July 2005 ASM3P2108A

rev 0.3


Alliance Semiconductor
2575 Augustine Drive • Santa Clara, CA • Tel: 408.855.4900 • Fax: 408.855.4999 • www.alsc.com
Notice: The information in this document is subject to change without notice.
Peak EMI Reducing Solution

Features

ƒ FCC approved method of EMI attenuation.
ƒ Generates a 1X low EMI spread spectrum clock of
the input frequency.
ƒ Input frequency range: 25MHz to 45MHz.
ƒ Internal loop filter minimizes external components
and board space.
ƒ Frequency deviation: - 1.3%( Typ) @32MHz.
ƒ Low cycle-to-cycle jitter.
ƒ 5.0V ± 5% operating voltage range.
ƒ TTL or CMOS compatible outputs.
ƒ Available in 8-pin SOIC and TSSOP Packages.

Product Description

The ASM3P2108A is a versatile spread spectrum
frequency modulator designed specifically for input clock
frequencies from 25MHz to 45MHz. The ASM3P2108A
can generate an EMI reduced clock from crystal, ceramic
resonator, or system clock.

The ASM3P2108A reduces electromagnetic interference
(EMI) at the clock source, allowing system wide reduction
of EMI of down stream clock and data dependent
signals. The ASM3P2108A allows significant system cost
savings by reducing the number of circuit board layers
ferrite beads, shielding and other passive components
that are traditionally required to pass EMI regulations.

The ASM3P2108A uses the most efficient and optimized
modulation profile approved by the FCC and is
implemented in a proprietary all digital method.

The ASM3P2108A modulates the output of a single PLL
in order to “spread” the bandwidth of a synthesized clock,
and more importantly, decreases the peak amplitudes of
its harmonics. This results in significantly lower system
EMI compared to the typical narrow band signal produced
by oscillators and most frequency generators. Lowering
EMI by increasing a signal’s bandwidth is called ‘spread
spectrum clock generation’.

Applications

The ASM3P2108A is targeted towards EMI management
for high speed digital applications such as PC peripheral
devices, consumer electronics and embedded controller
systems.




Block Diagram
GND
XIN
XOUT

Crystal
Oscillator


Frequency
Divider
Feedback
Divider
Modulation
Phase
Detector
Loop
Filter
VCO
Output
Divider
CLOCKOUT
PLL
V
DD