VCO

IN

N

f

f

M

=

´

IN

OUT

f

N

f

Pdiv

M

=

´

25

CDCE925, CDCEL925

www.ti.com

SCAS847I – JULY 2007 – REVISED OCTOBER 2016

Product Folder Links: CDCE925 CDCEL925

Submit Documentation Feedback

Copyright © 2007–2016, Texas Instruments Incorporated

Typical Application (continued)

Figure 16. Modulation Frequency (fm) and Modulation Amount

10.2.2 Detailed Design Procedure

10.2.2.1 Spread Spectrum Clock (SSC)

Spread spectrum modulation is a method to spread emitted energy over a larger bandwidth. In clocking, spread

spectrum can reduce Electromagnetic Interference (EMI) by reducing the level of emission from clock distribution

network.

CDCS502 with a 25-MHz Crystal, FS = 1, fOUT = 100 MHz, and 0%, ±0.5, ±1%, and ±2% SSC

Figure 17. Comparison Between Typical Clock Power Spectrum and Spread-Spectrum Clock

10.2.2.2 PLL Multiplier/Divider Definition

At a given input frequency (ƒIN), the output frequency (ƒOUT) of the CDCEx925 is calculated with Equation 1.

where

•

M (1 to 511) and N (1 to 4095) are the multiplier/divide values of the PLL

•

Pdiv (1 to 127) is the output divider

(1)

The target VCO frequency (ƒVCO) of each PLL is calculated with Equation 2.

(2)