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11-9
RF2516
Rev A17 060712
The schematic of the loop filter is:
The transfer function is:
where the time constants are defined as:
and
The frequency at which unity gain occurs is given by:
This is also the loop bandwidth.
If the phase margin (PM) and the loop bandwidth (
ω
LBW) are known, it is possible to calculate the time constants. These
are found using the equations4:
and
4 k
Ω
LOOP FLT
L
L
RESNTR+
RESNTR-
Fs
()
R
2
s
τ
2
1
+
⋅
s
τ
2
s
τ
1
1
)
+
⋅
(
⋅⋅
-------------------------------------------
⋅
=
τ
2
R
2
C
2
⋅
=
τ
1
R
2
C
1
C
2
⋅
C
1
C
2
+
-------------------⎠
⎞
⎝
⎛
⋅
=
ω
LBW
1
τ
1
τ
2
⋅
-------------------
=
τ
1
PM
()
sec
PM
()
tan
–
ω
LBW
--------------------------------------------------
=
τ
2
1
ω
LBW
2
τ
1
⋅
------------------------
=
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