LG1600KXH Clock and Data Regenerator
Data Sheet
June 1999
Theory of Operation
(continued)
FPD
OUT
FPD OUT
TIME
A. fck < f
B
–360°
–180°
0°
180°
360°
PHASE
FPD
OUT
TIME
12-3228(C)r.4
Figure 5. Frequency and Phase Detector
Characteristics
The frequency detector is not a separate function but
an integral part of the phase-lock loop. Any transition
between frequency and phase acquisition is completely
avoided. Figure 5 shows the output characteristics of
the FPD, which is essentially an extended range phase
detector. The two quadrature clock phases are used to
produce hysteresis, which extends the phase detector
range to ±270°. The extended range gives the phase
detector a static frequency sensitivity as demonstrated
in Figure 6. For clock frequencies lower than the bit rate
(the phase is increasing), the top trajectory of the dia-
gram in Figure 6 is followed. When the VCO frequency
exceeds the bit rate, the lower trajectory applies. Since
the linear part of the phase detector produces a net-
zero output, in the first instance, positive pulses are fed
into the loop filter increasing the VCO frequency, while
in the latter case, the FPD produces negative pulses.
The wide, 540° range of the phase detector is also
responsible for the high jitter tolerance of the
LG1600KXH and an associated immunity to cycle slip
under high jitter conditions. The clock can be momen-
tarily misaligned as much as 270° but still return to its
original position. This property is extremely important
in synchronous systems, since a cycle slip would cause
misalignment of the demultiplexer following the circuit
resulting in a loss of frame condition. The LG1600KXH
can handle bit error rates up to 1e
–3
as a result of low-
frequency jitter.
B. fck > f
B
12-3229(C)r.3
Figure 6. Frequency Detector Operation
PLL Dimensioning
The LG1600KXH CDR employs a heavily damped
second-order phase-lock loop. A linear model of this
PLL is depicted in Figure 7. The conventional second-
order equation describing the jitter transfer of the PLL
is shown below:
2
ϕ
o
2ςω
n
s
+
ω
n
-
H
(
s
)
=
-----
(
s
)
=
-----------------------------------------
2
2
ϕ
i
s
+
2ςω
n
s
+
ω
n
where
ϕ
i
and
ϕ
o
denote the input and output phase,
respectively,
ς
is the PLL damping ratio and
ω
n
is the
natural frequency. For most clock recovery applications
a very high damping is required that renders the PLL
essentially as a first-order system with a slight peaking
that is generally undesirable. The second-order equa-
tion above does not provide much insight into the peak-
ing and bandwidth parameters.
ϕi
Ko
Kd
C
PHASE DETECTOR
Rx
SUM OF INTERNAL
AND EXTERNAL
LOOP FILTER
CAPACITANCE
VCO
ϕo
12-3230(F)r.4
Figure 7. Phase-Lock Loop Linear Model
4
Lucent Technologies Inc.
AGERE [ AGERE SYSTEMS ]
