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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?

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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.

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    \$\begingroup\$ Can you clarify "destroying your eye"? Also what is the mechanism that slows down the clock edges? Is it some RC time constant? \$\endgroup\$ – Samee87 May 18 '15 at 22:58
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    \$\begingroup\$ For the eye, I think this is what @Some Hardware Guy means. \$\endgroup\$ – Bence Kaulics May 18 '15 at 23:01
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    \$\begingroup\$ No if were just talking emi there's no golden value only what works best for your system. From a purely emi standpoint the largest resistor that let's your system still work reliably would be what you want. There are other reasons to use series terminating resistors such as ringing, reflection termination, and matching driver output impedance to trace impedance. I think those topics are covered well elsewhere in the site though. \$\endgroup\$ – Some Hardware Guy May 18 '15 at 23:13
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    \$\begingroup\$ @BattleHamster, RE: eye patterns, see this old question. \$\endgroup\$ – The Photon May 18 '15 at 23:15
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    \$\begingroup\$ The lazy person's approach to controlled impedance is to assume all transmission lines are 50 ohms. 22 ohms is "half" of that. basebandhub.com/pdfs/series_emissions.pdf \$\endgroup\$ – pjc50 May 19 '15 at 14:29

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