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ICS810251 Datasheet(PDF) 10 Page - Integrated Device Technology |
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ICS810251 Datasheet(HTML) 10 Page - Integrated Device Technology |
10 / 15 page ICS810251I Data Sheet VCXO AND SYNCHRONOUS ETHERNET JITTER ATTENUATOR ICS810251AGI REVISION B OCTOBER 5, 2012 10 ©2012 Integrated Device Technology, Inc. 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 (CL). 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 CL 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 (CTUNE). If the crystal CL is greater than the total external capacitance, the VCXO will oscillate at a higher frequency than the crystal specification. If the crystal CL 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 CL is dependant on the characteristics of the VCXO. The recommended CL 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 FL_3OVT and FL_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 XTAL_IN XTAL_OUT RS C S C P C TUNE C TUNE 25MHz Symbol Parameter Typical Units kVCXO VCXO Gain 15000 Hz/V CV_LOW Low Varactor Capacitance 9.8 pF CV_HIGH High Varactor Capacitance 22.7 pF Bandwidth Crystal Frequency (MHz) RS (k)CS (µF) CP (µF) 246Hz (Low) 25 0.4 10 0.01 616Hz (Mid) 25 1.0 10 0.001 1000Hz (High) 25 1.65 10 0.001 Symbol Parameter Test Conditions Minimum Typical Maximum Units Mode of Oscillation Fundamental fN Frequency 25 MHz fT Frequency Tolerance ±20 ppm fS Frequency Stability ±20 ppm Operating Temperature Range -40 +85 0C CL Load Capacitance 10 pF CO Shunt Capacitance 4 pF CO / C1 Pullability Ratio 220 240 ESR Equivalent Series Resistance 20 Drive Level 1mW Aging @ 25 0C ±3 per year ppm |
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