SP8853A/B
Cx
R2
C2
R2
C1
C1
R1
R1
FROM
CHARGE
PUMP
FROM
CHARGE
PUMP
−
−
R3
TO VCO
C3
TO VCO
+
+
Fig. 11 Standard form of third order loop filter
Fig. 12 Modified form of third order loop filter
where Ku, K0, N and vn are as defined for the second order
loop and F0 is the phase margin, normally set to 45°. These
values can now be substituted in equation (3) to obtain a value
for C1 and in equations (4) and (5) to determine values for C2
and R2.
For Fig. 11,
For Fig. 12,
Substituting for C2:
t2 = R2 C11
t2 = R2 (C11C2)
t3 = C2R2
t3
R2
= R2 C11t3
Example
Calculate values for a third order loop with parameters as
for the second order loop and F0 = 45°.
t22t3
C1
or, R2=
From equation (5):
1
2tan 45°1
cos 45°
7·6873102421·31831024
0·015331026
t3 =
=
500Hz32p
0·4142
=
R2 = 41·627kΩ
3161·6
t3
t3 = C2R2 =
t3 = 131·8µs
R2
1·31831024
41627
C2 = 3·17nF
From equation (4):
=
1
t2 =
(500323p)231·31831024
For Fig. 12,
t1 = C1R1
t2 = 768·7µs
1·5331025
Using these values in equation (3):
or, C1 =
103
1
7·963102332p310MHz/V
3[A]2
t1 =
C1 = 0·0153nF
80003(50032p)2
11vn2 t22
t2 = C1R2
where A =
=
11vn2 t32
11(50032p)23(7·68731024 2
11(50032p)23(1·31831024 2
7·68731024
1·5331028
or, R2 =
)
)
R2 = 50·242kΩ
1
2
500141·6 6·832
7·89611010
t1 =
t3 = C2R3
1·1714
Since the values of C2 and R3 are independent of the other
components, either can be chosen and the other determined.
Assuming that R3 = 1kΩ, then
= 6·3343102632·415
t1 = 15·3µs
1·31831024
1·5331025
C2 =
Now, since t1 = C1R1 and R1 =1kΩ, C1
C1 = 0·0153µF
=
103
103
C2 = 0·01318µF
11
MITEL [ MITEL NETWORKS CORPORATION ]
