82583V Datasheet, PDF (329/374 Pages) Intel Corporation – Intel® 82583V GbE Controller

Design Considerations—82583V GbE Controller
11.3.1.7
Shunt Capacitance
The shunt capacitance parameter is relatively unimportant compared to load
capacitance. Shunt capacitance represents the effect of the crystal’s mechanical holder
and contacts. The shunt capacitance should equal a maximum of 6 pF.
11.3.1.8
Equivalent Series Resistance
Equivalent Series Resistance (ESR) is the real component of the crystal’s impedance at
the calibration frequency, which the inverting amplifier’s loop gain must overcome. ESR
varies inversely with frequency for a given crystal family. The lower the ESR, the faster
the crystal starts up. Use crystals with an ESR value of 50 Ω or better.
11.3.1.9
Drive Level
Drive level refers to power dissipation in use. The allowable drive level for a Surface
Mounted Technology (SMT) crystal is less than its through-hole counterpart, because
surface mount crystals are typically made from narrow, rectangular AT strips, rather
than circular AT quartz blanks.
Some crystal data sheets list crystals with a maximum drive level of 1 mW. However,
Intel Ethernet controllers drive crystals to a level less than the suggested 0.3 mW
value. This parameter does not have much value for on-chip oscillator use.
11.3.1.10 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.
11.3.1.11 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.
11.3.1.11.1 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.
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