82541PI(ER) and 82562GZ(GX) Dual Footprint LOM Design Guide
Application Note (AP-468)
11
3.1.1.10
Aging
Aging is a permanent change in frequency (and resistance) occurring over time. This parameter is
most important in its first year because new crystals age faster than old crystals. Use crystals with a
maximum of ±5 ppm per year aging.
3.1.2
Reference Crystal
The normal tolerances of the discrete crystal components can contribute to small frequency offsets
with respect to the target center frequency. To minimize the risk of tolerance-caused frequency
offsets causing a small percentage of production line units to be outside of the acceptable frequency
range, it is important to account for those shifts while empirically determining the proper values for
the discrete loading capacitors, C1 and C2.
Even with a perfect support circuit, most crystals will oscillate slightly higher or slightly lower than
the exact center of the target frequency. Therefore, frequency measurements (which determine the
correct value for C1 and C2) should be performed with an ideal reference crystal. When the
capacitive load is exactly equal to the crystal’s load rating, an ideal reference crystal will be
perfectly centered at the desired target frequency.
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.
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.
