the "simplified equivalent" circuits I've found for the box!I found this:http://literature.cdn.keysight.com/litweb/pdf/04396-90046.pdf... which describes the impedance measurement feature of the 4396A. Figure 5-2 (page 5-4), but it shows the 'A' input (current measurement) as though it were floating, and that seems unlikely.There's also the Impedance Measurement Handbook:http://cp.literature.agilent.com/litweb/pdf/5950-3000.pdf... which says the 4395A+43961A uses the RF I/V method, and figure 2-14 (page 2-16) shows a diagram - again, simplified - which includes a transformer, and which does show all the connections to the analyser as being referenced to ground. It looks more plausible.The question still remains, though, as to what exactly is inside the box. A spec for the transformer would be nice. Even just the proper turns ratio! Or maybe it doesn't matter, if it just affects the electrical load seen by the RF generator, and the V and I measurements retain their proper relationship regardless.

author: imett
author_rank: Contributor
timestamp: March 07, 2016, 09:08:17 am
content: hi, there is a hp patents who explain the construction , i think, the news is only that the current measure it is make across a 50 with a balun build with different ferrite bead an also with a semirigid cable winding into a ferrite core pot to give a high permeability at low freq. thus the v measure it is made with the voltage across the dut and a long semi rigid coax to compensate the current path lenght.now my question, there is a way to find the floppy disk for the 010 option?

author: AndyC_772
author_rank: Super Contributor
timestamp: March 07, 2016, 09:34:23 am
content: I guess a little update is probably in order for the benefit of anyone else with a 4395A who may stumble across this thread...I ended up using two methods to measure impedance.The way I'm currently using, is to put the unit into NA rather than ZA mode, and to put the DUT in series between the RF output and one of the inputs. It'll do the maths internally to work out the effective series impedance, and provided I calibrate the setup first using a short wire instead of the DUT, it works well.Bear in mind at this point, that I'm interested in the range from a few kHz up to single digit MHz only.There's a really helpful site here:http://www.prc68.com/I/4395A.shtml... which describes how to make impedance measurements this way.The other option I tried was to buy a 41951-69001 test adapter, which is built for a different analyser but which will connect up to the 4395A with cables. HP do, of course, say this doesn't work, but I beg to differ. On the experiment I tried - around 1 MHz IIRC - the plots obtained using this adapter were pixel perfect compared to the same device measured in network analyser mode. I was quite stunned by the correlation between the two.I can't help with the Option 010 I'm afraid... but maybe you can do without it?

author: asilva10
author_rank: Contributor
timestamp: April 27, 2017, 12:44:31 am
content: Hi AndyC_772,if possible, can you share how you make the connection of 41951A with 4395A?Thanks!!