Help: Strange glitch seems to be killing my RF modules - Page 1
Forum section: electronics / projects

author: Psi
author_rank: Super Contributor
timestamp: February 12, 2023, 03:07:15 am
content: For the past while ive been having an odd issue where sometimes the RF modules would die and I'd have no idea why.I think I managed to capture on the scope when it happens. Or at least there is something that looks very suspicious as the culprit but I'm not sure why it happens.When power is connected (normally 2 AAAs but this time its PSU at 3.0V) i can see a rather large and overvoltage spike of 3.7V on the REG_EN line to the RF transceiver this is having this failure problem.REG_EN should be low until the MCU sets that output high 100ms later after 3.3V power is stable.See diagram of the 3.7V pulse I'm talking about.C1 yellow is the 3.3V boost regulator starting up, scope probe is connected to this at the RF module VCC pin.C2 purples is the REG_EN line connected at the RF module pin.Note that only C1 had a ground clip at the time. C2 did not and was only grounded through the C1 ground clip.I will try repeat it with better grounding but for the movement this is all I have.I've also posted the schematic, does anyone have any clues what might be causing this 3.7V?

author: Psi
author_rank: Super Contributor
timestamp: February 12, 2023, 03:18:28 am
content: EDIT:Sadly i think that pulse was just bad probing, can't seem to get anything when i improved the grounding on channel 2.Damn, this issue is really starting to annoy me.oh well, i guess i have to look elsewhere.

author: Psi
author_rank: Super Contributor
timestamp: February 12, 2023, 03:36:15 am
content: Strange question.With soldering irons that use the curie heat tech and run at 13.56MHz. Are there any risks when soldering sensitive RF chips that the soldering irons high power 13mhz RF might somehow get into the chip and destroy/damage it?Maybe in combination with an ungrounded desk anti-static mat that a pcb being soldered is resting on?

author: Psi
author_rank: Super Contributor
timestamp: February 12, 2023, 04:02:34 am
content: The RF chip in question is the EM9209, in case anyone has used it before or knows of any issues with it.https://www.emmicroelectronic.com/product/proprietary-protocols/em9209Given what i've seen happen today with testing I am thinking the issue maybe relayed to how the chip is powered on.Something to do with sequencing.The RF chip/module seems to draw a constant 20-40mA more current (and sometimes fail totally) if you have a 100k pull-downon REG_EN, so that input is low(disabled) at power on, and only pulled high 100ms later by the MCU followed by getting SPI initialized.Verses much less current and "seemingly" less failures- If you have a 100K pull-upon REG_EN, so it is high(enabled) at power on, and then within 4ms of MCU boot start REG_EN is pulled low. Then the MCU waits 100ms. Then it's set high and the chip gets SPI initialized.Strange.

author: TurboTom
author_rank: Super Contributor
timestamp: February 12, 2023, 10:35:14 am
content: What's the purpose of C13 at the gate of the battery power switch? This capacitor will cause a power spike on the rail everytime the battery is connected. Depending on the layout, this may result in all kind of ringing on the rails, possibly resulting in overvoltage spikes. I'ld rather have the cap connected across R15. But maybe you've placed it there for a reason.

author: Psi
author_rank: Super Contributor
timestamp: February 12, 2023, 10:44:06 pm
content: Quote from: TurboTom on February 12, 2023, 10:35:14 amIt just adds lag so if you press a button quickly power stays on long enough to start the MCU up.Starting from the state of battery connected but Q2 off, so nothing is on...Then you press a button down. That causes a momentary current flow through C4/C5 to turn on Q3 and then Q2.Now we have power to the Vreg and that begins to start up, but the MCU has not had time to startup yet, (power/clock startup delay).If the user lets go of the button at this stage Q3 turns off, Q2 turns off and power is cut.The MCU has not had time to execute code and set the PWR_HOLD line high to keep power on.So C13 is adding some delay so after you let go a button the power stays on for a short time. Enough for the MCU to boot up.I could move it to the gate of Q3 though, doesn't have to be where it is now.

