User`s guide

Phase Noise Theory
Phase noise is the random fluctuation of phase in a clock signal and is typically used to specify the quality
of a PLL circuit. In the frequency domain, phase noise appears as sidebands of the carrier frequency
(Figure a). Phase noise is commonly shown as a single side band at frequencies offset from the carrier
(Figure b). Typical domain of interest is frequencies within a few kilohertz of the carrier.
Frequency
Power
f
0
Phase Noise
Carrier
Offset Frequency (Hz)
Phase Noise (dBc/Hz)
a) b)
Figures a and b. Phase noise in frequency domain.
Phase noise is expressed as the normalized power spectral density (PSD) on a decibel scale. It is defined by
Eq. 1
()
(
)








∆
=
fP
fP
fL
0
10
log10
,
where
f is frequency offset from the carrier, P
0
is the power of the carrier, P ( f ) is the noise power
spectrum, and ∆
f is the resolution bandwidth of the noise power spectrum. The units of phase noise are
dBc/Hz. Typical regions of interest are frequencies within a few kilohertz offset from the carrier. For the
time interval measurement systems, the phase noise power is given by
Eq. 2
() ()()
2
0
2
2
2
1
0
ftfVfP ∆= π
,
where,
f
0
and V
0
are the carrier frequency and peak voltage, respectively, and ∆ t ( f ) is the FFT N-clock
jitter spectrum acquired from the Low Frequency Modulation tool. The power of the carrier is
P
0
= V
0
2
/2.
The corresponding phase noise is
Eq. 3
()
(
)








∆
∆
=
f
ftf
fL
c
22
10
log10 ,
where ∆ f is the resolution bandwidth of the jitter spectrum.
Section 4 - GigaView © WAVECREST Corporation 2005
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