R&S ZVC VNA questions and repair - Page 1
Forum section: electronics / testgear

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: January 19, 2022, 06:20:46 am
content: I was able to buy a reasonably priced ZVC VNA, this is the 8 GHz version of the ZVR. Mine has the passive bridge option, includes step attenuators on all four channels, and has the external test set option. I found a couple of other threads on here, linked below:https://www.eevblog.com/forum/repair/rohde-schwarz-zvr-vna-repairhttps://www.eevblog.com/forum/testgear/rohde-schwarz-zvr-network-analyzerhttps://www.eevblog.com/forum/testgear/second-hand-vna-info-requiredIt looks like when the instrument is uncalibrated, with a straight cable between PORT 1 and PORT 2, I measure S21 of -10 dB, with some variation across the band. S12 measures +10 dB or so. After doing the recommended full two-port cal (TOM), the measured S21 and S12 look quite good. Measuring a cavity filter I had on hand showed good response, however the S21 had a lower dynamic range than S12.I found theservice manualsfor this series of VNAs on this forum, sadly they seem to lack schematics, or a detailed block diagram. It does have a troubleshooting guide, which shows the expected trace for the a1, a2, b1 and b2 receivers with a cable between the two ports. The expected trace shows -20 dB from 20 kHz to 4 GHz, dropping to -30 dB beyond that.I actually measured about 10 dB worse than that over the full band for a1, with the high band (4-8 GHz) looking even worse.The b2 trace looked even more strange, with a pronounced jump of 10 dB at about 5 MHz.To verify the output of the two ports, I had the VNA do a very slow sweep and connected a spectrum analyzer to the two ports with a peak hold. This indicated that the output power was flat to within 1 dB over the span of 1 MHz to 6 GHz (limits of my spectrum analyzer). So this means that the output power is fairly level. I also enabled external test set, and measured the same flatness at the a1 output.Next, I measured all of the virtual test points (using the service menu) and compared against the expected readings listed in the service manual. The synthesizer, LO generators and source modules checked out OK. However, the output stage level control measurements were way off from the expected readings, as were the control voltage and level control loop reference voltages. Finally, the front-end self test measurements were way off as well; these measurements are made using a diode detector at the IF output of the four receivers. The self test was recommended at a source level of -10 dBm, but I was able to measure the expected voltage when increasing the source level to 0 dBm (since the reference receiver was measuring ~10 dB low).So I'm confused where the instrument may be broken - the output stage is clearly generating the right power levels (according to my spectrum analyzer), but failing some self tests. As far as I can tell, the reference measurements are made after the output stage, so that seems to indicate that the receivers are bad. Any advice on further tests to perform?I sure hope it's not the receivers, since those have unobtanium hybrid modules...

author: pmx
author_rank: Contributor
timestamp: July 12, 2025, 07:13:11 pm
content: Hi, I read your post, the problem seams complex.Did you succeed in repairing you ZVC ?I'm currently considering buying a ZVR (4MHz) with a problem on port 2, and I found your 2022 post.BTW, I ca'nt find the service manuals, the link now points to a page qying to contact R&S USA... looks R&S don't want amateurs to repair 20+ years old VNAs... like we could buy a new R&S VNA that costs almost the price of a car !!I you still have these documents, I'm interested, it may help me decide if I have a way to repair / recalibrate the VNA, of if it is now no more than an heavy doorstop (you mentionned unobtainum hybrid circuits)Thanks in advance.

author: pmx
author_rank: Contributor
timestamp: July 12, 2025, 07:29:39 pm
content: OK, I found the 3 service manuals.Looks like you have used all the self-test options ?

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: July 13, 2025, 09:48:55 pm
content: I wasn't able to narrow down the issues. However, my instrument does work acceptably at higher bands (above around 500 MHz) so I'm using it as-is. If I set the frequency span to include frequencies below ~500 MHz, I cannot get a valid calibration. Without schematics, it's hard to figure out the internals of the instrument, especially since it has so many options installed. Please let me know if you come across any scans of the service manuals that include the schematics.The instrument doesn't really have an automated self-test, but you can get it to display the measurements of various internal analog bus nodes and compare them to expected values when the instrument is configured per the self-test table. I ran these for the parts of the instrument that were suspect (generator, output stage, receivers). The generator measurements mostly checked out, but the output stage and receivers were off by quite a bit, especially at the lower bands. Both of those, unfortunately, are in hybrid modules that are not really serviceable. I also don't have any block diagrams for them to figure out possible replacements.I had written up some notes while I was working on this, I've attached them here.

author: pmx
author_rank: Contributor
timestamp: July 25, 2025, 06:49:09 pm
content: Thanks for the document, I will read it thoroughly !I'm glad you can use it, albeit only above 500MHz.After some hesitations, I finally pulled the trigger on a ZVR with the 21 22 23 24 and 25 hardware options, but also with a serious coupler reflectivity problem on Port 1. I will receive it next week.S11 and S21 are not usable, but with some creativity, you can still use it like a very big and very heavy nanoVNA, but with the huge benefit of a very good dynamics range.

author: pmx
author_rank: Contributor
timestamp: August 05, 2025, 09:47:52 pm
content: Just got the VNA and tried it quickley.First impression : it is HUGE !I suspect there are problems with the attenuators, and/or one of the couplers. Hard to tell.Very likely, somebody exceeded the input power (or DC) limit, overheating the input stages.But once you calibrate the VNA (Thru, Open Short, oad, the usual way), most of the problems mostly "magically" seems to disappear, at the expanse of some dynamics.I'm not sure there are no side effects, though.The situation is less catastrophic that it could have been, but I really want to have a look at the problem.In a repair video on Youtube (Model ZVE, IIRC), we can see that the couplers are of the toroïdal type.BTW, I've not figured how to open the case (I don't see screws in the "usual" places).

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: August 07, 2025, 05:43:23 pm
content: Haha, that matches my first impression as well! At least its not as heavy as an 8566...Mine has pretty much the same issues, although if I include frequencies below about 500 MHz, the calibrations always fail. If the start frequency is > 500 MHz, the calibration completes and I get good measurements, but the SNR on S21 and S12 are quite different. I had previously suspected the receivers, though it could just as well be the couplers.I have to go take a look, but I recall the screws to open the unit are in the rear 'feet' (if you were using the instrument vertically). With those removed, the covers slide off. I would recommend replacing the hard drive with an SSD/CF card as soon as possible.The ebay pictures for my instrument showed the span set in the VHF region, and that was how the instrument powered up the first time I used it. I wonder if it was used for testing VHF power amps, and someone accidentally keyed the PA while it was connected to port 2.Could you post a link to the Youtube repair video?

author: pmx
author_rank: Contributor
timestamp: August 07, 2025, 06:25:21 pm
content: The video is hard to find if you search for a ZVR, as it is in fact about a ZVCE, the 8GHz version.As ypu probably know, the C suffix means 8GHz (R is 4GHz), the extra E suffix means 3 receiver channels (instead of 4 for the ZVR and the ZVC).Part 1 :Yes, part 1, even if the first images say "part 2"...part 2 :The guy is an Ham Radio operator from the Netherlands.

author: pmx
author_rank: Contributor
timestamp: August 07, 2025, 06:27:08 pm
content: BTW, the broken N connector in the video behaved like a hi-pass filter (capacitive coupling)