diy HP 35676 Reflection/Transmission adapter clone - Page 1
Forum section: electronics / projects

author: fenugrec
author_rank: Frequent Contributor
timestamp: December 10, 2021, 03:06:23 pm
content: Hi,in the third recursion level of a project dependency stack, I decided I needed to measure impedances at low frequencies (<10MHz, but specifically below the usual 100-300kHz limit of most impedance test kits), and to do that I would build a copy of the HP 35767 Reflection/Transmission test kit, originally designed to go on a HP 3577 VNA.I have a different instrument, an HP 4195A (a 10Hz - 500MHz VNA), that also has a slightly different port configuration than the 3577 : it has two identical "sides", each with Source out, and R + T inputs. The 3577 has Source out, and three inputs (Ref, A and B)... not sure exactly if it ever uses that B port for anything other than some S-param measurements maybe ? But whatever.Back to the 35767; some nice fellow posted the schematicshere:[ Specified attachment is not available ]I decided to build it inside one of these 4-way couplers that can be found cheap on greedbay when they're older / undocumented / broken :I ripped out all the extra components, leaving a nice 22mm-dia PCB with convenient traces and pads ! They probably used the same base unit+PCB for a whole line of products, and I'm going to bet the traces are pretty close to 50-ohm (there is a full ground plane on the opposite side).I needed to parallel some resistors to get within 1% of the values on the schematic, but otherwise a fairly straightforward build:Sadly, I'm way out of my depth in this type of thing and I don't understand how this circuit works. I kindof see T attenuators going to Ref and A, but I tried to run the numbers and simulate in LTspice , and nothing adds up to 50 ohms anywhere, no matter if I open / short / terminate each port...The 3577 ports can be set to High-Z but I believe for this accessory they must be set to 50-ohm.I was also going to figure out the exact theoretical value for each resistor so my parallel combinations would be closer to nominal, in case HP just selected the closest E96 value as they seem to have done.Conceptually, I would expect this to do two things - split power (50/50 ?) to the reference port , and sample the... reverse directional coupled signal, to send to the A port ?Anyway, as a good sorcerer's apprentice, I just tested the thing without understanding it. I even used the worst possible SMA-to-BNC-to-alligator O/S/L for the occasion, and then measured some plain inductors / capacitors. Numbers mostly made sense so.... success ?Any comments on the theory, use, or my implementation of this thing will be greatly appreciated. Thanks !

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: December 11, 2021, 01:38:48 am
content: Hi,I have also made a DIY version of the HP 35767A.Here is a picture:It is documented here:https://www.eevblog.com/forum/testgear/cloning-the-hp35676a-reflection-transmission-test-set-for-the-3577ab-vna/msg3126264/#msg3126264I found:On the 3577A the spacing between the R input and the A input is 1.700 inches.On the 4395A the spacing between the R input and the A input is 1.600 inches.Regards,Jay_Diddy_B

author: bob91343
author_rank: Super Contributor
timestamp: December 11, 2021, 02:33:27 am
content: I have an HP 35689A if that would help you.

author: fenugrec
author_rank: Frequent Contributor
timestamp: December 11, 2021, 02:42:27 am
content: Quote from: Jay_Diddy_B on December 11, 2021, 01:38:48 amWhy yes, and it's way nicer than mine... I daresay those extended N connectors probably cost an arm though!QuoteAh thanks, I will process that. So it is doing some kind of directional coupling... I'll try those tests with unterminated attenuators, see what I obtain.