MT-008 Datasheet, PDF (3/10 Pages) Analog Devices – Converting Oscillator Phase Noise to Time Jitter

MT-008
PHASE
NOISE
(dBc/Hz)
1
f³
1
f²
1 _{"FLICKER" PHASE NOISE}
f
"WHITE" PHASE NOISE
1 _{CORNER FREQUENCY}
f
FREQUENCY OFFSET, f_m, (LOG SCALE)
Figure 3: Oscillator Phase Noise in dBc/Hz vs. Frequency Offset
Note that the phase noise curve is somewhat analogous to the input voltage noise spectral density
of an amplifier. Like amplifier voltage noise, low 1/f corner frequencies are highly desirable in
an oscillator.
We have seen that oscillators are typically specified in terms of phase noise, but in order to relate
phase noise to ADC performance, the phase noise must be converted into jitter. In order to make
the graph relevant to modern ADC applications, the oscillator frequency (sampling frequency) is
chosen to be 100 MHz for discussion purposes, and a typical graph is shown in Figure 4. Notice
that the phase noise curve is approximated by a number of individual line segments, and the end
points of each segment are defined by data points.
A = AREA = INTEGRATED PHASE NOISE POWER (dBc)
PHASE
NOISE
(dBc/Hz)
A = 10 log₁₀(A1 + A2 + A3 + A4)
RMS PHASE JITTER (radians) ≈
A/10
2•10
RMS JITTER (seconds)
A/10
2•10
≈
2 π f_O
f_O = OSCILLATOR FREQUENCY (100MHz)
A1
INTEGRATE TO ≈ 2 f_O = 200MHz
A2
A3
A4
10k
100k
1M
10M
100M
1G
f_{m FREQUENCY OFFSET (Hz)}
Figure 4: Calculating Jitter from Phase Noise
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