Electronic – How serial resistors actually reduce EMI


I am recently working with a GSM based system, and there was this advice in the datasheet of the GSM module:

22 Ω resistors should be connected in series between the module and
the SIM card so as to suppress the EMI spurious transmission and
enhance the ESD protection.

I tried to do a little search and I have found a document, PCB Design Guidelines
For Reduced EMI
, it has similar statement in it, but no explanation.

Put a 50 –100 Ω resistor in series with every output pin, and 35 –50 Ω
resistor on every input pin.

An other part says:

(Series Termination, Transmission Lines)

Series resistance is an inexpensive solution to termination and
ringing problems, and is the preferred method for microcomputer-based
systems where minimizing the differential-mode noise is also a

One more possibly related part:

Impedance Matching at Inputs
and again, the series resistance is the most likely solution. Resistance placed at the driver increases
the output impedance, as seen by the trace and the input pin, thus
matching the high impedance of the input

I have found something in this document as well, Understanding Radiated EMI it says:

Add series resistance? May help
– Less current (good and bad current) flows through high impedance
– May reduce EMI by reducing currents flowing outside IC

All in all, I need a little clarification about the topic, so my question is:

How serial resistors actually reduce EMI and what is the principle?

Best Answer

Well one it slows down the rise time of your signal which reduces the high frequency content. So that could help you if you didn't need your edges to be that fast.

It also lowers the current flowing through the trace and back through its return path which would lower the strength of the field created around it (and radiated out).

I would add that I've often seen this added lazily, or perhaps by the "emi group" after the design is done. It's fine if you didn't need the edge rates but if you're trying to go fast slowing down your edges or destroying your eye may not be what you want.