EL4585C Elantec Semiconductor, Inc. (acquired by Intersil), EL4585C Datasheet - Page 11

• Manufacturer Part Number: EL4585C
• Description: Horizontal Genlock, 8FSC
• Manufacturer: Elantec Semiconductor, Inc. (acquired by Intersil)

creases T decreases For LDET to be low at
lock
high frequency noise from the charge pump The
effect these components have on time to lock is
illustrated below
Lock Time
Let T
but so does lock time Decreasing T decreases
damping and speeds up loop response but in-
creases overshoot and thus increases the number
of hunting oscillations before lock Critical damp-
ing (
decreased damping also decreases loop stability
it is sometimes desirable to design slightly over-
damped (
stability
Frequency
26 602
27 0
29 5
35 468
21 476
24 546
28 636
LC Loop Filter Components (Approximate)
(MHz)
l
e
e
T
R
1) occurs at minimum lock time Because
l
3
k
C
l
3
50nS C
1) trading lock time for increased
As T increases damping increases
Typical Loop Filter
(k )
100
100
100
100
100
100
100
R2
4
is used mainly to attenuate
(k )
R3
39
39
43
51
30
36
43
( F)
0 01
0 01
0 01
0 01
0 01
0 01
0 01
C3
0 001
0 001
0 001
0 001
0 001
0 001
0 001
( F)
4585 – 16
C4
11
PCB Layout Considerations
It is highly recommended that power and ground
planes be used in layout The oscillator and filter
sections constitute a feedback loop and thus care
must be taken to avoid any feedback signal influ-
encing the oscillator except at the control input
The entire oscillator filter section should be sur-
rounded by copper ground to prevent unwanted
influences from nearby signals Use separate
paths for analog and digital supplies keeping the
analog (oscillator section) as short and free from
spurious signals as possible Careful attention
must be paid to correct bypassing Keep lead
lengths short and place bypass caps as close to
the supply pins as possible If laying out a PCB
to use discrete components for the VCO section
care must be taken to avoid parasitic capacitance
at the OSC pins 3 and 5 and FILTER out (pin
7) Remove ground and power plane copper
above and below these traces to avoid making a
capacitive connection to them It is also recom-
mended to enclose the oscillator section within a
shielded cage to reduce external influences on the
VCO as they tend to be very sensitive to ‘‘hand
waving’’ influences the LC variety being more
sensitive than crystal controlled oscillators In
general the higher the operating frequency the
more important these considerations are Self
contained VCXO or VCO modules are already
mounted in a shielding cage and therefore do not
require as much consideration in layout Many
crystal manufacturers publish informative litera-
ture regarding use and layout of oscillators which
should be helpful
Xtal Loop Filter Components (Approximate)
Frequency
26 602
27 0
29 5
35 468
21 476
24 546
28 636
(MHz)
Horizontal Genlock 8 F
(k )
100
100
100
100
100
100
100
R2
EL4585C
(M )
R3
4 3
4 3
4 3
4 3
4 3
4 3
4 3
(pF)
C3
68
68
68
68
68
68
68
(pF)
SC
C4
6 8
6 8
6 8
6 8
6 8
6 8
6 8