AD9577BCPZ-R7 datasheet(25/44 Pages) AD

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Data Sheet

AD9577

Rev. 0 | Page 25 of 44

TERMINOLOGY

Phase Jitter and Phase Noise

An ideal sine wave can be thought of as having a continuous

and even progression of phase with time from 0° to 360° for each

cycle. Actual signals, however, display a certain amount of

variation from ideal phase progression over time, which is

called phase jitter. Although many causes can contribute

to phase jitter, one major cause is random noise, which is

characterized statistically as being Gaussian (normal) in

distribution.

This phase jitter leads to a spreading out of the energy of the

sine wave in the frequency domain, producing a continuous

power spectrum. This power spectrum is usually reported as

a series of values whose units are dBc/Hz at a given offset in

frequency from the sine wave (carrier). The value is a ratio

(expressed in dB) of the power contained within a 1 Hz band-

width with respect to the power at the carrier frequency. For each

measurement, the offset from the carrier frequency is also given.

It is meaningful to integrate the total power contained within

some interval of offset frequencies (for example, 12 kHz to

20 MHz). This is called the integrated phase noise over that

frequency offset interval and can be readily related to the time

jitter due to the phase noise within that offset frequency interval.

Phase noise has a detrimental effect on error rate performance

by increasing eye closure at the transmitter output and reducing

the jitter tolerance/sensitivity of the receiver.

Time Jitter

Phase noise is a frequency domain phenomenon. In the

time domain, the same effect is exhibited as time jitter. When

observing a sine wave, the time of successive zero crossings

vary. In a square wave, the time jitter is seen as a displacement

of the edges from their ideal (regular) times of occurrence. In

both cases, the variations in timing from the ideal are the time

jitter. Because these variations are random in nature, the time

jitter is specified in units of seconds root mean square (rms) or

1 sigma of the Gaussian distribution.

Additive Phase Noise

It is the amount of phase noise that is attributable to the device

or subsystem being measured. The phase noise of any external

oscillators or clock sources has been subtracted. This makes it

possible to predict the degree to which the device affects the

total system phase noise when used in conjunction with the

various oscillators and clock sources, each of which contributes

its own phase noise to the total. In many cases, the phase noise

of one element dominates the system phase noise.

Additive Time Jitter

It is the amount of time jitter that is attributable to the device

or subsystem being measured. The time jitter of any external

oscillators or clock sources has been subtracted. This makes it

possible to predict the degree to which the device will affect the

total system time jitter when used in conjunction with the

various oscillators and clock sources, each of which contributes

its own time jitter to the total. In many cases, the time jitter of

the external oscillators and clock sources dominates the system

time jitter.

Random Jitter Measurement

On the AD9577, the rms jitter measurements are made by

integrating the phase noise, with spurs disabled. There are two

reasons for this. First, because the part is highly configurable, any

measured spurs are a function of the current programmed state

of the device. For example, there may be a small reference spur at

the PFD frequency present on the output spectrum. If the PFD

operates at 19.44 MHz (which is common for telecommunications

applications), the resulting jitter falls within the normal 12 kHz

to 20 MHz integration bandwidth. When the PFD operates

above 20 MHz, the deterministic jitter is not included in the

measurement. As another example, for PLL2, the value of the

chosen FRAC and MOD values affects the amplitude and

location of a spur, and therefore, it is not possible to configure

the PLL to provide a general measurement that includes spurs.

The second, and more significant reason, is due to the statistical

nature of spurious components. The jitter performance information

of the clock generator is required so that a jitter budget for the

complete communications channel can be established. By

knowing the jitter characteristics at the ultimate receiver, the

data bit error rate (BER) can be estimated to ensure robust data

transfer. The received jitter characteristic consists of random

jitter (RJ), due to random perturbations such as thermal noise,

and deterministic jitter (DJ), due to deterministic perturbations

such as crosstalk spurs. To make an estimate of the BER, the

total jitter peak-to-peak (TJ p-p) value must be known. It is the

total jitter value that determines the amount of eye closure at

the receiver and, consequently, the bit error rate. The TJ p-p

value is specified for a given number of clock edges. For

example, in networking applications, the TJ is specified for 112

clock edges. The equation for the total jitter peak-to-peak is

TJ p-p = DJ p-p + 2 × Q × RJ rms

(1)

where the Q factor represents the ratio of the expected peak

deviation to the standard deviation in a Gaussian process for a

given population (of edge crossings). For 112 clock edges, Q is

7.03; therefore, for networking applications, the total jitter peak-

to-peak is estimated by

TJ p-p = DJ p-p + 14.06 × RJ rms

(2)

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