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Down Spread 3%

11.3dB

7dB

Center Spread + 0.4%

Down Spread 3%

11.3dB

11.3 dB

7dB

7 dB

Center Spread + 0.4%

SSC Modulation Amount

SSC Modulation Frequency

CDCE906

SCAS814 – NOVEMBER 2005

SSC also is bypass-able (Byte 25, Bit [6:4]), which powers-down the SSC output and set it to logic low state. The

non-SSC output of PLL2 is not affected by this mode and can still be used.

SSC is an effective method to reduce electro-magnetic interference (EMI) noise in high-speed applications. It

reduces the RF energy peak of the clock signal by modulating the frequency and spread the energy of the signal

to a broader frequency range. Because the energy of the clock signal remains constant, a varying frequency that

broadens the overtones necessarily lowers their amplitudes. Figure 15 shows the effect of SSC on a 54-MHz

clock signal for DSP

Figure 15. Spread Spectrum Clocking With Center Spread and Down Spread

The peak amplitude of the modulated clock is 11.3 dB lower than the non-modulated carrier frequency for down

spread and radiated less electro-magnetic energy.

In SSC mode, the user can select the SSC modulation amount and SSC modulation frequency. The modulation

amount is the frequency deviation based to the carrier (min/max frequency), whereas the modulation frequency

determines the speed of the frequency variation. In SSC mode, the maximum VCO frequency is limited to 167

MHz.

The CDCE906 supports center spread modulation and down spread modulation. In center spread, the clock is

symmetrically shifted around the carrier frequency and can be

±0.1%, ±0.25%, and ±0.4%. At down spread, the

clock frequency is always lower than the carrier frequency and can be 1%, 1.5%, 2%, and 3%. The down spread

is preferred if a system can not tolerate an operating frequency higher than the nominal frequency (over-clocking

problem).

Example:

Modulation Type

Minimum

Center

Maximum

Frequency

Frequency

Frequency

A

±0.25% center spread

53.865 MHz

54 MHz

54.135 MHz

B

1% down spread

53.46 MHz

—

54 MHz

C

53.73 MHz

53.865 MHz

54 MHz

(1)

A down spread of 0.5% of a 54-MHz carrier is equivalent to 59.865 MHz at a center spread of

±0.25%.

The modulation frequency (sweep rate) can be selected between 30 kHz and 60 kHz. It also based on the VCO

frequency as shown in the SSC Modulation Frequency Selection as shown on page 17. As shown in Figure 16,

the damping increases with higher modulation frequencies. It may be limited by the tracking skew of a

downstream PLL. The CDCE906 uses a triangle modulation profile which is one of the common profiles for SSC.

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