How to Reduce Noise When Measuring 20MHz frequency using relay - Page 1
Forum section: electronics / projects

author: ddsa
author_rank: Contributor
timestamp: March 15, 2025, 03:31:51 am
content: Hello everyone,I'm currently making a test machine for testing (voltage, frequency) PCBs. There are 5 test points on PCB where I need to measure frequency 20MHz. So, I'm using relay to switch around test pointand connect to the probe of my oscilloscope (Keysight DSOX1202a). I've found out that relay causes a lot of noise and I'm wondering if I could get help on how to reduce noise.I've tried using SSR type relay, but noise still occurs.Do you guys know any other relay types that are stable for measuring 20MHz or something else that I could use instead of relay?Thanks in advance..

author: moffy
author_rank: Super Contributor
timestamp: March 15, 2025, 04:10:26 am
content: Why would a relay introduce noise? and what type of noise are you talking about? There are plenty of analogue switches some of which into the GHz range, but you would need to specify the peak to peak voltage of the 20MHz signal. The simplest might be a low capacitance reed relay, but more info is needed.

author: ddsa
author_rank: Contributor
timestamp: March 15, 2025, 04:48:06 am
content: I don't know why relay causes noise as well.. as you can see in the picture, the frequency value is 20MHz which is what I need. however, when there's a noise, the wave becomes blurry and the frequency value drops significantly and loses signal as well.

author: Bud
author_rank: Super Contributor
timestamp: March 15, 2025, 05:19:38 am
content: Show how you wire the relay to the test ponts and to the scope.

author: ddsa
author_rank: Contributor
timestamp: March 15, 2025, 05:33:59 am
content: I'm at my home now so I can't take a picture but here's how I tested itTest point -> wire -> relay -> wire -> probeand also, there's more noise when I connect other test points to measure frequency

author: coppercone2
author_rank: Super Contributor
timestamp: March 15, 2025, 05:53:52 am
content: solder a wire across the relay to see if the noise goes away

author: ddsa
author_rank: Contributor
timestamp: March 15, 2025, 07:05:49 am
content: yeah I've already tried this and the noise goes away. That's why I concluded that noise occurs from relay. However, since I need to switch around the TPs, I need to find a way to find a relay type that's able to handle 20MHz or something else instead of relay

author: PCB.Wiz
author_rank: Super Contributor
timestamp: March 15, 2025, 07:46:20 am
content: Quote from: ddsa on March 15, 2025, 03:31:51 amThat sounds like capacitive loading, you may need a probe buffer before you switch into any cable loads.Quote from: ddsa on March 15, 2025, 03:31:51 amRF relays easily do that, but they are not cheap.What signal swings do you have?An analog switch might be good enough? A xx4052 can select 1 of 4SSR's tend to have higher current and higher voltage ratings than analog SW, but that also costs in capacitive Cout.

author: ddsa
author_rank: Contributor
timestamp: March 15, 2025, 08:26:48 am
content: Can you recommend some RF relays that I can use for this case? I've never used this beforeThank you

author: coppercone2
author_rank: Super Contributor
timestamp: March 15, 2025, 08:29:56 am
content: what relay are you using now?Did you also make sure the relay is good? It might just be messed up.I have a amplifier that does 1 MHz and it has no problems with a regular relay.https://na.industrial.panasonic.com/products/relays-contactors/mechanical-signal-relays/lineup/high-frequency

author: ddsa
author_rank: Contributor
timestamp: March 15, 2025, 08:35:20 am
content: Yeah the relay works well when measuring voltage.I'm currently using normal coil type relay but since there's noise, I've also tried using SSR type relay which didn't work as well.

author: coppercone2
author_rank: Super Contributor
timestamp: March 15, 2025, 08:36:45 am
content: Did you test to make sure the relay is not damaged from the factory. It could have slipped through QC.I am kind of wondering if its a chinese untested relay and then a counterfeit SSR and then your driving yourself insane because all the parts are bogus.Like you never said WHAT is the model # of the relay you are using and how it tested (coil and contact resistance, current draw, etc)

author: ddsa
author_rank: Contributor
timestamp: March 15, 2025, 08:38:32 am
content: Yeah I've already tested it.Can the length of the connection affect as well? It's approximately 3.5-4 meters long which I think is quite long

author: coppercone2
author_rank: Super Contributor
timestamp: March 15, 2025, 08:40:00 am
content: its a radio frequency circuit and lots of things can effect it but you should tell us the part # and data sheet of the relay and tell us what your test results were for measuring the different things you can measure to determine if a relay is good.and the same goes for the SSR.we are shooting in the dark here because we don't know how bad it is or what has been ruled out, how it has been ruled out and also what it is, is this a transmitter? a antenna? a feedback loop?without details its really al over the placeand who knows if it can effect it, I don't even know what it is, what kind of conductor you are using, etc. You can route a radio signals through hundreds of meters no problem if its correct.

