Cloning the HP35676A Reflection Transmission Test Set for the 3577A/B VNA - Page 1
Forum section: electronics / testgear

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: July 08, 2020, 05:02:36 pm
content: Hi,In the testing of the a CM/DM Noise Separator I designed, I needed to test a frequencies lower than the HP34677A S-parameter Test Set. The S-parameter is designed to work down 100kHz.I found that HP also produced the 35676A Reflection Transmission Test Set. The R/T Test Set will work down to 5Hz.You can buy the R/T Test Set on the usual places for about $300-500.Note:The 'B' Version is not an improvement on the A version, the A is 50and the B is 75.Information SearchI found a nice summary of the 35676A:Link:http://www.hparchive.com/Manuals/HP-35676-SCHEMATIC.pdfI will attach a copy of the document to the end of this message.I looks to be a pretty simple circuit, just 7 resistors and four connectors. Most of the challenges are in mechanical construction.SchematicI entered the schematic into KICAD. The test points are connected to the chassis mounted connectors.PCB DesignThe coplanar Wave Guides are 50.Complete ProjectHere is a picture showing the circuit board mounted inside a Hammond 1550P Diecast box.Mechanical DesignThe mechanical design was done using Fusion 360. I started with a simplified drawing of the HP3577A front panel:I downloaded the STEP file for the Hammond 1550P box and added the holes for the connectors and the PCB:Here is a picture showing the assembly:and the project installed on the HP3577A:TestingAfter installation I did the one port full calibration with Open, Short and Load.At high frequencies the results agreed with my S-parameter Test Set. At lower frequencies the R/T Test Set produced the expected results.I have attached the pdf file.Regards,Jay_Diddy_Bkicad schematic.JPG(29.48 kB. 687x379 - viewed 1393 times.)

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: July 08, 2020, 05:32:35 pm
content: Hi,This is a picture of an original one. They were supplied in a case with cables, a load and a short for calibration:The screws that hold it together are under the top label.The kit was about $1800 USD in 1989 about $3600.00 in today's money. This probably why they didn't want you to look insideRegards,Jay_Diddy_B

author: brob
author_rank: Newbie
timestamp: August 12, 2020, 05:00:05 pm
content: Hi JayWould you be able to post any DXF, or PCB layout?I am looking at making the same test set for my 4395A. Looks like you milled your PCB, I would also be looking at doing the same thing. I have not worked with controlled impedance PCBs before.Any help would be appreciatedThanks

author: bson
author_rank: Unknown
timestamp: August 13, 2020, 01:00:59 am
content: Very cool, I'd like one of these. My 3577A is one of my most used instruments! What connectors did you use, and care to share the KiCad project?

author: KE5FX
author_rank: Super Contributor
timestamp: August 13, 2020, 01:47:38 am
content: I'll confess I don't understand how this circuit achieves any sort of directivity, much less 40 dB. All I see are a couple of T attenuators at the reference and receiver ports, with a bypassed resistor between them. If the test port is shorted, the A receiver channel gets nothing from the source. If the test port is open, the A receiver gets everything Where's the balanced-bridge element?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 13, 2020, 02:00:54 am
content: Quote from: bson on August 13, 2020, 01:00:59 amHi,I can't be 100% sure what the connectors are. The word you are looking for is 'extended' teflon. I bought them from eBay a few years ago.These connectors from Amphenol should work. You may have to trim the extended dielectric.Regards,Jay_Diddy_B72224_24_customer_drawing-1279450.pdf(55.1 kB - downloaded 233 times.)72260_10_customer_drawing-1279511.pdf(62.92 kB - downloaded 186 times.)

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 13, 2020, 02:09:41 am
content: Quote from: KE5FX on August 13, 2020, 01:47:38 amYes, it is pretty hard to understand how this works so well for a simple circuit.The HP 87512A/B are similar devices. The 'A' is 50and the 'B' is 75.Link:http://www.miedema.dyndns.org/co/2016/4395a/trans-reflect/HP-87512B-tear-down-schema-en-manual.pdfA lot of the 'magic' happens in the VNA calibration routine.I can testify that they work relatively well, although not as good as the s-parameter test set. They work at lower frequencies than the s-parameter test sets.Regards,Jay_Diddy_B

author: KE5FX
author_rank: Super Contributor
timestamp: August 13, 2020, 02:34:16 am
content: It must be a variation on what Joel Dunsmore's book calls a 1+gamma reflectometer, except that the signal at the b1 receiver (labeled 'A' here) actually is allowed to go to zero when the DUT is shorted.This sort of topology requires near-perfect stability since directivity errors are magnified immensely. Probably a safe-enough assumption at VHF and below, but I'm still surprised they didn't go with anactive bridgeof some sort (to incorporate another thread by reference).

