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參數(shù)資料
| 型號： | SK12439PJT |
| 元件分類： | 時鐘產(chǎn)生/分配 |
| 英文描述： | 800 MHz, OTHER CLOCK GENERATOR, PQCC28 |
| 封裝： | PLASTIC, LCC-28 |
| 文件頁數(shù)： | 2/18頁 |
| 文件大小： | 233K |
| 代理商： | SK12439PJT |
10
Revision 1/February 9, 2001
www.semtech.com
HIGH-PER.ORMANCE PRODUCTS
SK12439
ADVANCED
Application Information (continued)
Figure 7. RMS PLL Jitter vs. VCO Frequency
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 SK12439 was collected in this manner.
Figure 8 shows the jitter as a function of the output
frequency. For the SK12439, 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 87.5 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.
Figure 8. RMS Jitter vs. Output Frequency
25
20
15
10
5
0
400
500
600
700
800
N = 2
N = 4
N = 8
RMS
Jitter
(ps)
Output Frequency (MHz)
25
20
15
10
5
0
25
50
75
100
125
150
175
200
225
250
275
300
325
350
375
400
6.25 ps Reference
Output Frequency (MHz)
RMS
Jitter
(ps)
The jitter data from Figure 7 does not include the
performance of the SK12439 when the output is in the
divide-by-one mode.
In the divide-by-one mode, the
SK12439 output jitter distribution is bimodal. Since a
bimodal distribution cannot be accurately represented with
an RMS value, peak-to-peak values of jitter for the divide-
by-one mode are presented.
Figure 9 shows the peak-to-peak jitter of the SK12439
output in divide-by-one mode as a function of output
frequency. Notice that, as with the other modes, the jitter
improves with increasing frequency. The ±65 ps shown
in the datasheet table represents a conservative value of
jitter, especially for the higher VCO, and thus output
frequencies.
Figure 9.
Peak-to-Peak Jitter vs. 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.
140
120
100
80
60
40
400
500
600
700
800
Spec Limit
N = 1
Output Frequency (MHz)
Peak-to-Peak
Jitter
(ps)
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