AN279
E
STIMATING
P
ERIOD
J
ITTER
1. Introduction
This application note reviews how RMS period jitter may be estimated from phase noise data. This approach is
useful for estimating period jitter when sufficiently accurate time domain instruments, such as jitter measuring
oscilloscopes or Time Interval Analyzers (TIAs), are unavailable.
FROM
P
HASE
N
OISE
2. Terminology
In this application note, the following definitions apply:
Cycle-to-cycle jitter—The short-term variation in clock period between
adjacent
clock cycles. This jitter
measure, abbreviated here as J
CC
, may be specified as either an RMS or peak-to-peak quantity.
Jitter—Short-term variations of the significant instants of a digital signal from their ideal positions in time
(Ref: Telcordia GR-499-CORE). In this application note, the digital signal is a clock source or oscillator.
Short-
term
here means phase noise contributions are restricted to frequencies greater than or equal to 10 Hz
(Ref: Telcordia GR-1244-CORE).
Period jitter—The short-term variation in clock period over
all
measured clock cycles, compared to the average
clock period. This jitter measure, abbreviated here as J
PER
, may be specified as either an RMS or peak-to-peak
quantity. This application note will concentrate on estimating the RMS value of this jitter parameter.
The illustration in Figure 1 suggests how one might measure the RMS period jitter in the time domain. The first
edge is the reference edge or trigger edge as if we were using an oscilloscope.
Clock Period
Distribution
σ
J
PER
(RMS) =
σ
T =0
T = T
PER
Figure 1. RMS Period Jitter Example
Phase jitter—The integrated jitter (area under the curve) of a phase noise plot over a particular jitter bandwidth.
Phase noise data may be recorded as either SSB phase noise L(f) in dBc/Hz or phase noise spectral density
S
φ
(f) in rad
2
/Hz where:
S
ϕ
(
f
)
L
(
f
) ≡
-------------
2
RMS phase jitter may be expressed in units of dBc, radians, time, or Unit Intervals (UI).
Rev. 0.1 7/06
Copyright © 2006 by Silicon Laboratories
AN279
SILABS [ SILICON LABORATORIES ]
