ICS810251I Data Sheet

VCXO AND SYNCHRONOUS ETHERNET JITTER ATTENUATOR

ICS810251AGI REVISION B OCTOBER 5, 2012

10

©2012 Integrated Device Technology, Inc.

Choosing the correct external components and having a proper

printed circuit board (PCB) layout is a key task for quality operation

of the VCXO-PLL. In choosing a crystal, special precaution must be

frequency, accuracy and temperature range must also be

considered. Since the pulling range of a crystal also varies with the

package, it is recommended that a metal-canned package like HC49

be used. Generally, a metal-canned package has a larger pulling

range than a surface mounted device (SMD). For crystal selection

information, refer to the VCXO Crystal Selection Application Note.

resonating frequency and is closely related to the VCXO tuning

range. The total external capacitance seen by the crystal when

installed on a board is the sum of the stray board capacitance, IC

package lead capacitance, internal varactor capacitance and any

VCXO will oscillate at a higher frequency than the crystal

capacitance, the VCXO will oscillate at a lower frequency than the

crystal specification. In either case, the absolute tuning range is

balances the tuning range by centering the tuning curve.

The frequency of oscillation in the third overtone mode is not

necessarily at exactly three times the fundamental frequency. The

mechanical properties of the quartz element dictate the position of

the overtones relative to the fundamental. The oscillator circuit may

excite both the fundamental and overtone modes simultaneously.

This will cause a nonlinearity in the tuning curve. This potential

problem is why VCXO crystals are required to be tested for absence

of any activity inside a +/-200 ppm window at three times the

crystal Characteristics table.

The crystal and external loop filter

components should be kept as

close as possible to the device.

Loop filter and crystal traces

should be kept short and

separated from each other. Other

signal traces should be kept

separate and not run underneath

the device, loop filter or crystal

components.

VCXO Characteristics Table

VCXO-PLL Loop Bandwidth Selection Table

Crystal Characteristics

LF0

LF1

XTAL_IN

XTAL_OUT

C

S

C

P

C

TUNE

C

TUNE

25MHz

Symbol

Parameter

Typical

Units

VCXO Gain

15000

Hz/V

Low Varactor Capacitance

9.8

pF

High Varactor Capacitance

22.7

pF

Bandwidth

Crystal Frequency (MHz)

246Hz (Low)

25

0.4

10

0.01

616Hz (Mid)

25

1.0

10

0.001

1000Hz (High)

25

1.65

10

0.001

Symbol

Parameter

Test Conditions

Minimum

Typical

Maximum

Units

Mode of Oscillation

Fundamental

Frequency

25

MHz

Frequency Tolerance

±20

ppm

Frequency Stability

±20

ppm

Operating Temperature Range

-40

+85

Load Capacitance

10

pF

Shunt Capacitance

4

pF

Pullability Ratio

220

240

ESR

Equivalent Series Resistance

20



Drive Level

1mW

±3 per year

ppm