GSM antenna causing interference - Page 1
Forum section: electronics / projects

author: Mad ID
author_rank: Regular Contributor
timestamp: August 31, 2015, 11:10:10 am
content: Hi,I'm developing a module which has GSM modem on board (SIM800C) and a Taoglas PA.25 GSM antenna.Here is the photo:You can see the antenna on the right, and the MCU on the left.The problem is that when the GSM power amplifier is transmitting the RF signal causes my MCU to draw almost short circuit current from the LDO. The LDO goes into current limit during transmission.I'm sure it's the RF signal because I tried with other antenna on a small coax and there is no problem when I move the antenna away from the board. As soon as I reduce the distance the MCU goes crazy. It's definitely the MCU becuase when I remove it and turn on the modem the problem is gone.To confirm the current draw I placed Dave's uCurrent in series with the LDO and can see current spikes during transmission. I know exactly when the modem is transmitting via the RF_SYNC pin of the modem and the spikes match exactly with RF_SYNC.I tried cutting all the MCU traces going to the modem and no result.I know that the antenna is transmitting at 2W+ but I never had a problem like this. What could be the issue? Where do I start? I tried everything. I even placed a small grounded shield can over the MCU..no result. I tried on three different boards..the problem is the same only the amplitude changes.It's probably picking up RF bursts via some MCU GPIO traces..but my scope cannot find it...My first task is to find what's wrong with these boards. My second task is to learn what I did wrong and avoid similar issues in the futureHelp!, thanks.

author: PChi
author_rank: Frequent Contributor
timestamp: August 31, 2015, 01:34:21 pm
content: Tricky, If it is the MCU then decoupling all the pins to a ground plane or possibly a grounded shield over the MCU or a piece of copper tape stick to the top of the MCU connected to it's ground pins with RF decoupling capacitors (a capacitor with a self resonant frequecy > GSM frequency, 22pf, 47pF, 100 pf are typical values).

author: Wilksey
author_rank: Super Contributor
timestamp: August 31, 2015, 10:09:17 pm
content: Post your schematic, more specifically, the PSU section for the GSM module and MCU.

author: Mad ID
author_rank: Regular Contributor
timestamp: September 01, 2015, 08:40:06 am
content: Quote from: Wilksey on August 31, 2015, 10:09:17 pmHere you go:GPRS is powered from the +3V7 Buck-Boost converter. The converter also powers the +3V1 LDO.I have noticed something interesting. If I put 100nF close to the output of the LDO the problem is mitigated by 2-3 times. I have a small resistor in series with the output cap because datasheet states couple tens of mR needed for stability.Do you see something wrong with my power supply?

author: PA0PBZ
author_rank: Super Contributor
timestamp: September 01, 2015, 10:10:20 am
content: Do you have the mandatory cap (100 uF) directly on the power input of the GSM module?This thing draws like 2A bursts while transmitting.

author: Mad ID
author_rank: Regular Contributor
timestamp: September 01, 2015, 10:13:36 am
content: Quote from: PA0PBZ on September 01, 2015, 10:10:20 amNop, it's not needed.My DC/DC has very good transient response, only 100mV drop at 2A impulse load.With bad transient response 100uF would not help, more like 1000uF would be needed.Any way, GSM modem voltage is stable.

author: Berni
author_rank: Super Contributor
timestamp: September 01, 2015, 10:54:52 am
content: The documentation for these GSM modules recommends a large amount of capacitance at the power pins. The transients are very sharp and the current draw is in very large pulses. This can not only deteriorate performance but also cause problems elsewhere. A large loop area to the source of the modules power might create strong AC magnetic fields that can affect other things. Also there could be large transients being pushed in to the main ground plane/traces, making it swing by 100s of mV in some places. That last one is potentially the worst as it could render a lot of analog circuitry completely non functional. There also might be potential RF energy radiating back in to the power supply where it can cause things like regulator ICs to act strange.A rule of thumb when you have devices that use large sharp transient loads is to have low ESR capacitance close by to take the brunt of the edge and bring power to it over a ferite choke so that the transients cant make it out to the main power rails and to reduce sharp transients in the ground path (But don't use ferite chokes on ground unless you absolutely know what you are doing).PCB Design can become a bit of an art sometimes because there are situations where the real world parasitic properties of PCB traces matter a lot. Sometimes it can even mean the difference between a design working or not.EDIT: By the way in your case it seams like the most likely cause is radiating RF power back in to the circuit. The way grounds are done might be the cause.