author: TurboTom
author_rank: Super Contributor
timestamp: February 13, 2023, 08:49:02 am
content: I'm aware that C13 adds delay to the reaction time (especially when powering down) the active power switch. But since this switch is implemented as a P-Channel MOSFET, pulling it down (momentarily) through C13 will enable the power. Initially, before connecting the battery to your circuitry, I'ld assume C13 is discharged. Now when you connect the power source (insert the battery), C13 will pull the gate of Q2 low until it's charged through R15. As long as the voltage across R15 is above Q2's threshold, all the spikes from connecting the battery will be directly fed to the positive rail of your power supply circuitry. If this is a problem or not is somthing that I cannot answer. But connecting C13 across R15 would immediately eliminate this issue.

author: TheDefpom
author_rank: Frequent Contributor
timestamp: February 17, 2023, 11:06:31 pm
content: A couple of things come to mind for me, one is to clamp the input voltage out of the main voltage regulator by adding a 3.3V Zener (or TVS diode) across its output to suppress any spikes that could be there.The other thing is the RF side, are you sure that the antenna system is well tuned for the operating frequency and is not suffering from excessive reflected power or any matching issues?

author: Psi
author_rank: Super Contributor
timestamp: February 18, 2023, 02:23:44 am
content: Quote from: karpouzi9 on February 17, 2023, 11:16:14 pmA lot of the high-end irons use the RF tech, i'm 99.5 sure its fine and not the issue, but it just occurred to me so i though id ask.Quote from: TheDefpom on February 17, 2023, 11:06:31 pmI have checked the 3.3V rail with a scope many times. Apart from that pulse that turned out to just be bad probing i've not see any over voltage problems. But yeah, i could add one just to be safe.The antenna is a copy of one i found online, the range is excellent so I'm pretty sure its not tuned that badly.It's this onehttps://www.electronics-lab.com/designing-with-an-inverted-f-2-4-ghz-pcb-antenna/

author: Psi
author_rank: Super Contributor
timestamp: February 18, 2023, 02:29:21 am
content: My current suspicion based on when they fail more vs less often is something to do with voltage startup and how quickly the voltage rises.I have a theory that if the voltage rises too slowly all the internal RF registers get set randomly before the thing has enough voltage for the internal MCU to start and init them correctly. So if the voltage rises too slowly you get a period of time when the RF module maybe outputting continuous or something and it burns out. I plan to do some testing with a pfet to control power so i can wait some time before turn on power to the RF module

author: coppercone2
author_rank: Super Contributor
timestamp: February 18, 2023, 03:07:51 am
content: what kind of part could overload so quickly from a soft error. It should be able to put out continuous at max power all day unless you have a seriously poor thermals and its a huge pace of shitwhat you say is plausible if the control system is not bounded properly. Kind of like using a 15v control signal for 3.3V maximum and hoping the software does not let it go above 3.3, usually these things are prevented on a very low level..... or the load is mega improper. I don't see that chip burning itself out from a soft error.That is really bad if that kind of bug got through their design review for silicon lol, essentially latch up burnout. It implies there is like a 'illegal' state (illegal duty cycle) going on.. mfao its like a business burning itself down because of a malfunctioning bureaucracy, forgot to put a circuit breaker on the copy machine after a garbled phone call came in demanding a 50000 copies by the end of the day.I would get some more parts from a different distributor or something before jumping to that conclusion, my gut feeling says they should have strongly considered this possibility during design review. Actually who am I kidding, someone said 'that cant happen'.

