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IMIFS741

Low EMI Spectrum Spread Clock

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Approved Product

FS741

Low EMI Spectrum Spread Clock

Figure 10, at the right shows the 3rd. Harmonic of the 10 MHz clock in Figure 9. The big difference here is that

the bandwidth of the 3rd. harmonic is 3 times greater

than the bandwidth at the fundamental frequency.

Since the energy is spread over a much wider

bandwidth, the peak energy reduction will be greater.

As can be seen in this picture, the difference

between the modulated and un-modulated peaks is

approximatly 8 dB. With the bandwidth of the

fundamental at 2% or 200 Khz, the bandwidth at the

3rd. harmonic will be 600 KHz.

Figure 10.

Modulation Rate

The FS741 moves from max to min frequencies of its bandwidth at a pre-determined rate and profile. The

modulation frequency is determined by the input frequency and the range that is selected. The FS741 has four

input frequency operating ranges, 4 - 8 MHz, 8 - 16 MHz, 16 - 40 MHz and 40 - 68 MHz. The modulation rate is

determined by a divider that results in 1/60, 1/120, 1/240 and 1/480 of the input frequency in each range,

respectively. Refer to the chart on page 9.

Figure 11. Frequency Modulation Profile

The Xin reference clock determines the modulation frequency but the internal SSCG control logic determines the

actual modulation profile. It is very important to note that the Bandwidth of the clock modulation is determined by

the values of the loop filter applied to pin 4.

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 13 of 16

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