14©2016 Integrated Device Technology, Inc Revision B March 3, 2016
810001-21 Data Sheet
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 dependent 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
third overtone mode is not
necessarily at exactly three times
the fundamental frequency. The
mechanical properties of the
quartz element dictate the
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 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
VCXO-PLL Loop Bandwidth Selection Table
LF0
LF1
ISET
XTAL_IN
XTAL_OUT
R
S
C
S
C
P
R
SET
C
TUNE
C
TUNE
19.44MHz
Symbol Parameter Typical Units
k
VCXO
VCXO Gain 6.6 kHz/V
C
V_LOW
Low Varactor Capacitance 15 pF
C
V_HIGH
High Varactor Capacitance 29 pF
Bandwidth Crystal Frequency (MHz) M R
S
(k)C
S
(µF) C
P
(µF) R
SET
(k)
6Hz (Low) 27 4004 175 4.7 0.01 8.0
80Hz (Mid) 27 1000 150 0.22 0.001 2.21
475Hz (High) 27 92 125 0.1 0.0001 3.3
