FS6128-01 查看數據表（PDF） - Unspecified

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FS6128-01

PLL Clock Generator IC with VCXO

Unspecified 

FS6128-01

PLL Clock Generator IC with VCXO

AMERICAN MICROSYSTEMS, INC.

April 2000

Table 2: Pin Descriptions

Key: AI = Analog Input; AO = Analog Output; DI = Digital Input; DIU = Input with Internal Pull-Up; DID = Input with Internal Pull-Down; DIO = Digital Input/Output; DI-3 = Three-Level Digital Input,

DO = Digital Output; P = Power/Ground; # = Active Low pin

PIN

TYPE

NAME

DESCRIPTION

1

AI

XIN

VCXO Feedback

2

P

VDD

Power Supply (+3.3V)

3

AI

XTUNE

VCXO Tune

4

P

VSS

Ground

5

DO

CLK

Clock Output

6

-

n/c

No Connection

7

DO

VSS

Ground

8

AO

XOUT

VCXO Drive

3.0 Functional Block Description

3.1 Phase-Locked Loop (PLL)

The on-chip PLL is a standard frequency- and phase-

locked loop architecture. The PLL multiplies the reference

oscillator to the desired frequency by a ratio of integers.

The frequency multiplication is exact with a zero synthe-

sis error.

3.2 Voltage-Controlled Crystal

Oscillator (VCXO)

The VCXO provides a tunable, low-jitter frequency refer-

ence for the rest of the FS6128 system components.

Loading capacitance for the crystal is internal to the

FS6128. No external components (other than the reso-

nator itself) are required for operation of the VCXO.

Continuous fine-tuning of the VCXO frequency is accom-

plished by varying the voltage on the XTUNE pin. The

value of this voltage controls the effective capacitance

presented by the VCXO to the crystal.

When using a crystal with a VCXO, it is important that the

crystal load capacitance (as specified in Table 4: Oper-

ating Conditions be matched to the load capacitance as

presented by the VCXO. The crystal must be specified

with the correct load capacitance to obtain the maximum

tuning range.

The oscillator operates the crystal resonator in the paral-

lel-resonant mode. Crystal warping, or the “pulling” of the

crystal oscillation frequency, is accomplished by altering

the effective load capacitance presented to the crystal by

the oscillator circuit. The actual amount that changing the

load capacitance alters the oscillator frequency will be

dependent on the characteristics of the crystal as well as

the oscillator circuit itself.

Specifically, the motional capacitance of the crystal (usu-

ally referred to by crystal manufacturers as C1), the static

capacitance of the crystal (C0), and the load capacitance

(CL) of the oscillator determine the warping capability of

the crystal in the oscillator circuit.

A simple formula to obtain the warping capability of a

crystal oscillator is:

∆f

(

ppm)

=

C1 × (C L2 − C )L1 ×106

2 × (C0 + C L2)× (C0 + C )L1

where CL1 and CL2 are the two extremes of the applied

load capacitance.

EXAMPLE: A crystal with the following parameters is

used. With C1 = 0.02pF, C0 = 5pF, CL1 = 13pF, and CL2

=35pF, the coarse tuning range is

∆f

=

0.02 × (35 −13)×106

2 × (5 + 35)× (5 + 13)

≈

305

ppm

.

ISO9001

2

4.24.00

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