I'm currently having trouble getting a product through EMC. Radiated emissions were comfortably under the limit, but conducted are failing badly.
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):
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.
Edit Added PCB Layout after comments requested layout