Measuring Lan Reference Frequency Using A Frequency Counter - Intel 82562EZ Design Manual

Appendix A Measuring LAN Reference Frequency Using a
Frequency Counter
A.1 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).
Note: Intel recommends a frequency tolerance of ±30 (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).
•
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.
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.
A.2 Required Test Equipment
•
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
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