lu82562ex Intel Corporation, lu82562ex Datasheet - Page 59

• Manufacturer Part Number: lu82562ex
• Description: Dual Footprint
• Manufacturer: Intel Corporation

Appendix A Measuring LAN Reference Frequency Using a
A.1
A.2
Note: Intel recommends a frequency tolerance of ±30 (ppm).
Required Test Equipment
Frequency Counter
Background
To comply with IEEE specifications for 10/100 Mbps and 10/100/1000Base-T Ethernet LAN, the
transmitter reference frequency must be correct and accurate within ±50 parts per million (ppm).
Most Intel LAN devices will operate properly with a 25.000 MHz reference crystal, provided it
meets the recommended requirements for frequency stability, equivalent series resistance at
resonance (ESR), and load capacitance.
Most circuits for series resonant crystals include two discrete capacitors (typically C1 and C2),
with values between 5 pF and 36 pF.
The most accurate way to determine the appropriate value for the discrete capacitors is to install the
approximately correct values for C1 and C2. Next, a frequency counter should be used to measure
the transmitter reference frequency (or transmitter reference clock).
This Appendix provides instructions and illustrations that explain how to use a frequency counter
and probe to determine the Ethernet LAN device transmit center frequency. An example describing
how to calculate the frequency accuracy of the measured and averaged center frequency with
respect to the target center frequency is also included.
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If the transmitter reference frequency is more than 20 ppm below the target frequency, then the
values for C1 and C2 are too big and should be decreased.
If the transmitter reference frequency is more than 20 ppm above the target frequency, then the
values for C1 and C2 are too small and should be increased.
Tektronix CMC-251, or similar high resolution, digital counter
Tektronix P6246, or similar high bandwidth, low capacitance (less than 1 pF) probe
Tektronix 1103, or similar probe power supply or probe amplifier
BNC, 50-ohm coaxial cable (less than 6 feet long)
System with power supply and test software for the LAN circuit to be tested
82562EZ(EX)/82547GI(EI) Dual Footprint Design Guide
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