author: bookaboo
author_rank: Frequent Contributor
timestamp: September 01, 2015, 11:17:54 am
content: Quote from: Mad ID on September 01, 2015, 10:13:36 amAre you sure about the stability of that voltage? I've found that the current draw varies with inverse proportion to signal strength on these modules. Probably the module trying harder and transmitting at a higher power when signal is poor. It could be a very short transient sending your micro or p/s into a brownout.Even if it's not the issue now you will find lack of capacitance becomes an issue as your battery depletes, the GSM module will cause transient voltage dips as the battery impedance increases, even though the battery may still have plenty of capacity to do other things. In some applications I've used 40mF supercaps to prevent this.I'd also check that you have used star routing for GSM power and routing.

author: Mad ID
author_rank: Regular Contributor
timestamp: September 01, 2015, 11:20:35 am
content: Quote from: bookaboo on September 01, 2015, 11:17:54 amHi, yes I'm sure.Since the LDO is powered form the DC/DC i have placed oscilloscope probes at the input of the LDO and the output.The input voltage is always high enough to allow for the dropout.This would not explain why I see >1A current spikes from the LDO! during the transmission which is on the other power rail..

author: bookaboo
author_rank: Frequent Contributor
timestamp: September 01, 2015, 11:24:22 am
content: A brownout could do weird things, try totally seperate supplies so that the micro supply is unaffected by the GSM module. If you still have a problem then the issue is radiated EMI, if the problem is removed then it's a powersupply/regulation issue.

author: Mad ID
author_rank: Regular Contributor
timestamp: September 01, 2015, 11:26:34 am
content: Quote from: bookaboo on September 01, 2015, 11:24:22 amI have tried to replace the LDO with a Rigol channel with current limit at 300mA. No problems in that case. But this could also be the case if the Rigol has a lot of capacitance and can supply the short-circuit current?Interesting thing when I place 100pF directly at the output and input pins of the LDO problem is mitigated by 5x.

author: bookaboo
author_rank: Frequent Contributor
timestamp: September 01, 2015, 11:44:40 am
content: Could be the LDO is going into self oscillation, nothing to do with what it's supplying but could be the input/output capacitance is not correct. My guess is that something is induced either on the input or output of that LDO that sends it crazy. Try a diode from the 3.1V to the 3.7V, ive seen that work. Although a better plan of attack might be looking at killing the source of the noise rather than treating the symptoms.

author: Berni
author_rank: Super Contributor
timestamp: September 01, 2015, 12:47:34 pm
content: Most lab power supplies have fairly large output caps to give them good transient and noise performance.If adding 100pF helps then you have a regulator osculation problem. Your current filtering caps are ether too high ESR or too low ESR or the wrong capacitance. Or it has something it doesnet like near it.

author: Mad ID
author_rank: Regular Contributor
timestamp: September 01, 2015, 01:33:13 pm
content: Quote from: Berni on September 01, 2015, 12:47:34 pmI'm trying to isolate the problem. The caps are per datasheet together with the ESR. That's why I have 0R47 in series with the 10uF.

author: Wilksey
author_rank: Super Contributor
timestamp: September 01, 2015, 02:30:28 pm
content: Not enough bulk capacitance to suppress the transmission bursts.Stick a couple of large capacitors on the output of the LDO and I bet your problems will go away.I have a reference 5v -> 3.6v GSM board from a supplier and there are 2 x 1500uF 6.3V caps on the output of the LDO.

author: Mad ID
author_rank: Regular Contributor
timestamp: September 01, 2015, 02:32:19 pm
content: Quote from: Wilksey on September 01, 2015, 02:30:28 pmBut why? The LDO is not powering the modem! Modem is powered by DC/DC converter.

author: Berni
author_rank: Super Contributor
timestamp: September 01, 2015, 02:43:01 pm
content: Yes but the power source that is powering the modem is also powering the LDO. The sharp transients can then feed noise to the LDO that might tip it over the edge in to the unstable region. Regulator ICs are pretty ineffective against high frequency components of noise, those are best taken care of using passive filters.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 01, 2015, 02:50:34 pm
content: Try 100pF from FB to GND, at the input to the LDOs. This has the effect of delaying the feedback slightly, but it shouldn't be enough to cause instability; the main benefit is RF filtering, a lack of which upsets the internal state of the controller chip.RF gets into everything, and ideally you should have small bypass caps *everywhere*. Or shielding.If you have another project like this, you might even consider building the PCB as a four-layer construction with the outer layers for GND/VCC and traces routed inside. This is a pain to rework and troubleshoot, but has the best radiated susceptibility short of enclosing the whole thing in shield cans.Tim

author: Mad ID
author_rank: Regular Contributor
timestamp: September 01, 2015, 03:44:29 pm
content: Quote from: T3sl4co1l on September 01, 2015, 02:50:34 pmI have put 100pF from Vout to FB because in this case is worked better.I have a working solution now, included three things:1) 2k2 ferrite bead on the input (for some reason it helps a lot)2) 100nF at the output together with already present 10uF3) 100pF from Vout to FBProblem is resolved or at least it's 5x better. This is the most strangest issue. Looks like my LDO Vout/Vin is picking up noise causing it to oscillate. Caps were per datasheet from the beginning. Any more info on why this would happen?

