Electronic – SIM900 Conducted Emissions Fail


I'm currently having trouble getting a product through EMC.
Radiated emissions were comfortably under the limit, but conducted are failing badly.

Here are the results:
enter image description here

The noise appears to be largely common mode, with the exception of the first few big spikes which are differential.
The first of the big spikes is at the SMPS switching frequency. I'm guessing the next 3 are harmonics of that.

Here's a schematic of the relevant/noisy parts of the circuit (the rest of the cicuit appears to be very quiet. The noise is only a problem when these parts are enabled):

enter image description here

Anyone have any ideas on how I might tackle this?


The cable is a pair of discrete un-shielded wires, ~2 meters long, connected to the 'BATT' input and GND.
The BATT input is the power supply input to the module, nominally 12V DC (vehicle battery).

The components (along with a micro and a few other bits) are all on the same PCB.
PCB is 4-layer with an ground plane on an internal layer. Unused areas on top and bottom layers are GND filled.
The module enclosure is plastic/ABS.

The SIM900 is a GPRS module. It was powered for the test and talking to the micro via UART, but not connected to a mobile network.

Added PCB Layout after comments requested layout

enter image description here

Best Answer

As a rule High frequency signals will always take the path of lowest impedance to get back to the source (or radiate). I can't speak for your whole setup because you have currently only posted a schematic.

I can tell you this, if its conducted emissions that is you problem on the battery cable, then you are making a nice antenna out of you battery cable. So applying the rule: the current's most preferred and lowest inductance path is through your cable and radiating out into the world. To find out if this is the case, increase the high frequency impedance of the cable. This can be done by adding inductance, either by ferrite or by adding inductance on the PCB with inductors that attenuate high frequency "noise".

If it is your board radiating, then you probably don't have a sufficient ground plane or you have made some nice slot or dipole antennas. But I'll digress on speculating since you haven't provided any information on the PCB.