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: February 18, 2023, 11:36:22 am
content: Quote from: TheDefpom on February 17, 2023, 11:06:31 pmI second this. This should fix the voltage spike.As for the reason of the spike, I am not sure but it looks like the duration of the spike is very close to 2.4GHz, which makes me think it may be related to the transmission. The radio chip consumes 36mA while transmitting which is not a lot.What kind of decoupling is present on the "remote PCB"? Is it high ESR type caps or what? The decoupling on VCC rail is a bit weak but I suppose with a DC-to-DC converter, its understandable.

author: nctnico
author_rank: Super Contributor
timestamp: February 18, 2023, 05:31:59 pm
content: Quote from: Psi on February 18, 2023, 02:29:21 amThat could be. I recently came across an LDO that needs a slow rising input otherwise the output will overshoot massively (like outputting 4.5V instead of 3.3V).Who puts such crap on the market?

author: coppercone2
author_rank: Super Contributor
timestamp: February 18, 2023, 05:47:36 pm
content: Quote from: nctnico on February 18, 2023, 05:31:59 pmthat cant happen!

author: wraper
author_rank: Unknown
timestamp: February 18, 2023, 06:20:54 pm
content: Quote from: nctnico on February 18, 2023, 05:31:59 pmIs output cap ESR requirement met for that LDO? It would not surprise me if something like that happens with LDO requiring high esr actually used with ceramic cap.

author: coppercone2
author_rank: Super Contributor
timestamp: February 18, 2023, 06:53:45 pm
content: The LDO would be short though, at least that is what I have seen. I think the OP would discover the bad regulator after the test. Unless some LDO don't burn out from that.

author: Psi
author_rank: Super Contributor
timestamp: February 18, 2023, 09:19:34 pm
content: The module does have an LDO built into the chip, and it does have some effect on the issue.The failures seem to happen more often when the module is first powered with its LDO enable line set low (LDO off).I see less failures if that enable line is high at startup (on), then I wait 100ms and toggle it low/high to cause a reset.Kind of like the RF section glitches, gets stuck in a state and since the LDO is off the internal MCU is off so never clears that state and something fries.

author: rfclown
author_rank: Frequent Contributor
timestamp: February 18, 2023, 10:22:44 pm
content: Quote from: Psi on February 12, 2023, 03:36:15 amRF irons like Metcal are not an issue. They are all that I've used at home and at work (and everyone else at work) for years. I work on RF.Grounding/static is a completely different issue. I'm lazy about it and haven't had it bite me, but that doesn't make it good practice. I wouldn't guess that's the problem here. If you zap a chip while soldering, it most likely won't work from the start, but stranger things have happened.

author: Psi
author_rank: Super Contributor
timestamp: February 18, 2023, 10:29:14 pm
content: Quote from: rfclown on February 18, 2023, 10:22:44 pmGood to know

author: coppercone2
author_rank: Super Contributor
timestamp: February 18, 2023, 11:29:38 pm
content: I did not read the datasheet because its big*the below is about the internal ldo*Does the chip bypass the LDO and draw power directly and operate when the LDO is turned on?If the chip is shut down, and then a turn on signal causes a failure, that means there is a faster voltage increase on the chip since the rail is stabilized. If the LDO is on while the voltage rail is settling, that means that there is a slower voltage increase on the chip.Like in terms of dI/dT. Not clear if its a part of the chip or the entire chip controlled by the internal LDO. In that case it sounds like it needs series impedance, not less impedance. If its a LDO for the RF section only while the digital section, then:And it also might mean that the LDO is absorbing a transient that occurs on power up, since its part of the load for your system, and if the LDO is disabled, there is no circuitry that fights the rail behavior. Like a turned on LDO might have some kind of internal protection circuitry that is protecting the rail as a active shunt load, possibly by undefined behavior that occurs on the power up cycle of the chip (the rising edge makes whatever circuitry go into some kind of accidental protection mode).Can you put a resistor in series with the main supply to see if the problem gets worse or better? This makes me think of a schmitt trigger.

author: Kim Christensen
author_rank: Super Contributor
timestamp: February 19, 2023, 12:43:29 am
content: QuoteYou're shorting the antenna system to power ground. It needs to be floating. Pin 11 "Antenna negative" of the module shouldn't be connected to "ground" Pin 8, because that would be bad. Look at the the pinout of the chip:

author: georges80
author_rank: Frequent Contributor
timestamp: February 19, 2023, 02:44:54 am
content: ... and the application in the datasheet mentions a 200 ohm pcb loop antenna and the need for a 1:4 balun if you want to interface to a 50 ohm antenna.

