ICS810251 Datasheet, PDF (10/15 Pages) Integrated Device Technology – VCXO and Synchronous Ethernet Jitter Attenuator

ICS810251I Data Sheet
VCXO AND SYNCHRONOUS ETHERNET JITTER ATTENUATOR
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.
VCXO Characteristics Table
Symbol Parameter
Typical
Units
k_VCXO
C_{V_LOW}
C_{V_HIGH}
VCXO Gain
Low Varactor Capacitance
High Varactor Capacitance
15000
9.8
22.7
Hz/V
pF
pF
VCXO-PLL Loop Bandwidth Selection Table
Bandwidth
Crystal Frequency (MHz)
246Hz (Low)
25
R_S (k)
0.4
616Hz (Mid)
25
1.0
1000Hz (High)
25
1.65
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.
R_S
C_P C_S
C_TUNE
25MHz
C_TUNE
LF0
LF1
XTAL_IN
XTAL_OUT
C_S (µF)
10
10
10
C_P (µF)
0.01
0.001
0.001
Crystal Characteristics
Symbol Parameter
Mode of Oscillation
f_N
Frequency
f_T
Frequency Tolerance
f_S
Frequency Stability
Operating Temperature Range
C_L
C_O
C_O / C₁
ESR
Load Capacitance
Shunt Capacitance
Pullability Ratio
Equivalent Series Resistance
Drive Level
Aging @ 25 ⁰C
Test Conditions
Minimum Typical Maximum
Fundamental
25
±20
±20
-40
+85
10
4
220
240
20
1
±3 per year
Units
MHz
ppm
ppm
0_C
pF
pF

mW
ppm
ICS810251AGI REVISION B OCTOBER 5, 2012
10
©2012 Integrated Device Technology, Inc.