STANDARD PRODUCT
PM4351 COMET
DATA SHEET
PMC-1970624
ISSUE 10
COMBINED E1/T1 TRANSCEIVER
The jitter attenuator generates the jitter-free 1.544 MHz or 2.048 MHz TCLKO
output transmit clock by adjusting TCLKO's phase in 1/96 UI increments to
minimize the phase difference between the generated TCLKO and input data
clock to TJAT (either BTCLK or the recovered clock). Jitter fluctuations in the
phase of the input data clock are attenuated by the phase-locked loop within
TJAT so that the frequency of TCLKO is equal to the average frequency of the
input data clock. For T1 applications, to best fit the jitter attenuation transfer
function recommended by TR 62411, phase fluctuations with a jitter frequency
above 5.7 Hz are attenuated by 6 dB per octave of jitter frequency. Wandering
phase fluctuations with frequencies below 5.7 Hz are tracked by the generated
TCLKO. In E1 applications, the corner frequency is 7.6 Hz. To provide a smooth
flow of data out of TJAT, TCLKO is used to read data out of the FIFO.
If the FIFO read pointer (timed to TCLKO) comes within one bit of the write
pointer (timed to the input data clock, BTCLK or RSYNC), TJAT will track the
jitter of the input clock. This permits the phase jitter to pass through
unattenuated, inhibiting the loss of data.
Jitter Characteristics
The TJAT Block provides excellent jitter tolerance and jitter attenuation while
generating minimal residual jitter. It can accommodate up to 61 UIpp of input
jitter at jitter frequencies above 5.7 Hz (7.6 Hz for E1). For jitter frequencies
below 5.7 Hz (7.6 Hz for E1), more correctly called wander, the tolerance
increases 20 dB per decade. In most applications the TJAT Block will limit jitter
tolerance at lower jitter frequencies only. For high frequency jitter, above 10 kHz
for example, other factors such as clock and data recovery circuitry may limit
jitter tolerance and must be considered. For low frequency wander, below 10 Hz
for example, other factors such as slip buffer hysteresis may limit wander
tolerance and must be considered. The TJAT block meets the stringent low
frequency jitter tolerance requirements of AT&T TR 62411 and thus allows
compliance with this standard and the other less stringent jitter tolerance
standards cited in the references.
The corner frequency in the jitter transfer response can be altered through
programming.
TJAT exhibits negligible jitter gain for jitter frequencies below 5.7 Hz (7.6 Hz for
E1), and attenuates jitter at frequencies above 5.7 Hz (7.6 Hz for E1) by 20 dB
per decade. In most applications, the TJAT block will determine jitter attenuation
for higher jitter frequencies only. Wander, below 10 Hz for example, will
essentially be passed unattenuated through TJAT. Jitter, above 10 Hz for
example, will be attenuated as specified, however, outgoing jitter may be
dominated by the generated residual jitter in cases where incoming jitter is
insignificant. This generated residual jitter is directly related to the use of a
PROPRIETARY AND CONFIDENTIAL
58
PMC [ PMC-SIERRA, INC ]
