7dB

CenterSpread±0.4%

9

th

Harmonic,f =60

m

DownSpread3%

9

th

Harmonic,f =60

m

11.3dB

C001

SSC Modulation Amount

SSC Modulation Frequency

CDCE706

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

Figure 19. Spread-Spectrum Clocking With Center Spread and Down Spread

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

spread and radiates less electromagnetic energy.

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

amount is the frequency deviation relative 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 CDCE706 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%, or ±0.4%. For down spread, the

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

preferred if a system cannot tolerate an operating frequency higher than the nominal frequency (overclocking

problem).

Example:

Minimum

Center

Maximum

Modulation Type

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

0.5% down spread(1)

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 is also based on the

VCO frequency as shown in the SSC Modulation Amount as shown in the Byte 25, Bits [6:4] table. As shown in

Figure 20, the damping increases with higher modulation frequencies. It may be limited by the tracking skew of a

downstream PLL. The CDCE706 uses a triangle modulation profile which is one of the common profiles for SSC.

26

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