REV. A

ADN2811

–9–

THEORY OF OPERATION

The ADN2811 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 yet a third loop, which

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-locked 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 also increases the delay through the phase shifter.

Both of these actions both 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 will be 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, and this zero is placed in the feedback

path and thus does not appear in the closed-loop transfer func-

tion. 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-locked and phase-locked loops together simultaneously

provide wideband jitter accommodation and narrow-band jitter

filtering. The linearized block diagram in Figure 10 shows the

jitter transfer function, Z(s)/X(s), is a second-order low-pass

providing excellent filtering. Note the jitter transfer has no zero,

unlike an ordinary second-order phase-locked loop. This means

that the main PLL loop has low jitter peaking (see Figure 11),

which makes this circuit ideal for signal regenerator applications

where jitter peaking in a cascade of regenerators can contribute

to hazardous jitter accumulation.

d/sc

psh

e(s)

X(s)

INPUT

DATA

Z(s)

RECOVERED

CLOCK

d = PHASE DETECTOR GAIN

o = VCO GAIN

c = LOOP INTEGRATOR

psh = PHASE SHIFTER GAIN

n = DIVIDE RATIO

JITTER TRANSFER FUNCTION

Z(s)

X(s)

1

+ s

+1

cn

do

n psh

o

=

TRACKING ERROR TRANSFER FUNCTION

e(s)

X(s)

+

do

cn

d psh

c

=

o/s

Figure 10. 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.

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 fre-

quency jitter. The internal loop control voltage remains small

for small phase errors, so the phase shifter remains close to the

center of its range and thus contributes little to the low fre-

quency 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 its 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, and so 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.

The gain of the loop integrator is small for high jitter frequen-

cies, so larger phase differences are needed to make the loop

control voltage big enough to tune the range of the phase

shifter. Large phase errors at high jitter frequencies cannot be

tolerated. In this region, the gain of the integrator determines

the jitter accommodation. Since the gain of the loop integrator

declines linearly with frequency, jitter accommodation is lower

with higher jitter frequency. At the highest frequencies, the loop

gain is very small and little tuning of the phase shifter can be

expected. In this case, jitter accommodation is determined by

the eye opening of the input data, the static phase error, and the

residual loop jitter generation. The jitter accommodation is

roughly 0.5 UI in this region. The corner frequency between the

declining slope and the flat region is the closed loop bandwidth

of the delay-locked loop, which is roughly 5 MHz.

JITTER PEAKING

IN ORDINARY PLL

ADN2811

Z(s)

X(s)

JITTER

GAIN

(dB)

o

n psh

d psh

c

Figure 11. Jitter Response vs. Conventional PLL