FS6128-07

VCXO

FS6128-07

PLL

XOUT

XIN

CLK

XTUNE

DIVIDER

Figure 2: Block Diagram

Table 2: Pin Descriptions

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

-

VDD

Power supply (+3.3V)

7

DO

VSS

Ground

8

AO

XOUT

VCXO drive

Input/Output; DI-3 = Three-Level Digital Input, DO = Digital Output; P = Power/Ground; # = Active Low Pin

3.0 Functional Block Diagram

3.1 Voltage-Controlled Crystal Oscillator (VCXO)

The VCXO provides a tunable, low-jitter frequency reference for the rest of the FS6128 system components. Loading capacitance for

the crystal is internal to the FS6128. No external components (other than the resonator itself) are required for operation of the VCXO.

Continuous fine-tuning of the VCXO frequency is accomplished by varying the voltage on the XTUNE pin. The value of this voltage

controls the effective capacitance presented to the crystal. The actual amount that this load capacitance change will alter the oscillator

frequency depends on the characteristics of the crystal as well as the oscillator circuit itself.

It is important that the crystal load capacitance is specified correctly to “center” the tuning range. See Table 5.

A simple formula to obtain the “pulling” capability of a crystal oscillator is:

where:

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