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3

4

5

6

7

8

9

10

11

12

30

40

50

60

3% Down Spread

2% Down Spread

fmodulation − kHz

+0.4 Center Spread

+0.25 Center Spread

Further EMI Reduction

11.3dB

7dB

Slew−Rate for Vccout = 2.5 V

11.3dB

11.3dB

7dB

7dB

Slew−Rate for Vccout = 3.3 V

nom−1

nom

nom+2

−3dB

6.4dB

nom−1

nom

nom+2

−2.5dB

5.6dB

CDCE906

SCAS814 – NOVEMBER 2005

The optimum damping is a combination of modulation amount, modulation frequency and the harmonics which

are considered. Note that higher order harmonic frequencies results in stronger EMI reduction because of

respective higher frequency deviation.

As seen in Figure 17 and Figure 18, a slower output slew rate and/or smaller output signal amplitude helps to

reduce EMI emission even more. Both measures reduce the RF energy of clock harmonics. The CDCE906

allows slew rate control in four steps between 0.6 ns and 3.3 ns (Byte 19-24, Bit [5:4]). The output amplitude is

Even a lower output supply voltage down to 1.8 V works, but the maximum frequency has to be considered.

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