AD9577BCPZ-R7 datasheet(36/44 Pages) AD

36 / 44 page

AD9577

Data Sheet

Rev. A | Page 36 of 44

Table 26 shows the relevant register names and programmable

ranges.

Table 26. Registers Used to Program SSCG Operation

Parameter

Range

NumSteps

BS2[7:0], BS3[7]

+1 to +511

FracStep

BS1[7:0]

−128 to 0

CkDiv

BS3[6:0]

+2 to +127

Because the register values need to be expressed as integers,

there are no guaranteed exact solutions; therefore, some

approximations and trade-offs must be made. The fact that

neither FracRange nor fMOD needs to be exact is exploited.

Note that the SSCG pin must be toggled every time the SSCG

parameters are adjusted for the changes to take effect.

Worked Example: Programming for fMOD = 31.25 kHz,

Downspread = −0.5%, fPFD = 25 MHz

Assume Nb = 100, MOD = 625, and FRAC = 198. In addition, a

large number of frequency steps are desired to cover −0.5%. The

objective is to find values for FracStep, NumSteps, and CkDiv

that result in the required frequency modulation profile.

The total feedback divider ratio is

NTOT = Nb +

MOD

FRAC

= 100 + 198/625 = 62,698/625

FracRange is set to −0.5% of 62,698, which results in an ideal

value of −313.5.

By rearranging Equation 12 and Equation 13, it results in













PFD

MOD

FracRange

CkDiv

FracStep

(14)

Putting in the values for FracRange, fMOD, and fPFD from the

previous information, the following results:

FracStep = CkDiv × (−0.78375)

(15)

An approximate solution must be found to Equation 15 that

produces an integer value for CkDiv, which gives a value that is

very close to an integer for FracStep. In this case, considering

CkDiv values in the range of 2 to 10 gives the FracStep values

shown in Table 27.

Table 27. CkDiv and FracStep Values Used in Worked Example

CkDiv

Ideal

FracStep

Rounded FracStep

FracStep Error

−1.5675

−2

21.6%

−2.35125

−2

17.6%

−3.135

−3

4.5%

−3.91875

−4

2.0%

−4.7025

−5

6.0%

−5.48625

−5

9.7%

−6.27

−6

4.5%

−7.05375

−7

0.77%

−7.8375

−8

2.0%

Both CkDiv and NumSteps must be integers. To minimize error,

CkDiv = 9 and FracStep = −7 was chosen. With a target for

FracRange = −313.5, Equation 12 is used to find the ideal value of

NumSteps = 44.79, which is rounded to 45. From Equation 12,

the actual used value for FracRange is

FracRange = −7 × 45 = −315

The accuracy of this solution needs to be verified. Putting the

derived values into Equation 13 gives

kHz

30.86

MHz

CkDiv

NumSteps

PFD

MOD

In addition, the percentage frequency deviation is obtained as

502

625

62698

625

315

100

−

TOT

MOD

FracRange

eviation

FrequencyD

The fMOD and the percentage frequency deviation are very close to

the target values. The register settings required for this example

are detailed in Table 29.

SSCG Register Summary

Table 28 summarizes the programmable registers required to set

up SSCG.

Table 28. Register Values for SSCG

Parameter

Range

NumSteps

BS2[7:0], BS3[7]

+1 to +511

FracStep

BS1[7:0]

−128 to 0

CkDiv

BS3[6:0]

+2 to +127

FRAC

BF0[7:0], BF1[7:4]

0 to +4094

MOD

BF1[3:0], BF2[7:0]

0 to +4095

BF3[5:0]

0 to +51

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