Hi,
I have a design that I've found to be radiating EMI with a spectrum analyzer and a few near field probes. My design currently doesn't have any EMI filtering. I'm thinking of using ferrite beads to reduce EMI. Other than the power and ground lines, I'm wondering where else would they help? Here are a couple of places I thought may help
- My design has a membrane keypad that connects via an FPC connector. The keypad acts like an antenna, and radiation is vastly reduced when I place it on a metal surface. Since right now I'm unable to change the mechanical design/material of the casing, I'm thinking of of putting ferrite beads on all of the keypad signals. These are very low frequency signals so I think should be fine?
- My design has a daughterboard that is connected with a 2x22 IDE type connector that transfers IDE signals. I also find that this connector leaks quite badly. However, since IDE signals are pretty high speed, I'm wondering if using regular ferrite beads will cause any signal problems?
Btw board boards have separate power and ground planes. The ground plane of the daughter board is connected via the ground pins of the IDE signal, and the daughter board actually helps to shield radiation coming from the main board. I've tested this with the probes with the daughter board over the main board(and not connected)...radiation is a lot less than if the main board was naked.
While I didn't design the board with EMC in mind, many of the SI design guides were followed, which help EMC too, from what I've read. These include a separate power/gnd plane, correct impedance matching with trace width/length/stackup configurations, trace length matching for the high speed signals, trace parallelism for the differential signals, decoupling caps as near to the chip as possible, etc. These are all according to the chip/IDE/USB design guidelines...wondering what else I can do to reduce the EMI radiation? I've tried pasting aluminum tape(helps), and EMI absorber sheets(doesn't help at all).
Thanks!