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IMIFS741

Low EMI Spectrum Spread Clock

Unspecified 

Approved Product

FS741

Low EMI Spectrum Spread Clock

FEATURES

• Reduces Systemic EMI.

• Modulates external clocks including crystals,

crystal oscillators and ceramic resonators.

• 3 - 5 Volt power supply.

• Modulation programmable with simple

external loop filter (LF).

• 4 to 68 MHz operating frequency range.

• Digitally controlled modulation.

• TTL/CMOS compatible outputs.

• Center Spread Modulation.

• Low short term jitter.

• Bi-Directional buffers for reduced pin count.

• Unmodulated Reference Clock Output.

• Low Power Dissipation;

3.3 VDC = 30 mw - typical

5.0 VDC = 100 mw - typical

• Available in 8 pin SOIC package.

APPLICATIONS

• Desktop/Laptop Computer

• Modems

• Scanners, Printers, Copiers, Fax Machines, MFP’s

• Disk and CD-ROM Drives

• Automotive and Embedded Systems

• Networking, LAN/WAN

• Digital Cameras, Games

• LCD displays

BENEFITS

• Reduction of radiated EMI

• Fast Time to Market

• Lower cost of compliance

• Programmable EMI reduction

• No degradation in Rise/Fall times

• Lower component and PCB layer count

PRODUCT DESCRIPTION

The IMI FS741 is a Frequency Spreading EMI Attenuator designed for the purpose of reducing Electro Magnetic

Interference (EMI) found in today’s high speed digital systems. The FS741 uses a proprietary technique to

modulate the output clock frequency, Modout. By modulating the frequency of the digital clock, measured EMI at

the fundamental and harmonic frequencies is greatly reduced. This reduction in radiated energy can significantly

reduce the cost of complying with regulatory requirements without degrading digital waveforms.

The IMI FS741 is a very simple device to use. By programming the two range select lines, RS0 and RS1, the

FS741 can operate over a very wide range of input frequencies. By utilizing IMI’s Bi-Directional Buffer design, the

pin count of the FS741 is kept to a minimum. Bi-Directional Buffers is a method of providing an input control

signal and an output driver circuit on the same pin. Bi-Directional Buffers is discussed further on page 6. The

FS741 also provides a buffered reference clock output, Refout, which can be turned on or off by controlling the

Ref_Off pin. The Refout clock can be used to drive system logic that can not tolerate Frequency Spreading, or as

a comparison of EMI reduction with respect to the Modout driver. The FS741 has a simple frequency selection

table that allows it to operate from 4 MHz to 68 MHz in four separate ranges. The bandwidth of the frequency

spread at Modout is determined by the values of the loop filter components. The modulation rate is determined by

the input frequency and the input frequency range selected. The bandwidth of the FS741 can be programmed

from as little as 0.3% up to as much as 4.0% by selecting the proper loop filter. It is for this reason that the FS741

uses an external loop filter (LF), in contrast to an internal loop filter type device which would severly limit the use of

a wide range of bandwidths.

International Microcircuits, Inc.

525 Los Coches St., Milpitas, 95035 408-263-6300, FAX 408-263-6571

http:/www.imicorp.com

4/5/1999

Rev. 2.1

Page 1 of 16

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