AN-09820 pages

AN-0982
APPLICATION NOTE
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The Residual Phase Noise Measurement
by David Brandon and John Cavey

INTRODUCTION

BACKGROUND INFORMATION

This application note describes a technique to evaluate
DUT noise by removing external noise sources. A residual
phase noise setup is used to isolate and measure the additive
phase noise contribution of a device. Designers use this information to select individual devices for their signal chain,
which, in aggregate, meet the phase noise requirements for
their complete system.

The setup shown in Figure 1 measures the additive phase noise
of the DUT. Note that two DUTs are used, each connected to a
common power supply and a common input clock. It will be
shown that the noise at each DUT output due to these common
noise sources is correlated. If the amplified DUT1 output signal
is denoted as E1 and the amplified and delayed DUT2 output
signal is denoted as E2, then the output phase noise can be
derived by simply modeling the phase detector as an analog
multiplier with a gain of KPD.

The benefit of a residual phase noise setup is that the effect of
noise sources external to the device under test (DUT), such
as power supplies or input clocks, can be canceled from the
measurement. Conversely, an absolute phase noise measurement includes the impact of power supply noise and reference
clock noise.
This application note includes actual phase noise measurement
plots of a clocked device to highlight the attributes of the residual
phase noise setup. In addition, it demonstrates how the additive
phase noise of a device can be used to identify the source of
noise-related issues in the signal chain.

E1 = EC1 cos[ωCt + θM1 cos(ωMt)]
E2 = EC2 sin[ωCt + θM2 cos(ωMt)]
EOUT = LPF {KPDE1E2} = KPDEC1EC2 sin [(θM2 – θM1)cos(ωMt)]
The signal powers are assigned ECX and the magnitude of the
phase modulation (noise) is assigned θMX with carrier frequency
ωC and modulating (offset) frequency ωM. Because superposition
applies, the phase noise intrinsic to the DUTs can be neglected
when considering the phase noise from external sources.

DUT 1

AMP
LPF

POWER
SPLITTER

PHASE
DETECTOR

0°

DUT 2

LNA

SPECTRUM
ANALYZER

AMP
FREQUENCY
90°

Figure 1. Residual Phase Noise Measurement Setup

Rev. 0 | Page 1 of 1

07838-001

CLOCK
SOURCE

MAGNITUDE
(dBc/Hz)

DUT POWER
SUPPLIES