author: PChi
author_rank: Frequent Contributor
timestamp: September 01, 2015, 05:42:09 pm
content: I have also come across situations were LDOs have picked up RF and misbehaved and 100 pF capacitors have been sprinkled around it. The impedance of the capacitor at the GSM frequency is the issue along with the layout. A mm or two of PCB track can have a large effect. I guess that the RF is being rectified by junctions / circuits in the LDO and causing output voltage shifts while the GSM signal is on. It's the same cause as the 'bumble bee' buzz that can be heard on audio circuits when Cell Phones are communicating.I second the suggestion by Tim of using ground planes. Without them decoupling RF just moves the signal around the board in an unpredictable way.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 01, 2015, 09:40:29 pm
content: The basic principle is, the internal structure of any IC has transistors, somewhere. (Obviously!) Transistors are nonlinear, so given enough RF, they can be driven on and off to some extent, regardless of the DC voltage that's supposed to be present.The most common manifestation of this is bipolar op-amp inputs. The input structures are B-E junctions, which tend to rectify RF. A few mV will do it. At low frequencies, they follow the input as usual, but at sufficiently high frequencies, the internal collector and emitter capacitances charge up, causing input bias shift.For LM358s, this is significant even in the 100s of kHz range (and harmonics thereof), so you want to be careful even in switching supplies using parts like this. (TL431 being a common example.)JFET and MOS op-amps are less sensitive, and digital circuitry (usually CMOS) even less so, but the same phenomenon still applies.With some very gross waves of the hand, you can estimate the amount of voltage introduced based on the voltage on the antenna (which you can, in turn, make some guesses about..), given the ratio of distances between antenna, trace and ground reference, and how close to 1/4 wave (or 2N+1 multiples) the trace length is (or 1/2 wave or multiples if low impedances are on both ends). I'm going to guess it's in the volt or fractional volt range here.Best solution, put that antenna as far away as possible.Tim

author: qno
author_rank: Frequent Contributor
timestamp: September 15, 2015, 08:53:34 pm
content: Do you have a 4 layer pcb with at least one layer an uninterrupted ground plane??

author: Mad ID
author_rank: Regular Contributor
timestamp: September 16, 2015, 07:39:46 am
content: Quote from: qno on September 15, 2015, 08:53:34 pmNo, 2-layer board but bottom layout is quite good ground plane. I have done 5 boards with GPRS on 2 layers without a problem before..this is new.

author: tggzzz
author_rank: Super Contributor
timestamp: September 16, 2015, 08:03:19 am
content: Quote from: Berni on September 01, 2015, 12:47:34 pm.... and "large" inductors between the PSU and load. Take 1nH/mm as a rule-of-thumb starting figure. Inductor + capacitor => resonance.

author: Mad ID
author_rank: Regular Contributor
timestamp: October 18, 2015, 07:49:44 pm
content: New system, same problem.I'm using TPS7A7001DDA to power a SIM900 with an onboard antenna. When the modem transmitts the LDO is shutting down with voltage dropping by 1.5V.If I take an old feature phone like Nokia 3310 and try to call somebody while holding the phone near the LDO, the LDO is shutting down. 900MHz RF is causing it to fail.With careful decoupling this can be fixed. There are test for RF susceptibility of ICs called DPI or Direct Power Injection.For the respin I included ferrite beads on the input and the output forming RC filters and hope they will dissipate most of the RF energy going into the LDO.Do you have more suggestions how to solve these issues?Cheers

author: AF6LJ
author_rank: Unknown
timestamp: October 18, 2015, 08:08:04 pm
content: I think the first thing I would have done is provided a proper grounded shield for all my non RF electronics.

author: Mad ID
author_rank: Regular Contributor
timestamp: October 18, 2015, 08:24:19 pm
content: Quote from: AF6LJ on October 18, 2015, 08:08:04 pmMost of the designs using GSM modems don't shield the digital circuitry...maybe I'm just unlucky with my LDO choice.