ICS813252I-02 Datasheet, PDF (15/22 Pages) Integrated Device Technology – VCXO JITTER ATTENUATOR & FEMTOCLOCK™ MULTIPLIER

ICS813252I-02
VCXO JITTER ATTENUATOR & FEMTOCLOCK™ 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 ). In addition, frequency, accuracy and
L
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 characteristic determines its
L
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’s C is greater than the total external capacitance,
L
the VCXO will oscillate at a higher frequency than the crystal
specification. If the crystal’s C is lower than the total external
L
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 is dependent on the
L
characteristics of the VCXO. The recommended C in the Crystal
L
Parameter Table balances the tuning range by centering the
tuning curve.
The VCXO-PLL Loop Bandwidth Selection Table shows R , C
S
S
and C values for recommended high, mid and low loop
P
bandwidth configurations. The device has been characterized
using these parameters. For other configurations, refer to the
Loop Filter Component Selection for VCXO Based PLLs
Application Note.
The crystal and external loop
filter components should be
kept as close as possible to the
LF0
LF1
ISET
R_S R_SET
device. Loop filter and crystal
traces should be kept short and
C_P C_S
separated from each other.
Other signal traces should be
kept separate and not run
under neath the device, loop
filter or crystal components.
C_TUNE
25MHz
XTAL_IN
XTAL_OUT
C_TUNE
VCXO CHARACTERISTICS TABLE
Symbol Parameter
k_VCXO
C_{V_LOW}
C_{V_HIGH}
VCXO Gain
Low Varactor Capacitance
High Varactor Capacitance
Typical
15,700
9.9
22.2
Unit
Hz/V
pF
pF
VCXO-PLL APPROXIMATE LOOP BANDWIDTH SELECTION TABLE
Bandwidth
10Hz (Low)
Crystal Frequency (MHz)
25MHz
R_S (kΩ)
121
C_S (µF)
1.0
90Hz (Mid)
25MHz
221
0.1
300Hz (High)
25MHz
680
0.1
C_P (µF)
0.01
0.001
0.0001
R_SET (kΩ)
9.09
2.21
2.21
CRYSTAL CHARACTERISTICS
Symbol Parameter
Mode of Operation
f_N
Frequency
f_T
Frequency Tolerance
f
Frequency Stability
S
Operating Temperature Range
C_L
C_O
C /C
O1
ESR
Load Capacitance
Shunt Capacitance
Pullability Ratio
Equivalent Series Resistance
Drive Level
Aging @ 25°C
Minimum Typical Maximum
Fundamental
25
±20
±20
-40
85
10
4
220
240
20
1
±3 per year
Units
MHz
ppm
ppm
°C
pF
pF
mW
ppm
IDT^™ / ICS^™ VCXO JITTER ATTENUATOR/MULTIPLIER
15
CS813252CKI-02 REV. A OCTOBER 22, 2008