IMIFS741 データシートの表示（PDF） - Unspecified

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IMIFS741

Low EMI Spectrum Spread Clock

Unspecified 

Approved Product

FS741

Low EMI Spectrum Spread Clock

Figure 13 is the equivalent internal oscillator circuit used in the FS741;

.

250 K

1

Xin

1

2

.

2

Xout

3 pF

5 pF

.

Figure 13.

Calculating dB Reduction

The dB reduction for a give frequency and spread can be calculated using a simple formula. This formula is only

helpful in determining a relative dB reduction for a given application. This formula assumes an ideal clock with

50% duty cycle and therfore only predicts the EMI reduction of odd harmonics. Other circumstances such as non-

ideal clock and noise will affect the actual dB reduction. The formula is as follows:

dB = 6.5 + 9(Log10(F)) + 9(Log10(P))

Where; F = Frequency in Mhz, P = total % spread (2.5% = .025)

Using a 50 Mhz clock with a 2.5% spread, the theoretical dB reduction would be;

db @ 50 Mhz (Fund) = 6.5 + 15.29 - 14.42 = 7.37

dB @ 150 Mhz (3rd) = 6.5 + 19.58 - 14.42 = 11.66

dB @ 550 Mhz (11th) = 6.5 + 24.66 - 14.42 = 16.74

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 15 of 16

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