author: ddsa
author_rank: Contributor
timestamp: March 15, 2025, 08:42:11 am
content: I'm at home right now and I can't check the relay model right now. I'm actually a programmer and I'm just trying to help out my wiring team to figure this problem out so I don't know a lot about the model names. However, we've made a lot of testing machines with the current relay that we are using and had no problemIt's just that it's our first time measuring high frequency.

author: coppercone2
author_rank: Super Contributor
timestamp: March 15, 2025, 08:43:24 am
content: I think its critical information otherwise you can just blindly try a 'rf rated' relay from panasonic or whatever and hope it works.here is my advice for radio frequency : devil is in the details and the concept is somewhat simple.Based on your description so many problems come to mind that its just going to be a wild goose chase IMO, things need to be totally ruled out and determined

author: PGPG
author_rank: Frequent Contributor
timestamp: March 15, 2025, 08:54:56 am
content: Quote from: ddsa on March 15, 2025, 08:38:32 amAs for suggestion:Quote from: coppercone2 on March 15, 2025, 05:53:52 amyou said:Quote from: ddsa on March 15, 2025, 07:05:49 amIf shorting the relay contacts with a piece of wire (I assume no other changes made) make noise go away than why you suspect any other source?

author: ddsa
author_rank: Contributor
timestamp: March 15, 2025, 09:00:41 am
content: SSR relay -https://www.cybertice.com/product/803/solid-state-relay-%E0%B9%82%E0%B8%8B%E0%B8%A5%E0%B8%B4%E0%B8%94%E0%B8%AA%E0%B9%80%E0%B8%95%E0%B8%97%E0%B8%A3%E0%B8%B5%E0%B9%80%E0%B8%A5%E0%B8%A2%E0%B9%8C-active-high-5v-2a-4-channel-%E0%B8%9A%E0%B8%AD%E0%B8%A3%E0%B9%8C%E0%B8%94%E0%B8%A3%E0%B8%B5%E0%B9%80%E0%B8%A5%E0%B8%A2%E0%B9%8C%E0%B9%81%E0%B8%9A%E0%B8%9A%E0%B9%84%E0%B8%A3%E0%B9%89%E0%B8%AA%E0%B8%B1%E0%B8%A1%E0%B8%9C%E0%B8%B1%E0%B8%AA-ssr-5v-2acoil relay - ZQWL-IO-1BZRC32this is from China

author: ddsa
author_rank: Contributor
timestamp: March 15, 2025, 09:01:29 am
content: ah yeah I was overthinking at the moment

author: Kleinstein
author_rank: Super Contributor
timestamp: March 15, 2025, 09:39:15 am
content: The problem looke like couling to another, lower frequency signal (maybe mains hum). That would not so much depend on the type of relay, but more a thing of the layout, supply and shielding. How critical the relay is also depends on the signal impedance. Normally RF uses 50 ohms or similar and is thus not that critical to capacitive coupling.20 MHz is not that high to require special RF relays, unless one has extreme demands on reflections / SWR.For the relays I would look for small types with gold covered contacts. This are usually small ones, like Fujitsu FTR-C1, Kemet EC2 / EE2 or Zettler AZ850.For small DC voltages (better than 10 µV precision needed) one would prefer latching types, other wise normal coils are also OK.Photomos switches as small SSRs can also work.If the voltage is limited and no isolation needed, CMOS switches like DG211/DG409 or cheaper 74HC4053 / 4052 could also work.

author: jonpaul
author_rank: Super Contributor
timestamp: March 15, 2025, 11:32:32 am
content: measurement: NO SSR.Only use dry contact rated relays prefer gold plated bifuracated contacts.Junk CCP Chi com will give BAD results.At RF we use Dow-Key (or equiv) coaxial BNC relays.J

author: mtwieg
author_rank: Frequent Contributor
timestamp: March 15, 2025, 12:32:36 pm
content: Quote from: ddsa on March 15, 2025, 09:00:41 amOk so apparently the "relay" is actually a huge PCBA with 32 relays on it. The manufacturer's website won't load for me so there's no clear information on its circuitry though (are the relays SPST, SPDT, DPDT? Contact material?).You haven't actually shown what the "noise" you're referring to looks like.But the relays themselves certainly are not the source of the "noise". It's likely due to one/both of the following:1. The wires carrying these signals are routed poorly, allowing them to pick up interference from the environment or each other.2. Same as above, but on the relay board itself.If you want help, you should share the following info:1. What the circuitry generating these signals is like.2. How these signalsand their return pathsare routed to/through the relays.3. What circuitry is on the other side of these relays.4. Where and how specifically you're measuring these waveforms.

author: CaptDon
author_rank: Super Contributor
timestamp: March 15, 2025, 01:31:31 pm
content: I hope you are not trying to pass 20MHZ through a solid state relay!!!!

