EL4585C
Horizontal Genlock 8 F
SC
Choosing Loop Filter
Components
The PLL VCO and loop filter can be described
as
transfer function C
v
e
F(V
C
) where V
C
is the re-
verse bias control voltage and C
V
is varactor ca-
pacitance Since F(V
C
) is nonlinear it is probably
best to build the VCO and measure K
VCO
about
2 5V The results of one such measurement are
shown below The slope of the curve is deter-
mined by linear regression techniques and equals
K
VCO
For our example K
VCO
e
9 06 Mrad s V
F
OSC
vs V
C
LC VCO
4585 – 13
Where
K
d
e
phase detector gain in A rad
F(s)
e
loop filter impedance in V A
K
VCO
e
VCO gain in rad s V
N
e
Total internal or external divisor (see 3 be-
low)
It can be shown that for the loop filter shown
below
C
3
e
K
d
K
VCO
C
3
2Ng
0
n
C
4
e
R
3
e
N
0
n2
10
K
d
K
VCO
5 Now we can solve for C
3
C
4
and R
3
C
3
e
K
d
K
VCO
(4 77e
b
5)(9 06e6)
e
e
0 01mF
N
0
n2
(1820)(5000)
2
C
3
e
0 001mF
10
2Ng
0
n
(2)(1820)(1)(5000)
e
e
42 1kX
(4 77e
b
5)(9 06e6)
K
d
K
VCO
4585 – 14
Where
0
n
e
loop filter bandwidth and
g
e
loop
filter damping factor
1 K
d
e
300mA 2
q
rad
e
4 77e-5A rad for the
EL4585C
2 The loop bandwidth should be about H-sync
frequency 20 and the damping ratio should be
1 for optimum performance For our example
0
n
e
15 734kHz 20
e
787 Hz 5000 rad S
3 N
e
910x2
e
1820 from table 1
N
e
F
VCO
28 636M
e
e
1820
e
910x2
F
Hsync
15 73426k
4 K
VCO
represents how much the VCO frequen-
cy changes for each volt applied at the control
pin It is assumed (but probably isn’t) linear
about the lock point (2 5V) Its value depends
on the VCO configuration and the varactor
C
4
e
R
3
e
We choose R
3
e
43kX for convenience
6 Notice R
2
has little effect on the loop filter de-
sign R
2
should be large around 100k and can
be adjusted to compensate for any static phase
error T
i
at lock but if made too large will
slow loop response If R
2
is made smaller T
i
(see timing diagrams) increases and if R
2
in-
10
ELANTEC [ ELANTEC SEMICONDUCTOR ]