author: Psi
author_rank: Super Contributor
timestamp: February 19, 2023, 09:59:29 am
content: Quote from: Kim Christensen on February 19, 2023, 12:43:29 amPin 10 (ANT-N) isn't being used and is not connected.but yes, pin 9 (ANT-P) is shorted to RFGND (pin12) through the antenna.hm.. I will trace out the castellated pad module and post the parts on it.Perhaps this is the problem. More testing required.

author: wraper
author_rank: Unknown
timestamp: February 19, 2023, 10:17:51 am
content: Quote from: Psi on February 19, 2023, 09:59:29 amYou thoroughly ignored what's in the datasheet and application note. Your circuit is totally broken. I'm surprised it works at all. How the thought even came to you that you can just leave an RF output unconnected and connect to GND what shouldn't be? Even without a capacitor in series. A brief glance in the datasheet (page 1, 6, 42) makes very obvious how it's done. Not to say there is an appnote.EDIT:I'm actually confused. You have RF2401 module in schematic which has a single antenna output. But then you write it's EM9209 and ANT-N pin isn't used. So what do you use actually? RF2401 module, bare EM9209, or what? RF2401 is not EM9209 based for sure.

author: georges80
author_rank: Frequent Contributor
timestamp: February 19, 2023, 12:20:20 pm
content: And if you are using the em9209, you need a balun to drive the 50 ohm inverted F antenna. As above, your antenna connection is totally messed up. Check their appnote for 50 ohm hookup. Also provide an accurate schematic ...

author: coppercone2
author_rank: Super Contributor
timestamp: February 19, 2023, 04:23:21 pm
content: well having a... floating antenna ground is very irregular. Does the chip have a transformer on board?

author: wraper
author_rank: Unknown
timestamp: February 19, 2023, 04:44:21 pm
content: Quote from: coppercone2 on February 19, 2023, 04:23:21 pmOf course you cannot leave particular antenna floating without GND. AFAIK most of chip antennas normally are not connected to ground (of course PCB GND copper pour is part of actual antenna). But it's not the main problem, and would not matter if done properly.QuoteIt requires a balun to feed unbalanced antenna. Look into datasheet.

author: Psi
author_rank: Super Contributor
timestamp: February 19, 2023, 08:42:18 pm
content: yep, I think the issue is that this module was intended only for a 'length of wire' antenna and I was not aware of the implications of that.Thanks everyone!I'm sorry i cannot provide some of the specific information people have requested, but there are reasons.

author: Psi
author_rank: Super Contributor
timestamp: February 20, 2023, 06:50:09 am
content: Ok, so shorted ant (at DC) was the issue. Seems the thing starts up a bit weird and some units put some DC on the RF output for a short time at power on.Some modules seem to latchup, or something similar at first power on. Others just draw a bit more current than normal and most don't seem to care and draw normal current.Removing the shorted antenna fixes modules that are are showing the problem.Quote from: georges80 on February 19, 2023, 12:20:20 pmThe module datasheet says it's designed to work with a 50 ohm antenna already, but the PCB has no balun. Which is what messed me up. In reality the module just has a crystal, few bypass caps and a few resistors and caps on the positive antenna chip output. (negative ant output is floating).Quote from: wraper on February 19, 2023, 10:17:51 amYes it is.To those asking why I've not read the EM9209 datasheet, I only found out the IC is a em9209 very recently due to the module manufacture shipping a small batch where the chip was not rebranded like it normal is. Otherwise I wouldn't know. I previously only had the datasheet for the module itself which is very simple and doesn't mention much about the antenna requirements.I have not posted the module datasheet because I really want to avoid naming the company in this thread as it might make things more difficult for me in the future if they find the thread and match it to me. I don't want to go into exactly why, just thought I should explain that. Please don't name the company if you find it from googling id like to keep google from indexing it to this thread if possible.Thanks everyone!Oh, one thing someone might know the answer for.Now that i know the problem i have to cut some PCB antenna tracks. Do you think its betterto cut the track in the A or B location (to stop the gnd short and keep most of the performance)

author: Psi
author_rank: Super Contributor
timestamp: February 20, 2023, 08:51:24 am
content: Different for me, 1st is antenna diagram and 2nd one is pic of the module itself.

