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where x = Average measured frequency in Hertz and
Example 1.
Given: The measured averaged center frequency is 124.99942 MHz (or 124,999,420 Hertz).
FrequencyAccuracy ppm
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.
FrequencyAccuracy ppm
y = Ideal reference frequency in Hertz
(
(
82563EB/82564EB LAN on Motherboard Design Guide
(
(
)
=
------------------------------- -
(
⁄
y 1000000
(
124999420 125000000
–
)
=
----------------------------------------------------------------
(
⁄
125000000 1000000
(
125000870 125000000
–
)
=
----------------------------------------------------------------
(
⁄
125000000 1000000
)
x y
–
)
)
=
–
4.64ppm
)
)
=
6.96ppm
)
61
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