MT-008 Datasheet, PDF (1/10 Pages) Analog Devices – Converting Oscillator Phase Noise to Time Jitter

MT-008
TUTORIAL
Converting Oscillator Phase Noise to Time Jitter
by Walt Kester
INTRODUCTION
A low aperture jitter specification of an ADC is critical to achieving high levels of signal-to-
noise ratios (SNR). (See References 1, 2, and 3). ADCs are available with aperture jitter
specifications as low as 60-fs rms (AD9445 14-bits @ 125 MSPS and AD9446 16-bits @ 100
MSPS). Extremely low jitter sampling clocks must therefore be utilized so that the ADC
performance is not degraded, because the total jitter is the root-sum-square of the internal
converter aperture jitter and the external sampling clock jitter. However, oscillators used for
sampling clock generation are more often specified in terms of phase noise rather than time jitter.
The purpose of this discussion is to develop a simple method for converting oscillator phase
noise into time jitter.
PHASE NOISE DEFINED
First, a few definitions are in order. Figure 1 shows a typical output frequency spectrum of a
non-ideal oscillator (i.e., one that has jitter in the time domain, corresponding to phase noise in
the frequency domain). The spectrum shows the noise power in a 1-Hz bandwidth as a function
of frequency. Phase noise is defined as the ratio of the noise in a 1-Hz bandwidth at a specified
frequency offset, f_m, to the oscillator signal amplitude at frequency f_O.
"CLOSE-IN"
PHASE NOISE
(LIMITS FREQUENCY RESOLUTION)
PHASE
NOISE
(dBc/Hz)
1Hz BW
BROADBAND
PHASE NOISE
(REDUCES SNR)
f_o f_m
f
Figure 1: Oscillator Power Spectrum Due to Phase Noise
Rev.A, 10/08, WK
Page 1 of 10