ICS810001-21 Datasheet, PDF (14/19 Pages) Integrated Circuit Systems – FEMTOCLOCKS-TM DUAL VCXO VIDEO PLL

ICS810001-21 Data Sheet
FEMTOCLOCK™ DUAL VCXO VIDEO PLL
VCXO-PLL EXTERNAL COMPONENTS
Choosing the correct external components and having a proper
printed circuit board (PCB) layout is a key task for quality operation of
the VCXO-PLL. In choosing a crystal, special precaution must be
taken with the package and load capacitance (C_L). In addition,
frequency, accuracy and temperature range must also be
considered. Since the pulling range of a crystal also varies with the
package, it is recommended that a metal-canned package like HC49
be used. Generally, a metal-canned package has a larger pulling
range than a surface mounted device (SMD). For crystal selection
information, refer to the VCXO Crystal Selection Application Note.
The crystal’s load capacitance C_L characteristic determines its
resonating frequency and is closely related to the VCXO tuning
range. The total external capacitance seen by the crystal when
installed on a board is the sum of the stray board capacitance, IC
package lead capacitance, internal varactor capacitance and any
installed tuning capacitors (C_TUNE).
If the crystal C_L is greater than the total external capacitance, the
VCXO will oscillate at a higher frequency than the crystal
specification. If the crystal C_L is lower than the total external
capacitance, the VCXO will oscillate at a lower frequency than the
crystal specification. In either case, the absolute tuning range is
reduced. The correct value of C_L is dependant on the characteristics
of the VCXO. The recommended C_L in the Crystal Parameter Table
balances the tuning range by centering the tuning curve.
The frequency of oscillation in the
LF0
third overtone mode is not
LF1
necessarily at exactly three times
ISET
the fundamental frequency. The
R_S R_SET
mechanical properties of the
quartz element dictate the
C_P C_S
position of the overtones relative
to the fundamental. The oscillator
circuit may excite both the
fundamental and overtone modes
simultaneously. This will cause a
nonlinearity in the tuning curve.
This potential problem is why
C_TUNE
19.44MHz
C_TUNE
XTAL_IN
XTAL_OUT
VCXO crystals are required to be
tested for absence of any activity inside a ±200ppm window at three
times the fundamental frequency. Refer to F_{L_3OVT} and F_{L_3OVT_spurs}
in the crystal Characteristics table.
The crystal and external loop filter components should be kept as
close as possible to the device. Loop filter and crystal traces should
be kept short and separated from each other. Other signal traces
should be kept separate and not run underneath the device, loop
filter or crystal components.
VCXO Characteristics Table
Symbol Parameter
k_VCXO
C_{V_LOW}
C_{V_HIGH}
VCXO Gain
Low Varactor Capacitance
High Varactor Capacitance
Typical
6.6
15
29
Units
kHz/V
pF
pF
VCXO-PLL Loop Bandwidth Selection Table
Bandwidth
Crystal Frequency (MHz)
M
6Hz (Low)
27
4004
80Hz (Mid)
27
1000
475Hz (High)
27
92
R_S (kΩ)
175
150
125
C_S (µF)
4.7
0.22
0.1
C_P (µF)
0.01
0.001
0.0001
R_SET (kΩ)
8.0
2.21
3.3
ICS810001DK-21 REVISION B APRIL 13, 2010
14
©2010 Integrated Device Technology, Inc.