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PRELIMINARY
XRT79L71
336
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
All of this means that if the Jitter Attenuator PLL receives a clock signal, that contains jitter with a frequency of
about 23Hz, the Jitter Attenuator PLL will reduce the amplitude of this "23Hz jitter component" by about 3dB or
50%. The "Pass-Band" of the Loop-filter (within the Jitter Attenuator PLL) is any frequency below 23Hz. This
means that the Jitter Attenuator PLL (within the XRT79L71) will not provide much attenuation on any jitter
(within the input In_CLK clock signal) that is of frequencies less than 23Hz. As a consequence, much of the
low-frequency jitter that appears at the "In_CLK" input, will also appear at the "Out_CLK" output pins. The
Jitter Attenuator PLL will provide more than 3dB of jitter attenuation for jitter with frequencies greater than
23Hz. The Jitter Transfer Characteristics (of the Jitter Attenuator PLL, within the XRT79L71) is such that for
frequencies greater than 23Hz, it imposes a -20dB/decade roll-off in the "Gain versus Frequency" curve (as
presented in Figure 156). Therefore, if the Jitter Attenuator PLL block receives a signal (via the "In_CLK_n"
input pin) that contains jitter which is of a frequency of 230Hz, then the Jitter Attenuation PLL will reduce the
amplitude of this jitter by 23dB or by 95%.
In general, the higher the frequency of the jitter (within the
In_CLK_n input signal), the greater the jitter amplitude will be attenuated.
DISABLING THE JITTER ATTENUATOR PLL
The XRT79L71 permits the user to disable the Jitter Attenuator PLL. If the Jitter Attenuator PLL is disabled,
then the Jitter Attenuator block will perform no jitter attenuation on the "In_CLK" input signal. More specifically,
the Jitter Attenuator FIFO Block will also be by-passed, and the signal path (through the Jitter Attenuator block)
will proceed to emulate the behavior of three wires. In this mode, the "In_CLK" input signal (within the Jitter
Attenuator block) will essentially be shorted to the "Out_CLK" output signal. Further, the "In_POS" input signal
will essentially be shorted to the "Out_POS" output signal; and the "In_NEG" input signal will essentially be
shorted to the "Out_NEG". Data residing on the "In_POS" and "In_NEG" input pins will not be clocked into the
Jitter Attenuator FIFO via the In_CLK input signal. Further, data will not be clocked out (via the "Out_POS" and
"In_NEG_n" output pins) upon the Out_CLK_n output signal. This data will simply propagate through the Jitter
Attenuator block; just as if it were three wires.
FIGURE 156. A SIMPLE ILLUSTRATION OF THE JITTER TRANSFER CHARACTERISTICS OF EACH JITTER ATTENUATOR
PLL (WITHIN THE XRT79L71)
Corner Frequency = 23Hz
10
23
230
0dB
-23dB
Jitter Frequency, Hz
Ji
tt
er
G
a
in
,d
B
-20dB/decade roll-off
Pass Band