ICS831721AGI REVISION A FEBRUARY 3, 2014

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©2014 Integrated Device Technology, Inc.

ICS831721I Data Sheet

DIFFERENTIAL CLOCK/DATA MULTIPLEXER

PCI Express Application Note

PCI Express jitter analysis methodology models the system

response to reference clock jitter. The block diagram below shows the

most frequently used Common Clock Architecture in which a copy of

the reference clock is provided to both ends of the PCI Express Link.

In the jitter analysis, the transmit (Tx) and receive (Rx) serdes PLLs

are modeled as well as the phase interpolator in the receiver. These

transfer functions are called H1, H2, and H3 respectively. The overall

system transfer function at the receiver is:

Ht s



H 3 s



H 1 s

 H 2 s



–





=

The jitter spectrum seen by the receiver is the result of applying this

system transfer function to the clock spectrum X(s) and is:

Ys



Xs

 H 3 s





H 1 s

 H 2 s



–





=

In order to generate time domain jitter numbers, an inverse Fourier

Transform is performed on X(s)*H3(s) * [H1(s) - H2(s)].

PCI Express Common Clock Architecture

For PCI Express Gen 1, one transfer function is defined and the

evaluation is performed over the entire spectrum: DC to Nyquist (e.g

for a 100MHz reference clock: 0Hz – 50MHz) and the jitter result is

reported in peak-peak.

PCIe Gen 1 Magnitude of Transfer Function

For PCI Express Gen 2, two transfer functions are defined with 2

evaluation ranges and the final jitter number is reported in rms. The

two evaluation ranges for PCI Express Gen 2 are 10kHz – 1.5MHz

(Low Band) and 1.5MHz – Nyquist (High Band). The plots show the

individual transfer functions as well as the overall transfer function Ht.

PCIe Gen 2A Magnitude of Transfer Function

PCIe Gen 2B Magnitude of Transfer Function

For PCI Express Gen 3, one transfer function is defined and the

evaluation is performed over the entire spectrum. The transfer

function parameters are different from Gen 1 and the jitter result is

reported in RMS.

PCIe Gen 3 Magnitude of Transfer Function

For a more thorough overview of PCI Express jitter analysis

methodology, please refer to IDT Application Note PCI Express

Reference Clock Requirements.