MC12429

TIMING SOLUTIONS

9

MOTOROLA

be applications in which overall performance is being

degraded due to system power supply noise. The power

supply filter and bypass schemes discussed in this section

should be adequate to eliminate power supply noise related

problems in most designs.

Jitter Performance of the MC12429

The MC12429 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.

Figure 8. RMS PLL Jitter versus VCO Frequency

0

5

10

15

20

25

200

250

300

350

400

N=1

N=2

N=4

VCO Frequency (MHz)

N=8

Figure 8 illustrates the RMS jitter performance of the

MC12429 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.

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. All of

the jitter data reported on the MC12429 was collected in this

manner.

Figure 9 shows the jitter as a function of the output

frequency. For the 12429 this information is probably of more

importance. The flat line represents an RMS jitter value that

corresponds to an 8 sigma

±25ps peak–to–peak long term

period jitter. The graph shows that for output frequencies

from 87.5 to 400MHz the jitter falls within the

±25ps

peak–to–peak specification. The general trend is that as the

output frequency is decreased the output edge jitter will

increase.

Figure 9. RMS Jitter versus Output Frequency

0

5

10

15

20

25

25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400

Output Frequency (MHz)

6.25ps Reference

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.