Mysterious Insertion Loss with a Phase Shifter - Page 1
Forum section: electronics / rf-microwave

author: ramenbytes
author_rank: Contributor
timestamp: June 14, 2022, 08:17:22 pm
content: Hello!I'm new to microwave, and I'm working on a project in microwave electronics for a summer internship. Basically, I'm using a voltage-controlled attenuator and phase-shifter to automate calibration of some equipment.The problem is that the phase shifter is showing much more insertion loss than I'd expect, based on the data sheet found here:https://www.mouser.com/datasheet/2/412/CMD297P34_Data_Sheet-1950445.pdfI'm expecting around -6dB with a 0V control voltage and a 7GHz sine as my input signal, based on this graph from the datasheet:However, using a scope to get the Vpp of the phase-shifted waveform and the untouched waveform and crunching the numbers yields about -25dB. Below is the what the VNA shows when directly connecting the phase shifter to it:And here is the two ports of the VNA shorted using the same cables I was using to test the phase shifter, plus a male-to-male connector to join them:Tests using the phase shifter were done using the evaluation board shown near the end of the datasheet and the control voltage pin tied to the ground pin, for 0V control voltage. Using a multimeter, I checked the DC resistance of the phase shifter ports to make sure they were DC blocked, and they are (meter shows overload).Any ideas as to what could be causing the extra 17.5 dB of insertion loss, or what I should check? I'm wondering if the evaluation board has more insertion loss than the IC alone, however 17dB more seems a bit much.Thanks for any input,ramenbytes

author: Marsupilami
author_rank: Frequent Contributor
timestamp: June 14, 2022, 08:40:06 pm
content: My first guess would be that either the chip is not powered on correctly or one of your RF connections is broken, not touching correctly. It can happen with the solderless edge launches.Does the phase shift function work despite the high loss you're seeing? If yes I'd look at the connectors, or even a broken solder joint on the chip. Get the microscope out! If you can get a good phase reading and it doesn't change with control voltage then there's something wrong with the chip on your EVB probably.

author: KE5FX
author_rank: Super Contributor
timestamp: June 14, 2022, 08:42:24 pm
content: How's the input and output return loss look -- meaning S11 and S22, with the phase shifter PCB connected to both ports of the VNA? If there's a problem with the board, it will likely show up as a discrepancy compared to the plots on page 5 of the data sheet. If you see poor S11 or S22 and the VNA has a time-domain option, that may give you a good clue as to where to look.In general, when phase shift causes an amplitude drop, I'd look for a matching problem. Something that's supposed to be Zo=50R may be reactive instead, and is being turned into an open or short circuit by the phase shifter. The problem withthatidea is that it's frequency-selective, while you're seeing poor performance everywhere. So... while you're looking at S21, verify that the expected phase shift is actually happening.Inspect the SMA connectors on the board carefully, as the center pins sometimes don't make good contact. An open center pin could account for the fact that you have worse loss at the low end.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: June 14, 2022, 08:50:55 pm
content: Quote from: KE5FX on June 14, 2022, 08:42:24 pmYes, do ↑ that ↑ first. I should've started with that.

author: ramenbytes
author_rank: Contributor
timestamp: June 14, 2022, 09:09:57 pm
content: QuoteInput return loss (S11):Output return loss (S22):QuoteAs for phase shifting, that function does appear to work. I hooked it up to a scope and was able to shift the phase back and forth by varying the control voltage. I've only done rough estimations of the control voltage delta vs phase shift, but so far it seems to be about what is expected. I could of course look more into that if it deserves attention.QuoteI'll check if it does, at least S11 looks pretty screwy with respect to the data sheet.I'll have to follow up with the other suggestions later, thanks for the input so far!

author: KE5FX
author_rank: Super Contributor
timestamp: June 14, 2022, 09:33:27 pm
content: Yeah, most likely you have an open SMA connector pin. If I had to guess I'd say it's on the input (port 1) side. The notch in S22 may correspond to a particular frequency where the phase shift is making the open look like 50 ohms.In a more complex debugging scenario, this is where the TDR would come in handy, but there's not much else that could be wrong in this case.

