Intel 82547EI, Design Manual

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82562EZ(EX)/82547GI(EI) Dual Footprint Design Guide
11
3.1.3 Reference Crystal Selection
There are several methods available for choosing the appropriate reference crystal:
•
If a Saunders and Associates (S&A) crystal network analyzer is available, then discrete crystal
components can be tested until one is found with zero or nearly zero ppm deviation (with the
appropriate capacitive load). A crystal with zero or near zero ppm deviation will be a good
reference crystal to use in subsequent frequency tests to determine the best values for C1 and
C2.
•
If a crystal analyzer is not available, then the selection of a reference crystal can be done by
measuring a statistically valid sample population of crystals, which has units from multiple
lots and approved vendors. The crystal, which has an oscillation frequency closest to the center
of the distribution, should be the reference crystal used during testing to determine the best
values for C1 and C2.
•
It may also be possible to ask the approved crystal vendors or manufacturers to provide a
reference crystal with zero or nearly zero deviation from the specified frequency when it has
the specified CLoad capacitance.
Note: For 82547GI(EI) devices, Intel® recommends choosing a crystal with a ESR value of 10
Ω
or less,
an equivalent Cload of 18 pF, and a maximum of 30 ppm frequency shift. Cload is defined to be the
load capacitance of the crystal, specified by the crystal vendor.
When choosing a crystal, customers must keep in mind that to comply with IEEE specifications for
10/100 and 10/100/1000Base-T Ethernet LAN, the transmitter reference frequency must be precise
within
±
50 ppm. Intel® recommends customers to use a transmitter reference frequency that is
accurate to within
±
30 ppm to account for variations in crystal accuracy due to crystal
manufacturing tolerance. For information about measuring transmitter reference frequency, refer to
Appendix A, “Measuring LAN Reference Frequency Using a Frequency Counter” .
3.1.4 Circuit Board
Since the dielectric layers of the circuit board are allowed some reasonable variation in thickness,
the stray capacitance from the printed board (to the crystal circuit) will also vary. If the thickness
tolerance for the outer layers of dielectric are controlled within ±17 percent of nominal, then the
circuit board should not cause more than ±2 pF variation to the stray capacitance at the crystal.
When tuning crystal frequency, it is recommended that at least three circuit boards are tested for
frequency. These boards should be from different production lots of bare circuit boards.
Alternatively, a larger sample population of circuit boards can be used. A larger population will
increase the probability of obtaining the full range of possible variations in dielectric thickness and
the full range of variation in stray capacitance.
Next, the exact same crystal and discrete load capacitors (C1 and C2) must be soldered onto each
board, and the LAN reference frequency should be measured on each circuit board.
The circuit board, which has a LAN reference frequency closest to the center of the frequency
distribution, should be used while performing the frequency measurements to select the appropriate
value for C1 and C2.
3.1.5 Temperature Changes
Temperature changes can cause the crystal frequency to shift. Therefore, frequency measurements
should be done in the final system chassis across the system’s rated operating temperature range.