Data Sheet ADN2819
Rev. C | Page 13 of 25
THEORY OF OPERATION
The ADN2819 is a delay-locked 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 that 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 the
input jitter. A separate phase control loop, comprised of the
VCO, tracks the low frequency components of the input jitter.
The initial frequency of the VCO is set by a third loop that
compares the VCO frequency with the reference 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-locked loops together track the phase of
the input data signal. For example, when the clock lags 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
data. The faster clock picks up phase while the delayed data
loses phase. Since the loop filter is an integrator, the static phase
error is driven to zero.
Another view of the circuit is that the phase shifter implements
the zero required for the frequency compensation of a second-
order phase-locked loop. This zero is placed in the feedback
path and therefore 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. Since this circuit has no zero in the
closed-loop transfer, jitter peaking is minimized.
The delay- and phase-locked loops together simultaneously
provide wideband jitter accommodation and narrow-band jitter
filtering. The linearized block diagram in Figure 15 shows that
the jitter transfer function, Z(s)/X(s), is a second-order low-pass
providing excellent filtering. Note that the jitter transfer has no
zero, unlike an ordinary second-order phase-locked loop. This
means the main phase-locked loop (PLL) has low jitter peaking
(see Figure 16), which makes this circuit ideal for signal
regenerator applica-tions where jitter peaking in a cascade of
regenerators can contribute to hazardous jitter accumulation.
d/sc
o/
s
psh
1/n
e(s)
X(s)
INPUT
D
A
TA
Z(s
)
RECOVERED
CLOCK
d = PHASE DETECT
OR GAIN
o = VCO GAIN
c = LOOP INTEGRATOR
psh = PHASE SHIFTER GAIN
n = DIVIDE RATI
O
JITTER
TRANSFER FUNCTION
Z(s)
X(s
)
1
s
2
+ s +1
cn
do
n ps
h
o
=
TRACKING ERR
OR
TRANSFER FUNCTION
e(s)
X(s
)
s
2
s
2
+ s
+
do
cn
d ps
h
c
=
02999-B-015
Figure 15. PLL/DLL Architecture
The error transfer, e(s)/X(s), has the same high-pass form as an
ordinary phase-locked loop. This transfer function is free to be
optimized to give excellent wideband jitter accommodation since
the jitter transfer function, Z(s)/X(s), provides the narrow-band
jitter filtering. See Table 4 for error transfer bandwidths and jitter
transfer bandwidths at the various data rates.
The delay-locked and phase-locked loops contribute to overall
jitter accommodation. 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, so the phase shifter remains close to the center of the range,
and therefore contributes little to the low frequency jitter
accommodation.
At medium jitter frequencies, the gain and tuning range of the
VCO are not large enough to track the input jitter. In this case,
the VCO control voltage becomes large and saturates, and the
VCO frequency dwells at one or the other extreme of the tuning
range. The size of the VCO tuning range therefore has only a
small effect on the jitter accommodation. The delay-locked loop
control voltage is now larger; thus, the phase shifter takes on the
burden of tracking the input jitter. The phase shifter range, in
UI, can be seen as a broad plateau on the jitter tolerance curve.
The phase shifter has a minimum range of 2 UI at all data rates.
