AD9546SLASHPCBZ datasheet(142/205 Pages) AD

142 / 205 page

AD9546

Data Sheet

Rev. 0 | Page 142 of 205

Figure 99 shows cascaded DPLL configuration and its operation.

The main difference between Figure 98 and Figure 99 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 99).

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 Bits[2:0] of

default, Bits[2:0] = 000 (binary), which assigns Translation

Profile 0.0 (for DPLL0) or Translation Profile 1.0 (for DPLL1)

as the inactive profile assignment, but the user can specify any

one of the six translation profiles as the inactive profile via

Bits[2:0]. Programming Bits[2:0] with a value of x, where x is 0

to 5 (decimal) selects the corresponding Translation Profile 0.x

or 1.x, where x can be any integer value from 0 to 5). Figure 99

indicates Translation Profile 0.b as the inactive profile to

demonstrate a nondefault example.

Consider the left side of Figure 99, which is the same as in

Figure 98 except for the fact that DPLL0 has Translation Profile B

assigned as its inactive profile. Like Figure 98, both DPLLs lock

to Reference Source 1. The middle portion of Figure 99 applies

when Reference Source 1 is lost. Like Figure 98, both DPLLs

lock to Reference Source 2. When Reference Source 2 is lost (as

shown in the right portion of Figure 99), 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 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 the role 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

APLL0

PFD

NCO

DPLL0

PFD

NCO

DPLL1

APLL1

REFERENCE

SOURCE 1

LOST

APLL0

NCO

DPLL0

DPLL1

FTW

TRANSLATION

PROFILE 0.a

TRANSLATION

PROFILE 1.z

PFD

NCO

DPLL0

PFD

NCO

DPLL1

APLL1

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 99. Cascaded DPLL Configuration

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