1Wireless Communication Technologies

Phase noise

A practical oscillator does not produce a carrier at exactly one frequency, but rather a carrier that is phase modulated by random phase jitter, which is called phase noise.

The oscillator output signal can be expressed by the equation

where describes the amplitude variation as a function of time and is the phase variation. corresponds to the phase noise.

A well-designed, high-quality oscillator is very amplitude stable and can be consider constant and all oscillator noise is due to .

2.5.3

)](cos[)()( tttAtS o

)(tA)(t )(t

)(tA)(t

2Wireless Communication Technologies

Phase noise

The easiest technique for measuring oscillator noise is to view the oscillator spectrum directly on a spectrum analyzer.

A simulation is shown in the following figure where the phase noise is 90 dBc/Hz at 100 kHz away.

2.5.3

-10 0 10-150

-100

-50

0

relative freq. (MHz) -- rew=.01MHz

rela

tive

pow

er

in d

Bc

phase noise PSD(P-100=90 dBc/Hz)

0 10 20 30-2

-1

0

1

2time domain waveform phas1.m

time (micro sec, fo=1MHz)

3Wireless Communication Technologies

Sensitivity to phase noise

Phase noise basically has two effects. First, it introduces a random phase variation that is common to all subcarriers.

The common phase error is strongly correlated from symbol to symbol, so tracking techniques or differential detection can be used to minimize the effects.

The second and more disturbing effect is that it introduces ICI, because the subcarriers

are no longer spaced at exactly 1/T in the frequency domain.

2.5.3

*

OFDM signal noisy oscillator

OFDM signal after mixing

fd