IMIFS741 Ver la hoja de datos (PDF) - Unspecified

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IMIFS741

Low EMI Spectrum Spread Clock

Unspecified 

Approved Product

FS741

Low EMI Spectrum Spread Clock

We see that the original 20 MHz reference clock is at the center Frequency,

Fc, and the minimum and maximum extremes are positioned symmetrically

about the center frequency. This type of modulation is called Center-

Spread. Figure 7 illustrates this as it is seen on a spectrum analyzer.

Fc = 20 MHz

Fmin =

19.8 MHz

Fmax =

20.2 MHz

Figure 7.

Figure 8 shows a 20 MHz clock as it would be seen

on an oscilloscope. The top trace is the non-

modulated reference clock, or the Refout clock at pin

7. The bottom trace is the modulated clock at pin 6.

From this comparison chart you can see that the

frequency is decreasing and the period of each

successive clock increasing. The Tc measurements

on the left and right of the bottom trace indicate the

max. and min. extremes of the clock. Intermediate

clock changes are small and accumulate to achieve

the total period deviation. The reverse of this Figure

would show the clock going from min. extreme back

to the high extreme.

Tc =49.50 ns.

Tc = 50.50 ns.

Figure 8. Period Comparison Chart

The FS741 is a center spread clock, meaning that it symetrically spreads above and below the reference

frequency. If a down spread clock is required for a specific application, refer to the datasheets for the SM530 and

the SG52x series of Low EMI clock generators.

Looking at figure 7, you will note that the peak amplitude of the 20 MHz non-modulated clock is higher than the

wideband modulated clock. This difference in peak amplitudes between modulated and unmodulated clocks is the

reason why SSCG clocks are so effective in digital systems. This figure refers to the fundamental frequency of a

clock. A very important characteristic of the SSCG clock is that the bandwidth of the harmonics is multiplied by the

harmonic number. In other words, if the bandwidth of a 20 MHz clock is 200 KHz, the bandwidth of the 3rd

harmonic will be 3 times 200 KHz, or 600 KHz. The amount of bandwidth is relative to the amount of energy in the

clock. Consequently, the wider the bandwidth, the greater the energy reduction of the clock.

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 11 of 16

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