author: wraper
author_rank: Unknown
timestamp: February 20, 2023, 09:31:55 am
content: Quote from: Psi on February 20, 2023, 06:50:09 amYou certainly cannot just cut this antenna. RF2401 datasheet says it needs 50 Ohm antenna. And it says nothing about antenna DC isolation from GND.

author: wraper
author_rank: Unknown
timestamp: February 20, 2023, 09:43:51 am
content: Comparing RF2401 datasheet reveals copy-paste from EM9209. But there is no Balun on RF2401 module. Dunno, maybe they consulted with chip manufacturer about this being OK for cost reduction.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 20, 2023, 09:50:25 am
content: Stupid question: why would anyone use a radio module which contains just the IC + oscillator + a few passives, but not the antenna nor matching components?Seems to me going for full module-less design removes uncertainties for not much added effort, or on the other hand, using a module with integrated antenna (or U.FL connector) saves from a lot of design work (and possibly simplifies certification).

author: wraper
author_rank: Unknown
timestamp: February 20, 2023, 09:51:12 am
content: Overall it's strange. Chip manufacturer in the datasheet says 200 Ohm impedance and that 4:1 balun is needed for 50 ohm antenna, then in the app note and in characterization in datasheet they use 2:1 balun 2450FB15A0100. Also this balun has direct connection to chip and GND, so it seems OK for chip antenna outputs having DC path to GND.https://www.emmicroelectronic.com/sites/default/files/products/datasheets/609014.pdfhttps://www.johansontechnology.com/datasheets/2450FB15A0100/2450FB15A0100.pdf

author: Psi
author_rank: Super Contributor
timestamp: February 20, 2023, 12:12:51 pm
content: Quote from: wraper on February 20, 2023, 09:31:55 amOh you can just cut it, i just didPretty sure the datasheet is just wrong.Range didn't seem to change much, actually got a bit better but I think that was just random test variation.The modules come with a small length of wire as the antenna, so that is what they are intended for and not a short to GNDQuote from: Siwastaja on February 20, 2023, 09:50:25 amIt's a low cost module, so meh. Now that i know how it works it's fine.It actually works really well. Range is excellent

author: Kim Christensen
author_rank: Super Contributor
timestamp: February 20, 2023, 04:35:57 pm
content: Quote from: wraper on February 20, 2023, 09:51:12 amStrange indeed. Especially since that Johnson balun is the isolated type and wouldn't ground the balanced RF pins of the chip if wired correctly. But alas, the application note you linked shows pin 2 (bias) being grounded.It could all be 100% clarified if Johanson provided a schematic/winding diagram of the balun's internals.Quote from the Johanson site:Quote

author: wraper
author_rank: Unknown
timestamp: February 20, 2023, 04:55:43 pm
content: Quote from: Kim Christensen on February 20, 2023, 04:35:57 pmWell, according to balun datasheet in should be directly grounded if there is no DC feed. As of chip datasheet I didn't see anything prohibiting DC connection to GND. The only thing I've seen that outputs have 170K bias to VDD_RXTX.

author: georges80
author_rank: Frequent Contributor
timestamp: February 20, 2023, 05:02:53 pm
content: Contact the manufacturer support and find out what they actually require.https://www.emmicroelectronic.com/sites/default/files/products/files/emrf9209sma-fs.pdfis another EM9209 module/datasheet (on their website) and that has a 200 ohm balun part number and schematic.Given the discrepancies in the EM info, I would reach out to their support for clarification, otherwise you're just 'experimenting' versus doing actual engineering.Cutting the Inverted F turns it into a piece of copper. If you don't know what you're doing with the antenna, then you are also just 'experimenting'. Yes, it'll radiate, but it is not running the chip to spec. Would you be happy if it now works just fine but fails in a month or two? Is this a home project or are you making a real product?Certainly you need a balun to get from the balanced drive of the EM chip to the unbalanced 50 ohm requirement. An inverted F is inherently a 50 ohm unbalanced antenna (if cut/layed out correctly).Contact the support group and get some facts to work with. They may even provide some engineering support for your design, in terms of the antenna, RF, layout considerations. Of course if FCC etc isn't important to you, then hack away...