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 13, 2020, 02:51:24 am
content: Hi,These measurement will give you an idea of how well this works. I set the VNA to measure S11 (reflection) and then I performed a full port calibration with open, short and load.I then measured some attenuators.First a 2x (6dB) attenuator. The reflection should be 12dB. 6dB attenuation, reflection and then 6dB attenuation.Now a 5x (14dB) attenuator. The answer should be 28dB.a 10x (20db) attenuator. The answer should be 40dB.and finally a 50a different one than the one I used for the calibration.These measurements were take with the clone of the HP35676A R/T test set.Regards,Jay_Diddy_B

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 14, 2020, 08:57:00 pm
content: Quote from: brob on August 12, 2020, 05:00:05 pmHi group,I have been working with forum member brob on this issue.The R/T test set for the HP 4395A was HP 87512A or 87512B. The 'B' versions are 75.There is is a subtle difference between the 4395A and the 3577A Network analyzer.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.I have made a second artwork that is for the 1.600 inch spacing.Regards,Jay_Diddy_B

author: bson
author_rank: Unknown
timestamp: August 14, 2020, 10:10:29 pm
content: I think it's just a 2-way coupling bridge. When the 2-port input impedance is 50Ω the receiver sees the calibrated 1x signal level. When it's less, the coupled signal drops. When it's more, it increases. So it operates on measuring loss rather than reflection per se. Since the main use of S11/S22 is to measure input and output impedances (and, yes, there are impedances at low frequencies as well, and being able to sweep and measure them both by frequency and amplitude is useful) it doesn't really matter how they're derived.

author: RomDump
author_rank: Regular Contributor
timestamp: November 13, 2020, 04:16:53 pm
content: Quote from: Jay_Diddy_B on August 14, 2020, 08:57:00 pmWondering if you can post the PCB Artwork?Thanks.

author: garrettm
author_rank: Frequent Contributor
timestamp: February 04, 2025, 06:42:23 am
content: I have both original 35676A and 87512A transmission / reflection test sets and it appears HP used the exact same design for both. 4-wire DC ohms measurements read within 1% of each other for all combinations (center-pin-to-center-pin of each port and center-pin-to-housing). At first I was afraid my 87512A was bad after looking at the linked PDF, but eventually realized it was for the 87512B. Whew! Anyways, it is somewhat surprising that HP managed to massage this circuit from 200 MHz with the 35676A all the way out to 2 GHz with the 87512A. I assume this is mainly due to SMT resistors being better for this application.I've attached photos of both original parts and a basic sanity test for ohms measurements at each port.The center-pin to center-pin measurements aren't needed (and are hard to measure), but could help isolate which resistor is bad. The following values were calculated from the 35676A schematic (both signal dividers were within 1% or better of these values):R_RA = 122.11 ΩR_RS = 98.23 ΩR_RB = 121.46 ΩR_SB = 64.12 ΩR_SA = 110.89 ΩR_AB = 83.94 Ω

author: danir
author_rank: Newbie
timestamp: June 26, 2025, 07:00:59 pm
content: I analyzed the original circuit (OriginalSchematic.png) and found, that the resisitance on the ports are not 50 ohm. When loading the unused ports with 50 ohm the following port resistances result:- X1 (to VNA Ref input): 54.6 ohm- X2 (from VNA generator output): 60.5 ohm- X3 (to VNA A input): 63.8 ohm- X4 (DUT test port): 60.7 ohmHowever these mismatches are removed by the calibration, but if a nonlinear device is connected that is sensitive to the output resistance of the the test port, this will lead to measurement errors. But fortunately if one is rebuilding the device the replacement of the original resistor values addresses this issue. The calculation is simple - there are 7 resistors; to solve for their values 7 conditions are needed:- The resistances on ports X1 to X4 must equal to 50 ohm- The attenuation from port X2 to port X1 must be 17dB- from X4 to X3 13dB- from X2 to X4 10dB as specified in HP's Operating and Service Manual.With the resulting resistors (see attached 50ohmSchematic.png) the nominal port resistances are all between 50 ohm and 50.17 ohm and the attenuation from X2 (VNA generator output) to the test port is 10.02dB.