AD9577BCPZ-R7 Analog Devices Inc, AD9577BCPZ-R7 Datasheet - Page 36

• Manufacturer Part Number: AD9577BCPZ-R7
• Manufacturer: Analog Devices Inc

Specifications of AD9577BCPZ-R7

• Lead Free Status / Rohs Status: Compliant

AD9577
Table 26 shows the relevant register names and programmable
ranges.
Table 26. Registers Used to Program SSCG Operation
Parameter
NumSteps
FracStep
CkDiv
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
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 f
Downspread = −0.5%, f
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
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
Putting in the values for FracRange, f
previous information, the following results:
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.
N
FracStep = CkDiv × (−0.78375)
FracStep
TOT
= Nb +
=
CkDiv
Register Name
BS2[7:0], BS3[7]
BS1[7:0]
BS3[6:0]
FRAC
MOD
×
MOD
⎛
⎜
⎜
⎝
= 100 + 198/625 = 62,698/625
2
PFD
×
needs to be exact is exploited.
FracRange
= 25 MHz
f
PFD
MOD
Range
+1 to +511
−128 to 0
+2 to +127
×
, and f
MOD
f
MOD
= 31.25 kHz,
PFD
⎞
⎟
⎟
⎠
from the
(14)
(15)
Rev. 0 | Page 36 of 44
Table 27. CkDiv and FracStep Values Used in Worked Example
CkDiv
2
3
4
5
6
7
8
9
10
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
The accuracy of this solution needs to be verified. Putting the
derived values into Equation 13 gives
In addition, the percentage frequency deviation is obtained as
The f
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
NumSteps
FracStep
CkDiv
FRAC
MOD
Nb
FracRange = −7 × 45 = −315
MOD
=
FrequencyD
f
MOD
625
100
and the percentage frequency deviation are very close to
Ideal
FracStep
−1.5675
−2.35125
−3.135
−3.91875
−4.7025
−5.48625
−6.27
−7.05375
−7.8375
=
×
×
62698
2
−
625
×
315
NumSteps
eviation
=
Register Names
BS2[7:0], BS3[7]
BS1[7:0]
BS3[6:0]
BF0[7:0], BF1[7:4]
BF1[3:0], BF2[7:0]
BF3[5:0]
−
f
Rounded FracStep
−2
−2
−3
−4
−5
−5
−6
−7
−8
PFD
. 0
502
=
×
100
CkDiv
%
MOD
×
FracRange
×
=
N
2
25
TOT
×
MHz
45
Range
+1 to +511
−128 to 0
+2 to +127
0 to +4094
0 to +4095
0 to +51
×
9
Data Sheet
FracStep Error
21.6%
17.6%
4.5%
2.0%
6.0%
9.7%
4.5%
0.77%
2.0%
=
30.86
kHz