EMC test failure: LC filter

Question

I am currently developing my first product which needs to fulfill EU EMC requirements.

Below you can see the first result out of the EMC lab (radiated emission test):

One can clearly see the 16 MHz clock frequency and the radiation out of the product seem to go over the power lines through multiple connectors from the main PCB.

This assumption also comes from the following results of a measurement without having the connectors attached:

So, now I am designing an update and beside optimizing the layout regarding the crystal, uC decoupling, trace length reductions, and provisions for ferrite beads on the signal lines, but I am not really sure about one countermeasure - an LC filter.

Here I would be very thankful to get some help.

Questions:

1. I want to add an LC filter on every power line right before the connector. But here I have the first issue - which cut-off frequency needs to be used? Is it the frequency of the first peak (32 MHz) or, as I read a lot, 1/10th of the first peak (3.2 MHz)?

2. Filter: I have a connector which has 1 x 12 V power and 3x switched low-side power (PWM/MOSFET). Does it make any sense to add filters to these lines, or is it just the power line which makes sense to filter?

Answer 1

Question 1 - If I'm interpreting your test results correctly, you are missing your spec by approximately 18 dB at (I think) 32 MHz. That means you need to attenuate that frequency by at least 18 dB. Hence the filter needs to be designed for a cutoff frequency of much less than 32 MHz, depending upon the order of the filter.

The peaks at 140 MHz and around 200 MHz are not directly related to the 16 MHz crystal oscillator. The spectrum of the oscillator should roll off with increasing frequency. So those frequencies are indicative of other parts of the circuit that are switching at ~140 MHz and ~200 MHz.

I know this solution probably isn't in your solution space, but this is one of the reasons we routinely shield cables, especially signal cables. Input power filtering is usually taken care of with appropriate common mode filters.

Answer 2

I want to add an LC filter on every power line right before the connector. But here I have the first issue - which cutoff frequency needs to be used? Is it the frequency of the first peak (32 MHz) or, as I read a lot, 1/10th of the first peak (3.2 MHz).

Do something easier first and add a ceramic 0.1uf cap between the terminals and ground of the connectors while you are testing. If you use wire, use a high gauge with lower inductance (not 30awg), the higher the better. Usually it's a good idea also in future designs to have connectors with the option for shielding in the case of radiating cables. Make sure all cables are twisted to minimize radiation.

Filter: I have a connector which has 1x 12 V power and 3x switched GND (PWM/MOSFET), does it make any sense to add filters to the GND lines or is it just the power line which makes sense to filter?

Have two things you can do. either make the impedance high enough in or before the cable the high frequency signals will not travel down the cable. Another option is to short the high frequency signals to ground with capacitance. One problem with this is you need to make sure that your capacitance does not have parasitic inductance (parasitic inductance arises from any copper traces, vias or even the leads of capacitors, which creates small amounts of series inductance that can block high frequency signals, these effects must be accounted for when creating a shunt to ground, at lol at the impedance graph of the shunt capacitor)