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.

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VCO FREQUENCY

(MHz)

N = 1

N = 2

N = 4

N = 8

SW01010

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.

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OUTPUT FREQUENCY (MHz)

6.25 ps REFERENCE (1 SIGMA)

SW01011

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.