Datasheet

AD9577 Data Sheet
Rev. 0 | Page 36 of 44
Table 27. CkDiv and FracStep Values Used in Worked Example
CkDiv
Ideal
FracStep Rounded FracStep
Tabl e 2 6 shows the relevant register names and programmable
ranges.
Table 26. Registers Used to Program SSCG Operation
Parameter
FracStep Error
2 −1.5675 −2 21.6%
Register Name Range
NumSteps
3 −2.35125 −2
17.6%
BS2[7:0], BS3[7] +1 to +511
FracStep
4 −3.135 −3
4.5%
BS1[7:0]
−128 to 0
5 −3.91875 −4
2.0%
CkDiv BS3[6:0] +2 to +127
6 −4.7025 −5 6.0%
7 −5.48625 −5
9.7%
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 f
MOD
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.
8 −6.27 −6 4.5%
9 −7.05375 −7
0.77%
10 −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
Worked Example: Programming for f
MOD
= 31.25 kHz,
Downspread = −0.5%, f
PFD
= 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.
FracRange = −7 × 45 = −315
The accuracy of this solution needs to be verified. Putting the
derived values into Equation 13 gives
The total feedback divider ratio is
kHz30.86
9452
MHz25
2
=
××
=
××
=
CkDivNumSteps
f
f
PFD
MOD
N
TOT
= Nb +
M
OD
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
f
fFracRange
CkDivFracStep
2
(14)
In addition, the percentage frequency deviation is obtained as
%502.0
625
62698
625
315100
100
−=
×
−×
=
×
×
=
TOT
NMOD
FracRange
eviationFrequencyD
The f
MOD
and the percentage frequency deviation are very close to
the target values. The register settings required for this example
are detailed in Table 29.
Putting in the values for FracRange, f
MOD
, and f
PFD
from the
previous information, the following results:
FracStep = CkDiv × (−0.78375) (15)
SSCG Register Summary
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.
Tabl e 2 8 summarizes the programmable registers required to set
up SSCG.
Table 28. Register Values for SSCG
Parameter Register Names 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
Nb BF3[5:0] 0 to +51