author: Kim Christensen
author_rank: Super Contributor
timestamp: February 20, 2023, 09:40:33 pm
content: Quote from: wraper on February 20, 2023, 04:55:43 pmThat's all true. It's really poor documentation for both the chip, module, and balun.But when I see a pin saying it's biased to X voltage, I wouldn't be connecting it to ground and expecting it to work properly. PSI has basically confirmed this for us. It would be interesting to see what DC voltage he now measures on the floating antenna.Also if you look at the block diagram of the IC, and then read the descriptions of the antenna pins (Positive and Negative antenna terminal), how they describe the antenna (Direct antenna interface 200 Ω differential), etc. it again seems to point away from grounding the RF outputs.I noticed something strange in the PDF georges80 just posted above me:

author: georges80
author_rank: Frequent Contributor
timestamp: February 20, 2023, 10:02:48 pm
content: ^ probably just a schematic drawing error. The PCB layout shows 2/5 connected to ground and the balun datasheet also states 2/5 go to ground.The EM 'information' is pretty poor in the appnote dept.But, bottom line: the datasheet specs 200 ohm diff drive that requires a 1:4 balun to go from balanced to unbalanced 50 ohm. The OP cutting the inverted F 'ground' stub and expecting it to function properly is lacking any RF knowledge.The OP needs to get hold of the EM support folk and get full clarification of how to connect the part to a 50 ohm inverted F antenna. Unless this is just an experiment/plaything and not going to be a production design.

author: wraper
author_rank: Unknown
timestamp: February 21, 2023, 07:17:50 am
content: Quote from: georges80 on February 20, 2023, 10:02:48 pmHe did what RF module datasheet suggests. Why module does not have a balun is another question.

author: georges80
author_rank: Frequent Contributor
timestamp: February 21, 2023, 10:34:50 am
content: Quote from: wraper on February 21, 2023, 07:17:50 amOk, yep, I see that. Doesn't help than xxxxRF named it an rf2401 module that has too much in common with the nordic nrf24L01 chip name.Looking closer at the module, it's a) clear that xxxxRF just copied the em9209 datasheet and b) they have some kind of basic network on the antenna pin(s). The problem is that the OP tried using an Inverted F antenna and it has a DC short as part of the topology. A typical rubber duck has an impedance of 50 ohms, but it doesn't have a DC short/path. Also a wire antenna cut to length and soldered to the module ANT pin would also not have a DC short/path. So, either wire or rubber duck would not cause the EM9209 shortcircuit heartache in its RF output.It's the Inverted F that brings the lack of a balun to the forefront with the xxxxRF module.By cutting the GND connection of the Inverted F, the OP has basically created a wire antenna, obviously not tuned to 1/4 wavelength, but even with the mismatch it would somewhat work. Range would be compromised and loading would be off - but then the whole xxxxRF module's antenna path is dubious.So, no blame to the OP since yes, he followed the module 'pinout' and tried using an inverted F antenna.

author: Psi
author_rank: Super Contributor
timestamp: February 21, 2023, 10:46:25 am
content: Quote from: georges80 on February 21, 2023, 10:34:50 amyeah, exactly.

author: rfclown
author_rank: Frequent Contributor
timestamp: February 21, 2023, 10:12:32 pm
content: In general, if you have a DC bias on the Tx output and an antenna which is a short circuit to ground, you put a DC blocking capacitor in the feed line. You wouldn't cut the antenna structure like the picture you showed. You'd cut the 50 ohm feed line and put a capacitor like 100 pF in series. Not uncommon when using a PIFA.