Philips Semiconductors Product data
PCK12429
25–400 MHz differential PECL clock generator
2002 Jun 03
10
Jitter performance of the PCK12429
The PCK12429 exhibits long term and cycle-to-cycle jitter which
rivals that of SAW based oscillators. This jitter performance comes
with the added flexibility one gets with a synthesizer over a fixed
frequency oscillator.
0
5
10
15
20
200 220 240 260 280 300 320 340 360 380 400
VCO FREQUENCY (MHz)
N = 1
N = 2
N = 4
N = 8
SW01010
RMS JITTER (ps)
Figure 5. RMS PLL jitter versus VCO frequency
Figure 5 illustrates the RMS jitter performance of the PCK12429
across its specified VCO frequency range. Note that the jitter is a
function of both the output frequency as well as the VCO frequency,
however the VCO frequency shows a much stronger dependence.
The data presented has not been compensated for trigger jitter, this
fact provides a measure of guardband to the reported data. In
addition the data represents long term period jitter, the cycle-to-cycle
jitter could not be measured to the level of accuracy required with
available test equipment but certainly will be smaller than the long
term period jitter.
The most commonly specified jitter parameter is cycle-to-cycle jitter.
Unfortunately with today’s high performance measurement
equipment there is no way to measure this parameter for jitter
performance in the class demonstrated by the PCK12429. As a
result, different methods are used which approximate cycle-to-cycle
jitter. The typical method of measuring the jitter is to accumulate a
large number of cycles, create a histogram of the edge placements,
and record peak-to-peak as well as standard deviations of the jitter.
Care must be taken that the measured edge is the edge immediately
following the trigger edge. The oscilloscope cannot collect adjacent
pulses, rather it collects pulses from a very large sample of pulses. It
is safe to assume that collecting pulse information in this mode will
produce period jitter values somewhat larger than if consecutive
cycles (cycle-to-cycle jitter) were measured. All of the jitter data
reported on the PCK12429 was collected in this manner.
Figure 6 shows the jitter as a function of the output frequency. For
the PCK12429, this information is probably of more importance. The
flat line represents an RMS jitter value that corresponds to an
8 sigma ±25 ps peak-to-peak long term period jitter. The graph
shows that for output frequencies from 125 to 400 MHz the jitter falls
within the ±25 ps peak-to-peak specification. The general trend is
that as the output frequency is decreased the output edge jitter will
increase.
0.00
5.00
10.00
15.00
20.00
25.00
25 50 75 100 125 150 175 200 225 250 275 300 325350 375 400
OUTPUT FREQUENCY (MHz)
6.25 ps REFERENCE (1 SIGMA)
SW01011
RMS JITTER (ps)
Figure 6. RMS jitter versus output frequency
The jitter data presented should provide users with enough
information to determine the effect on their overall timing budget.
The jitter performance meets the needs of most system designs
while adding the flexibility of frequency margining and field
upgrades. These features are not available with a fixed frequency
SAW oscillator.
