AD9545 datasheet(98/157 Pages) AD

98 / 157 page

AD9545

Data Sheet

Rev. A | Page 98 of 157

Figure 80 shows cascaded DPLL configuration and its operation.

The main difference between Figure 79 and Figure 80 is that

DPLL1 has Translation Profile 1.x configured to use the

feedback path of DPLL0 as a reference input. Setting up

Translation Profile 1.x in this way establishes the potential for

cascaded DPLL operation. The fact that Translation Profile 1.x

is set up to use the feedback path of DPLL0 as a reference input

identifies DPLL1 as the secondary DPLL and DPLL0 as the

primary DPLL in the cascaded DPLL configuration (see the

right side of Figure 80).

The existence of Translation Profile 1.x is one of two conditions

necessary to enable cascaded DPLL operation. The other

condition is that the primary DPLL must have an assigned

inactive profile. The user makes this assignment via the DPLLx

inactive profile index bit field (where x is 0 or 1) in Bits[D2:D0]

of Register 0x102A or Register 0x142A. By default, DPLLx

inactive profile index = 0, which assigns Translation Profile 0.0

or Translation Profile 1.0 as the inactive profile assignment, but

the user can specify any one of the six translation profiles as the

inactive profile by programming DPLLx inactive profile index =

n (where n is a value from 0 to 5). Figure 80 indicates

Translation Profile 0.b as the inactive profile to demonstrate a

nondefault example.

Consider the left side of Figure 80, which is the same as in

Figure 79 except for the fact that DPLL0 has Translation Profile B

assigned in its DPLL0 inactive profile index bit field. Like

Figure 79, both DPLLs lock to Reference Source 1. The middle

portion of Figure 80 applies when Reference Source 1 is lost.

Like Figure 79, both DPLLs lock to Reference Source 2. When

Reference Source 2 is lost (as shown in the right portion of

Figure 80), however, and assuming the user has configured a

translation profile (Translation Profile 1.x in this example) to

use the feedback path of the primary DPLL as a reference

source, the following events occur.

When DPLL0 loses its reference, it has no recourse but to

switch to holdover or freerun mode because no reference

sources are available. However, when Translation Profile 0.b

becomes inactive, because Translation Profile B is the inactive

profile index for DPLL0, the feedback path of the DPLL0 is

available as a valid reference for DPLL1. Although DPLL0

switches to holdover or freerun, DPLL1 does not because it has

an available reference source: the feedback path of the DPLL0

(DPLL1 may experience a momentary switch to holdover or

freerun mode before switching to cascaded DPLL operation).

The result is cascaded DPLL operation, in which the reference

input of DPLL1 (the secondary DPLL) connects to the feedback

path of DPLL0 (the primary DPLL). In this way, the two DPLL

outputs are phase locked, which preserves the original

requirement of phase locked DPLL outputs even when all

reference sources are lost.

The role of the primary and secondary DPLLs is reversible

because it is dependent on how the user programs the

translation profiles and assigns the inactive profile index.

For the cascaded DPLL configuration to become active, the

following conditions are necessary.

•

A translation profile associated with the secondary DPLL

that assigns the feedback path of the primary DPLL as a

reference input must exist.

•

The primary DPLL must have an inactive translation

profile assignment (by default, the inactive profile is

Translation Profile 0 unless otherwise specified).

•

The primary channel of the APLL must be calibrated and

indicate locked status.

•

The primary DPLL must be inactive, that is, either in

freerun or holdover operation.

DIGITAL

CROSS

POINT

MUX

APLL 0

PFD

NCO

DPLL 0

PFD

NCO

DPLL 1

APLL 1

REFERENCE

SOURCE 1

LOST

APLL 0

NCO

DPLL 0

DPLL 1

FTW

TRANSLATION

PROFILE 0.a

TRANSLATION

PROFILE 1.z

PFD

NCO

DPLL 0

PFD

NCO

DPLL 1

APLL 1

TRANSLATION

PROFILE 0.b

TRANSLATION

PROFILE 1.y

ALL

REFERENCE

SOURCES

LOST

DIGITAL

CROSS

POINT

MUX

DIGITAL

CROSS

POINT

MUX

PROFILE B IS NOW

INACTIVE, BUT DPLL0

BECOMES A VALID

REFERENCE SOURCE BY

VIRTUE OF THE INACTIVE

INDEX ASSIGNMENT

PFD

NCO

DPLL1 USES DPLL0 AS A

REFERENCE SOURCE

(DEFINED IN PROFILE 1.x)

TRANSLATION

PROFILE 1.x

DPLL0

CONFIGURED

WITH

INACTIVE

INDEX

ASSIGNED TO

PROFILE B

(PRIMARY)

(SECONDARY)

TDC

REFERENCE

SOURCES

TDC

REFERENCE

SOURCES

TRANSLATION PROFILE 0.a AND

TRANSLATION PROFILE 1.z ASSIGN

REFERENCE SOURCE 1

TRANSLATION PROFILE 0.b AND

TRANSLATION PROFILE 1.y ASSIGN

REFERENCE SOURCE 2

Figure 80. Cascaded DPLL Configuration

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