Intel 82541PI Design Manual page 80

82541PI(ER) and 82562GZ(GX) Dual Footprint LOM Design Guide
Example 2.
Given: The measured averaged center frequency is 125.00087 MHz (or 125,000,870 Hertz).
FrequencyAccuracy ppm
Note: The following items should be noted for an ideal reference crystal on a typical printed circuit
board.
•
If the transmitter reference frequency is more than 8 ppm below the target frequency, then the
values for C1 and C2 are too big and they should be decreased. When tests are performed
across temperature, it may be acceptable for the center frequency deviation to be a little greater
than 8 ppm.
•
If the transmitter reference frequency is more than 8 ppm above the target frequency, then the
values for C1 and C2 are too small and they should be increased. When tests are performed
across temperature, it may be acceptable for the center frequency deviation to be a little greater
than 8 ppm.
A.5 Direct Probing Test Method, Applicable for Most 10/100
Devices (Devices that do NOT support 1000Base-T)
Because probe capacitance can load the reference crystal affecting the measured frequency, it is
preferable to use a probe with less than 1 pF capacitance.
The probe should be connected between the X2 (or Xout) pin of the LAN device and a nearby
ground. Typically, it is possible to connect the probe pins across one of the discrete load capacitors
(C2 in Figure
72
(
20).
(
125000870 125000000 –
)
= ----------------------------------------------------------------
( ⁄
125000000 1000000
)
= 6.96ppm
)
Application Note (AP-468)