CDCE706 datasheet(27/40 Pages) TI

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fModulation − Modulation Frequency − kHz

G006

±0.4 Center Spread

±0.25 Center Spread

2% Down Spread

3% Down Spread

Further EMI Reduction

11.3dB

7dB

11.3dB

7dB

Nom – 1

Nom

Nom+2

–3dB

–2.5dB

6.4dB

5.6dB

Slew-RateforV

=2.5V

CCOUT

Slew-RateforV

=3.3V

CCOUT

C002

Nom – 1

Nom

Nom+2

CDCE706

www.ti.com ........................................................................................................................................... SCAS815I – OCTOBER 2005 – REVISED NOVEMBER 2008

Figure 20. EMI Reduction vs fModulation and fAmount

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

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

frequency deviation.

As seen in Figure 21 and Figure 22, 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 CDCE706

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

set by the two independent output supply voltage pins, VCCOUT1 and VCCOUT2, and can vary from 2.3 V to 3.6 V.

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

Figure 21. EMI Reduction vs Slew-Rate and VCCOUT

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