ICS813N252AKI-04 REVISION A MAY 24, 2011 17 ©2011 Integrated Device Technology, Inc.
ICS813N252I-04 Data Sheet VCXO JITTER ATTENUATOR & FEMTOCLOCK
®
NG MULTIPLIER
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 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 ±200 ppm 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
Crystal Characteristics
LF0
LF1
ISET
XTAL_IN
XTAL_OUT
R
S
C
S
C
P
R
SET
C
TUNE
C
TUNE
25MHz
Symbol Parameter Typical Units
k
VCXO
VCXO Gain 4.4 kHz/V
C
V_LOW
Low Varactor Capacitance 8 pF
C
V_HIGH
High Varactor Capacitance 16 pF
Bandwidth Crystal Frequency (MHz) M R
S
(kΩ)C
S
(µF) C
P
(µF) R
SET
(kΩ)
8Hz (Low) 25 3125 680 0.20 0.002 22
20Hz (Mid) 25 3125 470 0.20 0.002 5
75Hz (High) 25 3125 680 0.02 0.0003 2.2
Symbol Parameter Test Conditions Minimum Typical Maximum Units
Mode of Oscillation Fundamental
f
N
Frequency 25 MHz
f
T
Frequency Tolerance ±20 ppm
f
S
Frequency Stability ±20 ppm
Operating Temperature Range -40 85
0
C
C
L
Load Capacitance 10 pF
C
O
Shunt Capacitance 4 pF
C
O
/ C
1
Pullability Ratio 220 240
F
L_3OVT
3
rd
Overtone F
L
200 ppm
F
L_3OVT_spurs
3
rd
Overtone F
L
Spurs 200 ppm
ESR Equivalent Series Resistance 20
Ω
Drive Level 1mW
Aging @ 25
0
C ±3 per year ppm
