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參數(shù)資料
| 型號： | AD9548/PCBZ |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 12/112頁 |
| 文件大小： | 0K |
| 描述： | BOARD EVAL FOR AD9548 |
| 產品變化通告： | AD9548 Mask Change 20/Oct/2010 |
| 設計資源： | AD9548 Schematic AD9548 BOM AD9548 Eval Brd Layers |
| 標準包裝： | 1 |
| 主要目的： | 計時，時鐘發(fā)生器 |
| 嵌入式： | 否 |
| 已用 IC / 零件： | AD9548 |
| 主要屬性： | 62.5 ~ 450 MHz 輸出頻率 |
| 次要屬性： | SPI 和 I2C 兼容控制端口 |
| 已供物品： | 板 |
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Data Sheet
AD9548
Rev. E | Page 109 of 112
CALCULATING DIGITAL FILTER COEFFICIENTS
The digital loop filter coefficients (, , , and (see Figure 41))
relate to the time constants (T1, T2, and T3) associated with the
equivalent analog circuit for a third order loop filter (see Figure 68).
Note that AD9548 evaluation software contains a profile
designer that will compute these coefficients for you. The
user should not normally need to use these formulas.
R2
FROM
CHARGE
PUMP
TO
VCO
R3
C3
C1
C2
0
802
2-
0
42
Figure 68. Third Order Analog Loop Filter
The design process begins by deciding on two design
parameters related to the second order loop filter shown in
margin (.
R2
FROM
CHARGE
PUMP
TO
VCO
C1
C2
08
02
2
-04
3
Figure 69. Second Order Analog Loop Filter
An analysis of the second order loop filter leads to its primary
time constant, T1. It can be shown that T1 is expressible in terms
of fP and as
)
cos(
)
sin(
1
P
1
T
where
P
f
2
.
An analysis of the third order loop filter leads to the definition
of another time constant, T3. It can be shown that T3 is
expressible in terms of the desired amount of additional
attenuation introduced by R3 and C3 at some specified
frequency offset (fOFFSET) from the PLL output frequency.
OFFSET
ATTEN
3
T
1
10 10
where
OFFSET
f
2
.
Note that ATTEN is the desired excess attenuation in decibels.
Furthermore, ATTEN and ωOFFSET should be chosen so that
P
f
T
5
1
3
With an expression for T1 and T3, it is possible to define an
adjusted open-loop bandwidth (fC) that is slightly less than fP. It
can be shown that ωC (fC expressed as a radian frequency) is
expressible in terms of T1, T3, and θ (phase margin) as
1
)
tan(
1
)
tan(
2
3
1
3
1
3
1
3
1
3
1
3
1
C
T
It can also be shown that the adjusted open-loop bandwidth
leads to T2 (the secondary time constant of the second order
loop filter) expressed as
3
1
C
2
T
2
1
Calculation of the digital loop filter coefficients requires a
scaling constant, K (related to the system clock frequency, fS),
and the PLL feedback divide ratio, D.
S
f
K
33
2
125
,
578
,
517
,
30
1
V
U
S
D
where S, U, and V are the integer and fractional feedback
divider values that reside in the profile registers. Keep in mind
that the desired integer feedback divide ratio is one more than
the stored value of S (hence, the +1 term in the equation for D
in this equation). This leads to the digital filter coefficients
given by
2
2
1
2
C
3
C
1
C
1
2
C
T
K
T
D
T
2
1
S
T
f
1
32
1
S T
f
32
3
S T
f
32
Calculation of the coefficient register values requires the
application of some special functions described as follows:
The if() function
y = if(test_statement, true_value, false_value)
where test_statement is a conditional expression (for example, x
< 3), true_value is what y equals if the conditional expression is
true, and false_value is what y equals if the conditional
expression is false.
The round() function
y = round(x)
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