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 concern.
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?