author: Bud
author_rank: Super Contributor
timestamp: March 15, 2025, 02:36:02 pm
content: Quote from: ddsa on March 15, 2025, 08:38:32 amIt is not noise and not relay. You are upsetting the circuit by connecting long cable and pieces of wire to it. That introduces both inductance of the wires and capacitance of the cable. For instance, capacitance of RG174 coaxial cable (and you did not say if you even using coaxical cable) is roughly 100pF/m. So your 4m cable would load your 20MHz circuit with 400pF which is a lot. That is likely throwing the circuit off and it goes out of wack. That explains the change in frequency and the awful waveform. What you should do is either bring the oscilloscope close to the circuit and use a normal scope probe, or introduce a low load isolating buffer right at the relay common signal terminal so it isolates rest of the setup from the circuit.

author: fourfathom
author_rank: Super Contributor
timestamp: March 15, 2025, 07:38:39 pm
content: Quote from: CaptDon on March 15, 2025, 01:31:31 pmI use SSRs (Panasonic PhotoMOS) for frequencies beyond 100 MHz with excellent results. But I've also used small standard relays for RF as well -- it's a matter of layout and RF design techniques. It sounds like the OP has a layout / signal-routing and termination problem. But I can't explain why shorting the relay contacts makes the problem go away. I suspect we are not hearing the full story.

author: coppercone2
author_rank: Super Contributor
timestamp: March 15, 2025, 09:13:10 pm
content: Quote from: fourfathom on March 15, 2025, 07:38:39 pm

author: mtwieg
author_rank: Frequent Contributor
timestamp: March 16, 2025, 01:28:45 pm
content: Quote from: fourfathom on March 15, 2025, 07:38:39 pmInteresting, could you share the part number(s) you use? Looking at panasonic's website, it looks like many of these PhotoMOS parts are obsolete.QuoteI suspect they didn't actually short the relay at the relay's contacts (which would require soldering to their huge relay board), but instead just routed the wires around the relay board.

author: jonpaul
author_rank: Super Contributor
timestamp: March 16, 2025, 01:54:19 pm
content: mains hu: Sync scope on LINE , slow sweep to 10-20 mS/divLook for mains freq or 2x humBad grounds, bad probingj

author: fourfathom
author_rank: Super Contributor
timestamp: March 16, 2025, 04:32:57 pm
content: Quote from: mtwieg on March 16, 2025, 01:28:45 pmI'm using the 4-pin SMT Panasonic AQY282SX, which is a 60V/500mA part with <1 Ohm on-resistance (typ) and 0.8 pF I/O off-capacitance (typ). I get these through JLCPCB / LCSC, but I see that they are in stock at DigiKey, etc. I use these to switch in/out 50 Ohm termination and AC/DC coupling in a couple of designs working in the DC-100 MHz region.FWIW, Octopart says the lifecycle status is "production".

author: RFDx
author_rank: Regular Contributor
timestamp: March 17, 2025, 06:03:49 am
content: Quote from: fourfathom on March 16, 2025, 04:32:57 pmWith up to 80 pF across the SSR output terminals in its off-state perfectly fine for switching between DC/AC coupling but is it really suitable for switching a 50 Ohm termination in/out at frequencies up to 100 MHz or for probing different test points carrying 20MHz signals?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: March 17, 2025, 08:41:02 am
content: Quote from: ddsa on March 15, 2025, 04:48:06 amBelow are RF SSR's from Panasonic, they do have low Cout models,parts like AQY221N5TY are very cute little packages, ~1.1pf Cout, 20V, 2.8 Ohms, but you pay for the performance.

author: mtwieg
author_rank: Frequent Contributor
timestamp: March 17, 2025, 11:52:27 am
content: Interesting, they're claiming performance (Ron x Cout) on par with GaN FETs. Will have to sample a few to test.

author: fourfathom
author_rank: Super Contributor
timestamp: March 17, 2025, 05:09:23 pm
content: Quote from: RFDx on March 17, 2025, 06:03:49 amMy mistake -- you are correct about the output terminal off-state capacitance. I was looking at the I/O capacitance.So it's not a great part for general RF switching. In my applications that 80 pF isn't a significant factor.

author: mtwieg
author_rank: Frequent Contributor
timestamp: March 17, 2025, 06:13:21 pm
content: Quote from: fourfathom on March 17, 2025, 05:09:23 pmA lot of the datasheets I was browsing (like AQY282SX) didn't even mention the off-state switch capacitance, which is odd. In fact, if I search for that part number on panasonic's website, it doesn't even find a matchBut I did findthis FAQ pagewith some interesting info. Apparently many of these parts aren't based on optocouplers at all, and use capacitive coupling across the isolation barrier instead. And some parts have three separate switches in a tee network, to improve isolation at high frequencies. Not clear which part numbers these configurations correspond to, though... very messy documentation on Panasonic's part.