dp83241 National Semiconductor Corporation, dp83241 Datasheet - Page 10

• Manufacturer Part Number: dp83241
• Description: Cdd Device Fddi Clock Distribution Device
• Manufacturer: National Semiconductor Corporation

Loop Filter Calculations
Several constants need to be known in order to determine
the loop filter components They are the loop divide ratio N
the phase detector gain K
bandwidth W
and K
Grad V respectively N is equal to the VCO frequency divid-
ed by the reference frequency w
less than 1 20th of the reference frequency (times 2
rads) Having found all these constants the following equa-
tions are used to find the component values
For w
For a phase margin other than 57
The component equations above are not meant to provide
optimal solutions for all implementations
R
C
C
R
C
C
1
1
2
1
1
2
e
e
e
e
e
e
o
e
(1 1 N w
(3 K
(0 3 K
(Cosec w
(Tan w) ((2 K
(Sec w
for the DP83241 are fixed at 80 mA rad and 0 8
57 phase margin
p
o
K
p
o
b
K
and the phase margin w The constants K
o
) (N w
o
) (K
( (N w
a
Tan w) ((K
1) ((N w
p
p
K
o
K
2
o
o
o
)
) (N w
)
2
)
p
p
o
K
the VCO gain K
2 K
o
o
2
))
(N w
o
p
K
is recommended to be
o
))
o
2
))
o
the loop
q
p
10
Let us now design an example system with the following
characteristics
Since the VCO is twenty times the frequency of the refer-
ence frequency we get N
of the reference frequency or 2 62 x 10
From these values we get
Let us now design an example system with the following
characteristics
We will set w
1 0 x 10
From these values we get




12 5 MHz Crystal reference
250 MHz VCO
12 5 MHz Crystal reference
250 MHz External VCO with a gain of 40 MRad V
C
C
1
6
1
e
Rad
e
1400 pF C
470 pF C
o
to be 1 78th of the reference frequency or
2
2
e
e
e
47 pF and R
140 pF and R
20 We will set w
6
1
TL F 10385 – 27
Rad
e
1
e
6 8 kX
o
to be 1 30th
900X