The Importance of EMI Countermeasures in Clock Design

This may be a bit sudden, but when designing a product in which EMI noise from clocks is a concern, what measures do you take?
Perhaps many people have taken measures such as shielding the chip or using an RC filter.

In this article, we will introduce SiTime's clock solutions for products where EMI noise is a concern.
SiTime offers a lineup of products that can provide EMI countermeasures with a single chip, so please consider them if you are having problems with EMI.

We also introduce our product lineup for your reference.

Introduction

There are several types of EMI, but in this article we will introduce countermeasures against "noise from output lines" that is generated by "clocks."

Main sources of EMI noise

·power supply

·clock

・SoC or FPGA with built-in PLL​

・Signal crosstalk etc...

Oscillator-related noise

・Noise from the output line

・Noise from the power line

・Noise generated by the crystal oscillator itself

Introducing solutions using SiTime products

SiTime offers two methods for dealing with EMI:

1. Programmable oscillator with adjustable output strength

②Oscillator with spread spectrum function

Both devices allow for noise suppression on the clock output line, but ② oscillators with spread spectrum function are also effective in suppressing EMI in signals generated by the IC they drive.

1. Programmable oscillator with adjustable output drive capacity

SiTime products are programmable oscillators, and the output drive capacity can be selected from 8 patterns!

By adjusting the drive capacity, it is possible to adjust the "rise/fall time", which has the following advantages:

・EMI noise can be reduced!

・No need for damping resistors after the clock!

To combat EMI noise, you can suppress it by setting the output edge rate to a weaker value!

(In the diagram below, the blue line shows the weak edge rate that was actually set.)

Many customers compare several patterns when making prototypes, so please feel free to contact us if you have any sample requests.

↓↓Compatible products are here↓↓

type name	frequency	Frequency deviation (ppm)	Output format	Operating temperature range	Power-supply voltage	package size
SiT8008	1~110MHz	±20 ±25 ±50	LVCMOS	-20 to +70℃ -40 to +85℃	1.8V 2.5 to 3.3V 1.8 to 3.3V	2.0x1.6 2.5x2.0 3.2x2.5 5.0x3.2 7.0x5.0
SiT8009	115~137MHz	±20 ±25 ±50	LVCMOS	-20 to +70℃ -40 to +85℃	1.8V 2.5 to 3.3V	2.0x1.6 2.5x2.0 3.2x2.5 5.0x3.2 7.0x5.0
SiT2001	1~110MHz	±20 ±25 ±50	LVCMOS	-20 to +70℃ -40 to +85℃	1.8 2.5 to 3.3V	SOT23(2.9x2.8) * Foot ground
SiT2002	115~137MHz	±20 ±25 ±50	LVCMOS	-20 to +70℃ -40 to +85℃	1.8 2.5 to 3.3V	SOT23(2.9x2.8) * Foot ground

2) Oscillator with spread spectrum function

SiTime can also offer products with a spread spectrum function called SSXO.
Spread spectrum is a method of eliminating EMI noise by intentionally modulating the clock frequency, and has the following features:

- Up to 17dB reduction in fundamental frequency and up to 30dB reduction in harmonics possible!

-Wide range spread of up to 4% pk-to-pk is possible!

Oscillators with spread spectrum functionality are effective not only for EMI noise directly emitted from the clock output line, but also for EMI suppression of signals generated by the IC they drive.

↓↓Compatible products are here↓↓

type name	SiT9005
oscillator type	spread spectrum clock
output	LVCMOS (single-ended)
frequency	1MHz to 141MHz
Frequency deviation (ppm)	±20 ±25 ±50
Operating temperature range	-20 to +70℃ -40 to +85℃
Power-supply voltage	1.8V 2.5V to 3.3V
package size	2.0×1.6 2.5×2.0 3.2x2.5

Summary

This time, we introduced the following SiTime clock solutions for products where EMI noise is a concern.

1. Programmable oscillator with adjustable output strength

②Oscillator with spread spectrum function

I hope this article will be of some help to you. If you have any questions during the design process, please feel free to contact us.

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