GS1515-CQM Gennum Corporation, GS1515-CQM Datasheet - Page 10

• Manufacturer Part Number: GS1515-CQM
• Description: Hd-linx(tm) HDTV Serial Digital Reclocker
• Manufacturer: Gennum Corporation

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.
However,
capacitors can be used, over-damping
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 C
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.
TABLE 1: Loop Bandwidth Setting Options
522 - 23 - 03
A
B
RCP1
Open
50
because
CP1
Fig. 15 Typical Jitter Peaking
CCP3
Open
and C
1.0
relatively
CP2
capacitors should be no less
CCP1
1.0
5.6
larger
CCP2
C
1.0
5.6
CP1
of the loop
and
282.9kHz
25.72kHz
FACTOR
BW
C
CP2
10 of 17
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.
.
BW at 0.2UI JITTER
MODULATION
1.41MHz
129kHz
INDEX
Fig. 16 Typical Jitter Transfer Curve
Fig. 17 Typical Jitter Transfer Curve
at setting A in Table 1
at setting B in Table 1
ASYNCHRONOUS
340ms
60ms
SYNCHRONOUS
1.25µs
11.0µs