Rainbow-electronics MAX3679A Manual do Utilizador descarregar pdf (Página 8)

MAX3679A

+3.3V, Low-Jitter Crystal to LVPECL

Clock Generator

8 _______________________________________________________________________________________

Crystal Selection

The crystal oscillator is designed to drive a fundamental

mode, AT-cut crystal resonator. See Table 3 for recom-

mended crystal specifications. See Figure 4 for external

capacitance connection.

Crystal Input Layout and

Frequency Stability

The crystal, trace, and two external capacitors should

be placed on the board as close as possible to the

MAX3679A’s X_IN and X_OUT pins to reduce crosstalk

of active signals into the oscillator.

The layout shown in Figure 3 gives approximately 3pF

of trace plus footprint capacitors per side of the crystal

(Y1). The dielectric material is FR4 and dielectric thick-

ness of the reference board is 15 mils. Using a 25MHz

crystal and the capacitor values of C22 = 27pF and

C23 = 33pF, the measured output frequency accuracy

is -14ppm at +25°C ambient temperature.

Table 2. Output Divider Configuration

INPUT

SELA1/SELB1 SELA0/SELB0

NA/NB DIVIDER

0 0 ÷2*

1 0 ÷4

1 1 ÷5

0 OPEN ÷10

Table 3. Crystal Selection Parameters

PARAMETER SYMBOL MIN TYP MAX UNITS

Crystal Oscillation

Frequency

OSC

25 MHz

Shunt Capacitance C

2.0 7.0 pF

Load Capacitance C

18 pF

Equivalent Series

Resistance (ESR)

50 

Maximum Crystal

Drive Leve l

300 μW

Table 1. Output Frequency Determination

XO OR CMOS

INPUT

FREQUENCY

(MHz)

FEEDBACK

DIVIDER, M

VCO

FREQUENCY

(MHz)

OUTPUT

DIVIDER,

NA AND NB

OUTPUT

FREQUENCY

(MHz)

APPLICATIONS

÷2 312.5

÷4 156.25

÷5 125

25 25 625

÷10 62.5

Ethernet

*Maximum guaranteed output frequency is 160MHz for CMOS

and 320MHz for LVPECL output.

Figure 3. Crystal Layout

X_IN

X_OUT

27pF

25MHz CRYSTAL

= 18pF)

33pF

Figure 4. Crystal, Capacitors Connection

1 2 3 4 5 6 7 8 9 10 11

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