Intel 82541PI Design Manual page 79

A.4 Indirect Frequency Measurement and Frequency Accuracy
Calculation Steps
1. Make sure the system BIOS has the LAN controller enabled.
2. Connect the test equipment as shown in
3. Using the appropriate controls for your model of high resolution digital counter, make sure it
can display ~125.0000 MHz with at least four decimal places frequency resolution.
4. Enable the 125 MHz buffered reference clock.
5. Determine the center reference frequency as accurately as possible. This can be done by taking
30 to 50 different readings using the frequency counter and then calculating the average results
of the readings.
6. Calculate the accuracy of the measured and averaged center frequency with respect to an ideal
125.0000 MHz reference frequency.
where
Example 1.
Given: The measured averaged center frequency is 124.99942 MHz (or 124,999,420 Hertz).
FrequencyAccuracy ppm
Application Note (AP-468)
82541PI(ER) and 82562GZ(GX) Dual Footprint LOM Design Guide
FrequencyAccuracy ppm
x = Average measured frequency in Hertz and
y = Ideal reference frequency in Hertz
( )
Figure 19.
(
x y
( )
= ------------------------------- -
( ⁄
y 1000000
(
124999420 125000000 –
= ----------------------------------------------------------------
( ⁄
125000000 1000000
)
–
)
)
= – 4.64ppm
)
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