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ADN2804 Datasheet(PDF) 14 Page - Analog Devices

Part No. ADN2804
Description  622 Mbps Clock and Data Recovery IC with Integrated Limiting Amplifier
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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ADN2804 Datasheet(HTML) 14 Page - Analog Devices

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ADN2804
Rev. 0 | Page 14 of 24
THEORY OF OPERATION
The ADN2804 is a delay- and phase-locked loop circuit for
clock recovery and data retiming from an NRZ encoded data
stream. The phase of the input data signal is tracked by two
separate feedback loops, which share a common control voltage.
A high speed delay-locked loop path uses a voltage controlled
phase shifter to track the high frequency components of input
jitter. A separate phase control loop, composed of the VCO,
tracks the low frequency components of input jitter. The initial
frequency of the VCO is set by yet a third loop that compares
the VCO frequency with the input data frequency and sets the
coarse tuning voltage. The jitter tracking phase-locked loop
controls the VCO by the fine-tuning control.
The delay and phase loops together track the phase of the input
data signal. For example, when the clock lags the input data, the
phase detector drives the VCO to a higher frequency and
increases the delay through the phase shifter; both of these
actions serve to reduce the phase error between the clock and
the data. The faster clock picks up phase, whereas the delayed
data loses phase. Because the loop filter is an integrator, the
static phase error is driven to 0°.
Another view of the circuit is that the phase shifter implements
the zero required for frequency compensation of a second-order
phase-locked loop, and this zero is placed in the feedback path;
therefore, it does not appear in the closed-loop transfer
function. Jitter peaking in a conventional second-order phase-
locked loop is caused by the presence of this zero in the closed-
loop transfer function. Because this circuit has no zero in the
closed-loop transfer, jitter peaking is minimized.
The delay and phase loops together simultaneously provide
wideband jitter accommodation and narrow-band jitter
filtering. The linearized block diagram in Figure 17 shows that
the jitter transfer function, Z(s)/X(s), provides excellent second-
order low-pass filtering. Note that the jitter transfer has no zero,
unlike an ordinary second-order phase-locked loop. This means
that the main PLL loop has virtually no jitter peaking (see
Figure 18), making this circuit ideal for signal regenerator
applications, where jitter peaking in a cascade of regenerators
can contribute to hazardous jitter accumulation.
The error transfer, e(s)/X(s), has the same high-pass form as an
ordinary phase-locked loop. This transfer function can be
optimized to accommodate a significant amount of wideband
jitter, because the jitter transfer function, Z(s)/X(s), provides the
narrow-band jitter filtering.
X(s)
Z(s)
RECOVERED
CLOCK
e(s)
INPUT
DATA
d/sc
psh
o/s
1/n
d = PHASE DETECTOR GAIN
o = VCO GAIN
c = LOOP INTEGRATOR
psh = PHASE SHIFTER GAIN
n = DIVIDE RATIO
=
1
cn
do
s2
+
n psh
o
s+ 1
Z(s)
X(s)
JITTER TRANSFER FUNCTION
=
s2
s2
d psh
c
s
++
do
cn
e(s)
X(s)
TRACKING ERROR TRANSFER FUNCTION
Figure 17. PLL/DLL Architecture
ADN2804
Z(s)
X(s)
FREQUENCY (kHz)
JITTER PEAKING
IN ORDINARY PLL
o
n psh
d psh
c
Figure 18. Jitter Response vs. Conventional PLL
The delay and phase loops contribute to overall jitter accom-
modation. At low frequencies of input jitter on the data signal,
the integrator in the loop filter provides high gain to track large
jitter amplitudes with small phase error. In this case, the VCO is
frequency modulated, and jitter is tracked as in an ordinary
phase-locked loop. The amount of low frequency jitter that can
be tracked is a function of the VCO tuning range. A wider
tuning range gives larger accommodation of low frequency
jitter. The internal loop control voltage remains small for small
phase errors; therefore, the phase shifter remains close to the
center of its range and thus contributes little to the low
frequency jitter accommodation.


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