ICS874001I-05 Datasheet, PDF (12/17 Pages) Integrated Device Technology – PCI EXPRESS™ JITTER ATTENUATOR

ICS874001I-05
PCI EXPRESS™ JITTER ATTENUATOR
PCI EXPRESS APPLICATION NOTE
PCI Express jitter analysis methodology models the system
response to reference clock jitter. The below block diagram
shows the most frequently used Common Clock Architecture
in which a copy of the reference clock is provided ot both ends
of the PCI Express Link.
In the jitter analysis, the Tx and Rx serdes PLLs are modeled
as well as the phase interpolator in the receiver. These ransfer
functions are called H1, H2 and H3 respectively. The overall
system transfer function at the receiver is:
Ht(s) = H3(s) * [H1(s) - H2(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:
Y(s) = X(s) * H3(s) * [H1(s) - H2(s)]
In order to generate time domain jitter numbers, an inverse
Fourier Transform is performed on X(s) * H3(s) * [H1(s) - H2(s)].
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 to 50MHz) and
the jitter result is reported in peak-peak. For PCI Express Gen2,
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 below plots show the
individual transfer functions as function H1, F2 for H2, and f#
for H3. For a more thorough overivew of PCI Express jitter
analysis methodology, please refer to IDT Application Note, PCI
Express Reference Clock Requirements.
Tx Serdes
2.5 GHz
PLL
x25 Mult
H1(s)
100 MHz
X(s)
Reference
Clock
Rx Serdes
Phase
Aligner
PLL
H3(s)
x25 Mult
H2(s) 2.5 GHz
- _Y(s)
DQ
100 MHz
X(s)
System Transfer Function, Ht(s) = H3(s) * [H1(s) - H2(s)]
Reference Clock Spectrum seen by Receiver Sample Latch, Y(s) = X(s) * Ht(s)
IDT^™ / ICS^™ PCI EXPRESS JITTER ATTENUATOR
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ICS874001AGI-05 REV. A APRIL 20, 2009