Building a HP 41951A Impedance Test Kit - Page 1
Forum section: electronics / projects

author: paulek
author_rank: Newbie
timestamp: December 30, 2022, 04:17:42 pm
content: Hi to all,recently I got myself a HP 4195A Network/Spectrum/(Impedance) Analyzer, mainly for its Impedance analyzing capability.First of all, this beautys main reason is for me to learn more about passives. (EE Masters Student)The machine came without the (expensive) impedance test kit, so I thought about building it myself.I jigged up all the, what I thought necessary, components on a protoboard. That was everything but the DC-Bias circuitry and the stuff with the transformer.And the circuit worked good up to around 100MHz which I was already really happy about for THT components.Now my main question is, what is the function of the transformer section?(Schematic attached)Thanks in advance for any info on the section, or overall design tips for this build!

author: chilternview
author_rank: Frequent Contributor
timestamp: December 30, 2022, 06:58:15 pm
content: The 'compensating electrical length' is a coiled solid outer coaxial cable, with external dimension of about 1mm or less. The same cable is used in the transformer. Probably difficult to get hold of, and difficult to work with.

author: buta
author_rank: Contributor
timestamp: December 31, 2022, 06:59:23 am
content: Figure 3-1 from the 41951A Operation Note shows the block diagram, the transformer T1 seems to be part of the directional coupler." alt="" class="bbc_img" />

author: jonpaul
author_rank: Super Contributor
timestamp: December 31, 2022, 11:12:01 am
content: Bonjour BRAVO for the find!We had HP 4195A and test fixt 41951, 41952 since 1990s, amazing machines, made in Japan with Yokogawa.We used it to develop ultralow capacitance pulse Transformers 0.3 pF , 10 kHz..200 MHz BW.The 41951 has 100k..500 MHz BW, APC-7 connectors, and DC bias provision, hard to duplicate with any accuracy.Suggest to be patient and keep searching Ebay, local school and firm disposals, Ham fleas.Eventually you may get the real one.Meanwhile get some TR sets, and use as SA and NA, very versatile!Bon Chance,Jon

author: paulek
author_rank: Newbie
timestamp: December 31, 2022, 04:18:45 pm
content: Thanks for the insights Jon! I will definetly keep looking for the real deal.In the meantime and for curiousity reason I will still try to make something work.As this unit needs to be calibrated with the used test kit I assume much of my design mistakes will get covered up by it.Any ideas on how to test the actual performance of my kit?

author: jonpaul
author_rank: Super Contributor
timestamp: December 31, 2022, 07:24:57 pm
content: Rebonjour cher Paulek: 4195A is a Great machine, was fortunate to find one from ex HP in Colorado Spr, like new, good price.Afterwards, I saw the accessories like 41951, and asked the seller if he still had them.Looed in a close: EVERY bound loosleaf manual, 41951, 41952, test sixtures, CAL set in the Japanese black wood cases!Entire set of accessoris was ~ $200!As I recall, Test is as per HP 41951 manual,CAL requires a Precision short, Precision open, Precision 50 Ohm (APC-7 on HP)mode IMPset frq swp start end log lin etc.enter IMP CALSHORT >>enterOPEN>>enter50 Ohm >>Enterreturn to main menu, set for use CALtest 50 Ohm again, should be perfect Z= 50.0 and phase = 0 over the entire band.The 41951 is specified only 100 kHz (due to transformer in the dir coup) to 500 MHz.Bon chanceJon

author: coromonadalix
author_rank: Super Contributor
timestamp: December 31, 2022, 08:29:29 pm
content: I think right now you have 2 kits on ebay at high $$$$

author: G0HZU
author_rank: Super Contributor
timestamp: December 31, 2022, 09:26:07 pm
content: Transformer T1 looks to be a balun transformer and L1 will presumably be part of its compensation. The whole circuit looks to be a classic resistive bridge.However, in this case, HP have driven it via the balun and then sensed the reflection coefficient at the T port. It looks like HP have done it this way around because it allows a direct DC feed path to the DUT and this allows a DC bias to be fed to the DUT.The input S port feeds a classic 50 ohm divider and one arm goes off to the R port. The R port output is presumably part of the feedback for RF levelling. If the levelling stays in closed loop (as it should) then the impedance at the J1 node will be close to zero. This helps to define the source impedance that feeds the balun as it will then be defined by the accuracy of the two parallel 100R resistors R1 and R2 (and this will be a very well defined 50 ohms resistive).The little nanovnaH uses a similar bridge system, except it doesn't offer the option to feed in an external DC bias to the DUT. This means the nanovnaH uses the resistive bridge in a different way. It uses it in reverse. The nanovna doesn't need the balun as it uses a mixer/detector chip that already has balanced inputs.The source is fed to the top of the bridge in the case of the nanovnaH. i.e. it is backwards compared to the HP kit.I'm not sure how well the HP bridge will perform. At low frequencies (below 300kHz) I'd expect the nanovna to perform really well because it has no balun. This allows it to work down to just a few kHz.I'm not sure which would perform best across 1MHz through 50MHz. I doubt there would be much in it either way. Above 50MHz the nanovnaH starts to struggle a bit, but then again, I'd expect that HP setup to struggle a bit up above (say) 200MHz.The closed loop levelling system of the HP unit offers a performance benefit though. The auto levelling would help it to hold a good cal for quite a long time. The nanovnaH would drift over a fairly short time and would require regular calibrations to try and stay close to the HP kit.

author: jonpaul
author_rank: Super Contributor
timestamp: December 31, 2022, 09:48:50 pm
content: our 4195A and 41952 are fine as,specified 100khz...500 MHz, using the HP test fixtures on th3 APC-7 connect. One can work down to 10..20 kHz but not as accurate.Indeed the R1 port is the RX 50 ohm for leveling.The DC bias is supplied by the 4195A or externally, useful for characteristics of Varicaps, and other voltage sensitive capacitance.Test adapters are available for TH large or smallest binding posts, and several types,of SMD, all easily adjustable.EnjoyJon

author: paulek
author_rank: Newbie
timestamp: January 01, 2023, 02:03:06 pm
content: Puhh, it seems like I need to dive into this a lot deeper than expected. Thanks for the info! I will report back when I achieve something.

author: jonpaul
author_rank: Super Contributor
timestamp: January 01, 2023, 03:44:00 pm
content: Rebonjour Paulek! (Paul?)Indeed the HP 41951 imp method is unique and clever. Since 1992, using it 100 kHz..500 MHz down to 0.3 pF and 100 nH.Especially the APC-7 connector to define a measurement plane and the test fixtures are critical to accurate and repeatable measurements at 50..500 MHz.See the excellait HP/Agilent app notes and tech briefs on 4195A, Impedance measurements, etc.The HP memory project and BAMA may have them as PDF.Pix of 4195 setup and fixtures, APC-7 and CAL setThis Took many years and a lot of luck to accumulate!Your thoughts appreciated!Jon

author: paulek
author_rank: Newbie
timestamp: January 01, 2023, 04:04:41 pm
content: Hello Jon, this set looks so pristine! AmazingStructural integrity aside, couldn't one use a more modern APC-3.5 (SMA?) Connector instead of the APC-7?I will take a look at those documents for sure, after my vacation the coming week.Paul

author: jonpaul
author_rank: Super Contributor
timestamp: January 01, 2023, 04:33:46 pm
content: Paul, the APC-7 is super wideband and mechanilly robust. It's strong enough to supposed the test fixtures.I doubt if SMA, 3.5 mm can do so.If you can find APC-7 you have compatibles to the HP fixtures, TH and SMDJon