11
FN7174.3
May 9, 2008
smaller, tθ (see “Timing Diagrams” on page 5) increases,
and if R
2
increases, tθ decreases. For LDET to be low at
lock, |tθ| < 50 ns. C
4
is used mainly to attenuate high
frequency noise from the charge pump.
Lock Time
Let = R
3
C
3
. As t increases, damping increases, but so does
lock time. Decreasing t decreases damping and speeds up
loop response, but increases overshoot and thus increases
the number of hunting oscillations before lock. Critical
damping (ζ = 1) occurs at minimum lock time. Because
decreased damping also decreases loop stability, it is
sometimes desirable to design slightly overdamped (ζ > 1),
trading lock time for increased stability.
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 influencing the oscillator except at the
control input. The entire oscillator/filter section should be
surrounded 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 capacitors 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 recommended to
enclose the oscillator section within a shielded cage to
reduce external influences on the VCO, as they tend to be
very sensitive to “handwaving” 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 literature
regarding use and layout of oscillators which should be
helpful.
The VCO and loop filter section of the EL4583, EL4584,
EL4585 demo board can be implemented in Figures 18, 19
and 20.
TABLE 5. LC LOOP FILTER COMPONENTS (APPROXIMATE)
FREQUENCY
(MHz)
R
2
(kΩ)
R
3
(kΩ)
C
3
(µF)
C
4
(µF)
13.301 100 30 0.01 0.001
13.5 100 30 0.01 0.001
14.75 100 33 0.01 0.001
17.734 100 39 0.01 0.001
10.738 100 22 0.01 0.001
12.273 100 27 0.01 0.001
14.318 100 30 0.01 0.001
C
3
K
d
K
VCO
Nω
2
n
------------------------
4.77e 5–()6.05e6()
910()5000()
2
------------------------------------------------------
0.01µF== =
(EQ. 9)
C
4
C
3
10
-------
0.0001µF==
(EQ. 10)
R
3
2Nζω
n
K
d
K
VCO
------------------------
2()910()1()5000()
4.77e 5–()6.05e6()
------------------------------------------------------
31.5kΩ== =
(EQ. 11)
FIGURE 17. TYPICAL LOOP FILTER
TABLE 6. XTAL LOOP FILTER COMPONENTS
(APPROXIMATE)
FREQUENCY
(MHz)
R
2
(kΩ)
R
3
(MΩ)
C
3
(pF)
C
4
(pF)
13.301 100 4.3 68 6.8
13.5 100 4.3 68 6.8
14.75 100 4.3 68 6.8
17.734 100 4.3 68 6.8
10.738 100 4.3 68 6.8
12.273 100 4.3 68 6.8
14.318 100 4.3 68 6.8
EL4584