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: June 14, 2022, 11:19:08 pm
content: Quote from: ramenbytes on June 14, 2022, 09:09:57 pmThere's probably enough signal coupling capacitively across the open on port 1 so you can see the phase shifter operating properly. You must have a well equipped lab to have a scope capable of observing the phase shift of the 7 GHz signal! I would have used a VNA for this. As others said, inspect the center pin of the SMA end launch connector. The EVM BOM doesn't specify the connector type, some of the higher-spec end launches (eg: Southwest Microwave) don't have a soldered center pin, instead you torque down the mounting screws and the center pin makes contact with the PCB surface. You could have an open if the screws are loose, or there's corrosion on the pin or PCB.

author: ramenbytes
author_rank: Contributor
timestamp: June 15, 2022, 01:52:49 am
content: Quote from: KE5FX on June 14, 2022, 09:33:27 pmBetween this forum and discussion with colleagues, that's the working hypothesis on my end. Tomorrow I'll spend some more time pinning it down.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: June 15, 2022, 05:04:50 am
content: Quote from: radar_macgyver on June 14, 2022, 11:19:08 pmThe plots look typical for a Keysight VNA. But I agree port 1 is probably open (could also be a short-to-ground at the input, you can tell for sure by looking at S11 at low freq after a proper calibraiton to remove test lead phase shift). Possibly, but not likely, the input cap on the chip is not connected but I'd guess it is an integrated capacitor so broken solder joint at the SMA connector is my first guess.

author: ramenbytes
author_rank: Contributor
timestamp: June 15, 2022, 05:43:56 pm
content: Quote from: radar_macgyver on June 14, 2022, 11:19:08 pmYeah, I got lucky and through this internship have been able to play with equipment much nicer than what's at my home lab. Originally I was going to use a VNA, but that was occupied so I just used a scope with a pure sine split into two inputs instead. I did eventually get my hands on the VNA for the various plots I posted, as I think another user observed.Quote from: radar_macgyver on June 14, 2022, 11:19:08 pmAs it turns out, this does appear to be a Southwest Microwave part. I already tried to torque down the screws, but that didn't seem to make a difference. Time for the microscope!

author: ramenbytes
author_rank: Contributor
timestamp: June 15, 2022, 05:49:59 pm
content: Quote from: Joel_Dunsmore on June 15, 2022, 05:04:50 amI just probed the input with my DMM, and it does appear to be shorted to ground. For the check with S11, I assume I'm looking for essentially zero reflections? I'm guessing that if we see this, it indicates that low-frequency signals are all being shunted to ground while high-frequency signals encounter impedance-match funniness and so there is some partial transmission? I'm going to go grab the microscope and have a look at that pin.

author: ramenbytes
author_rank: Contributor
timestamp: June 15, 2022, 09:20:15 pm
content: Ok, used the microscope to look all over the signal line and did not find any apparent shorts, though my DMM tells me that electrically it's definitely shorted. The input connector pin also appears to be soldered down. Not the most inspiring solder job, but still, soldered. I also used my DMM's resistance measurements to "trace" the short, and it appears that I get the smallest resistance to ground when measuring between a ground pin on the chip and the chip's input signal pin.I hooked it up to the VNA again as suggested, and S11 from 10MHz (lowest the VNA goes) to 100MHz shows a flat line at 0dB.Any ideas? I think the chip is Class 1A ESD sensitive, and I know we haven't been observing any precautions, could that be it? Or maybe a solder bridge under the chip? I did see a solder bridge between two ground pins on the outside of the chip, so I'm suspicious of the underside.

author: KE5FX
author_rank: Super Contributor
timestamp: June 15, 2022, 11:05:58 pm
content: Quote from: ramenbytes on June 15, 2022, 09:20:15 pmIdeas:X-ray, if your company has it. Sounds like they have a good toy budget, so they should have an X-ray inspection machine around there somewhere.Apply 1-2V at high current (an amp or two) and use a thermal camera (see above) to see where the hot spot shows up.This is an excellent excuse to learn how to use the time-domain feature on your VNA, if the board is big enough and the VNA goes high enough in frequency.

