GS1515-CTM 데이터 시트보기 (PDF) - Gennum -> Semtech
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GS1515-CTM Datasheet PDF : 17 Pages
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lock time. Therefore, there is a trade-off between
asynchronous lock time and jitter peaking/stability. These
capacitors should be as large as possible for the allowable
lock time and should be no smaller than the allowed value.
With the recommended values, jitter peaking of less than
0.1dB has been measured at the lower loop bandwidth as
shown in Figure 15. At higher loop bandwidths, it is difficult
to measure jitter peaking because of the limitation of the
measurement unit.
Bandwidths lower than 129kHz at 0.2UI modulation have
not been characterized, but it is believed that the
bandwidth could be further lowered. Since a lower
bandwidth has less correction for noise, extra care should
be taken to minimize board noise. Figures 16 and 17 show
the two measured loop bandwidths at these two settings.
Table 1 summarizes the two bandwidth settings.
Fig. 15 Typical Jitter Peaking
However, because relatively larger CCP1 and CCP2
capacitors can be used, over-damping of the loop
response occurs. An accurate jitter peaking measurement
of 0.1dB for the GS1515 requires the modulation source to
have a constant amount of jitter modulation index (within
0.1dB or 1.2%) over the frequency range beyond the loop
bandwidth.
It has been determined that for 282.9kHzUI, the minimum
value of the CCP1 and CCP2 capacitors should be no less
than 0.5µF. For added margin, 1µF capacitors are
recommended. The 1µF value gives a lock time of about
60ms in one attempt. For 25.72kHzUI, these capacitors
should be no less than 5.6µF. This would result in 340ms of
lock time. If needed, extra margin could be built by
increasing these capacitors at the expense of a longer
asynchronous lock time.
.
Fig. 16 Typical Jitter Transfer Curve
at setting A in Table 1
Fig. 17 Typical Jitter Transfer Curve
at setting B in Table 1
TABLE 1: Loop Bandwidth Setting Options
RCP1
CCP3
CCP1
CCP2
A Open
Open
1.0
1.0
B
50
1.0
5.6
5.6
BW
FACTOR
282.9kHz
25.72kHz
BW at 0.2UI JITTER
MODULATION
INDEX
1.41MHz
129kHz
ASYNCHRONOUS
60ms
340ms
SYNCHRONOUS
1.25µs
11.0µs
10
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