AD9577BCPZ-R7 datasheet(35/44 Pages) AD

35 / 44 page

Data Sheet

AD9577

Rev. A | Page 35 of 44

MARGINING

By asserting the MARGIN pin, a second full frequency map can

be applied to the output ports. The values for the Na, V0, D0,

V1, and D1 parameters, and the Nb, FRAC, MOD, V2, D2, V3,

D3 parameters must be programmed over the I2C, although default

values exist. There are some limitations: the output buffer signal

formats cannot be changed, and the PLL2 fractional-N settings,

such as power-down of the SDM, and bleed settings cannot be

changed. The margining feature can be used to set higher than

nominal frequencies on each of the ports to test system robustness.

When the MARGIN pin signal level is changed, a new frequency

acquisition is performed.

SPREAD SPECTRUM CLOCK GENERATION (SSCG)

By asserting the SSCG (spread spectrum clock generator) pin,

PLL2 operates in spread spectrum mode, and the output

frequency modulates with a triangular profile. As the clock

signal energy spreads out over a range of frequencies, it reduces

the peak power at any one frequency when observed with a

spectrum analyzer through a resolution bandwidth filter. This

result improves the radiated emissions from the part and from

the devices that receive its clock.

The triangular-wave modulation is implemented by controlling

the divide ratio of the feedback divider. This is achieved by

ramping the fractional word to the SDM. Figure 43 shows an

example implementation. The PFD frequency, fPFD, is 25 MHz.

The starting VCO frequency, fVCO, is 25 MHz × (99 + 3072/4096),

giving 2.49375 GHz. By continuously ramping the FRAC word

down and up, this frequency is periodically reduced to 25 MHz ×

(99 + 1029/4096) = 2.481281 GHz. This results in a triangular

frequency modulation profile, with a peak downspread (that is,

peak percentage frequency reduction) of −0.5%. By controlling

the step size, number of steps, and the step rate, the modulation

frequency is adjusted.

fPFD

VCO

2.15GHz

2.55GHz

DIVIDE BY

80 TO 131

FEEDBACK

DIVIDER

SDM

3-BIT

FRAC

FRAC_TRIWAVE

SSCG

MOD

CKDIV

FRAC

FRACSTEP

NUMSTEPS

SSCG

TRIWAVE

GENERATOR

FRAC_TRIWAVE

fVCO

fVCO – 0.5%

TIME

VCO

FREQUENCY

TIME

FRAC_TRIWAVE = 3072

FRAC_TRIWAVE = 1029

Figure 43. Spread Spectrum Clock Generator with Triangular Wave

Modulation, fPFD = 25 MHz

Basic Spread Spectrum Programming

The SSCG is highly programmable; however, most applications

require that the frequency modulation rate be between 30 kHz

and 33 kHz and that the peak frequency deviation be −0.5%

downspread. The AD9577 supports downspread only, with a

maximum deviation of −0.5%.

The key parameters (which are not themselves registers) that

define the frequency modulation profile include the following:

•

fMOD, which is the frequency of the modulation waveform.

•

FracRange, which determines the peak frequency deviation

by setting the maximum change in the FRAC value from

the nominal.

The following equations determine the value of these parameters:

FracRange = FracStep × NumSteps

(12)

CkDiv

NumSteps

PFD

MOD

(13)

where the following are programmable registers:

•

NumSteps is the number of fractional word steps in half the

triwave period.

•

FracStep is the value of the fractional word increment/

decrement, while traversing the tri-wave.

•

CkDiv is the integer value by which the reference clock

frequency is divided to determine the update rate of the

triangular-wave generator, that is, the step update rate.

•

fPFD is the PFD frequency.

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