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: June 15, 2022, 11:22:21 pm
content: Quote from: ramenbytes on June 15, 2022, 05:43:56 pmThat would be the person who (literally) wrote the book on those VNAsMy first thought on the short was it's possible the part has an internal bias network that appears as a DC short, but the block diagram clearly shows an internal DC blocking cap. Maybe there was too much solder paste applied, as evidenced by the shorts between the grounding pins. Use X-ray inspection if you have it, else you could try your luck preheating the board and hot-air reflowing the part. The picture of the package in the datasheet indicates it has the wrap-around QFN pads, so it should be possible to inspect the solder for a properly formed heel.If that SMA connector is soldered down, there may be a solder bridge there, since the CPW traces on the board will have very small clearances. Using the TDR approach will give you a hint on where the short is physically located with respect to the SMA. Good hunting!

author: ramenbytes
author_rank: Contributor
timestamp: June 16, 2022, 04:00:38 pm
content: Quote from: KE5FX on June 15, 2022, 11:05:58 pmThanks for the pointers! I checked, and it turns out that we don't have an X-ray inspection machine or a thermal camera. We're a research lab, so I guess they haven't been strictly needed for stuff up 'till now. The second idea reminds me of the story I heard about an electrician who found shorts by connecting the circuit to high voltage and listening for the BOOM!The current VNA I'm using doesn't have the time-domain feature unlocked, but I'll ask around and see if another one does. I started looking into what you can do with time-domain analysis on a VNA, and it looks like it'd be very useful in this situation, not to mention cool, so I hope there is one lying around with the feature enabled.

author: ramenbytes
author_rank: Contributor
timestamp: June 16, 2022, 04:16:03 pm
content: Quote from: radar_macgyver on June 15, 2022, 11:22:21 pmI had no idea, cool!Quote from: radar_macgyver on June 15, 2022, 11:22:21 pmNo X-ray machine unfortunately, so if there is time later I might try reflowing the solder like you mentioned, possibly with a fine-tipped iron if I can't find a hot air station. On a slight tangent, bias networks that appear as a DC short are a new thing to me. Any good resources you recommend for learning more about the various bias networks?Quote from: radar_macgyver on June 15, 2022, 11:22:21 pmThat was my first thought, since it looked like the pin was on top of one of the ground plane traces. No such luck though, under the microscope there appears to be plenty of clearance.Quote from: radar_macgyver on June 15, 2022, 11:22:21 pmThanks! It's all been quite fun so far, if not occasionally perplexing, and I hope I'll find an excuse to actually pinpoint the issue instead of merely getting a second EVB.

author: ramenbytes
author_rank: Contributor
timestamp: June 16, 2022, 04:25:45 pm
content: Current status of everything right now is that we're chalking the issue up to either some soldering error or possibly a bad chip, and we'll order a new EVB and work on the parts of the project that aren't affected by the excess attenuation. This is only supposed to be a side project while working on other stuff, and I've spent about as much time as I can for now tracking down the issue. Several weeks from now if I have time and we haven't gotten a second EVB in by then we might try the other suggestions in this thread such as time-domain analysis and reflowing the solder.Thanks for everyone's input! It's definitely been educational, and when I hopefully pick this back up in a few weeks I'll have a decent idea of where to go from there.Cheers!

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: June 17, 2022, 07:39:41 am
content: For output biasing, most power amplifier ICs require a bias-T at the output to provide power to the drain or collector of the IC. I tried looking, but couldn't find any, but some ICs also require a gate bias, applied via a bias T to the input side of the amplifier. These appear as a DC short. Another case is where there's a flux-coupled balun (transformer) at the input. For example, see the LTC5594's RF input pin."Fundamentals of Microwave and RF Design" andMicrowave and RF Design (vol 5)by Michael Steer have an introduction to bias-T networks used to power amplifiers. Other references exist, sorry I'm away from home and unable to recall them by name!