Appendix A: Measuring LAN Reference Frequency Using a Frequency Counter. Intel Ethernet Controllers
Intel Ethernet Controllers are designed to provide high-performance networking capabilities for a variety of applications. These controllers support various Ethernet standards, including 10BASE-T, 100BASE-TX, and 1000BASE-T. They offer features such as auto-negotiation, flow control, and jumbo frame support. Intel Ethernet Controllers are used in a wide range of devices, including servers, workstations, routers, and switches.
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Intel Ethernet Controllers Timing Device Selection Guide
Appendix A: Measuring LAN Reference Frequency Using a
Frequency Counter
A.1
Background
To comply with IEEE specifications for 10/100 Mbps and 10/100/1000Base-T Ethernet LAN, the transmitter reference frequency must be correct and accurate within ±50 parts per million (ppm).
Note:
Intel recommends a frequency tolerance of ±30 (ppm).
Most Intel LAN devices will operate properly with a 25.000 MHz reference crystal, provided it meets the recommended requirements for frequency stability, equivalent series resistance at resonance (ESR), and load capacitance.
Most circuits for series resonant crystals include two discrete capacitors (typically C1 and C2), with values between 5 pF and 36 pF.
The most accurate way to determine the appropriate value for the discrete capacitors is to install the approximately correct values for C1 and C2. Next, a frequency counter should be used to measure the transmitter reference frequency (or transmitter reference clock).
•
If the transmitter reference frequency is more than 20 ppm below the target frequency, then the values for C1 and C2 are too big and should be decreased.
•
If the transmitter reference frequency is more than 20 ppm above the target frequency, then the values for C1 and C2 are too small and should be increased.
This Appendix provides instructions and illustrations that explain how to use a frequency counter and probe to determine the Ethernet LAN device transmit center frequency. An example describing how to calculate the frequency accuracy of the measured and averaged center frequency with respect to the target center frequency is also included.
A.2
A.3
Required Test Equipment
•
Tektronix CMC-251, or similar high resolution, digital counter
•
Tektronix P6246, or similar high bandwidth, low capacitance (less than 1 pF) probe
•
Tektronix 1103, or similar probe power supply or probe amplifier
•
BNC, 50-ohm coaxial cable (less than 6 feet long)
•
System with power supply and test software for the LAN circuit to be tested
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.
Application Note (AP-419)
7
Intel Ethernet Controllers Timing Device Selection Guide
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.
A.3.1
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.
•
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.
Note:
For the 82541GI(EI) and 82547GI(EI) devices, Intel® recommends choosing a crystal with a ESR value of 20
Ω or less, an equivalent Cload of 18 pF, and a maximum of 30 ppm frequency shift.
Cload is defined to be the load capacitance of the crystal, specified by the crystal vendor.
A.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.
Alternatively, a larger sample population of circuit boards can be used. A larger population will increase the probability of obtaining the full range of possible variations in dielectric thickness and the full range of variation in stray capacitance.
Next, the exact same crystal and discrete load capacitors (C1 and C2) must be soldered onto each board, and the LAN reference frequency should be measured on each circuit board.
8
Application Note (AP-419)
Intel Ethernet Controllers Timing Device Selection Guide
A.5
The circuit board, which has a LAN reference frequency closest to the center of the frequency distribution, should be used while performing the frequency measurements to select the appropriate value for C1 and C2.
Temperature Changes
Temperature changes can cause the crystal frequency to shift. Therefore, frequency measurements should be done in the final system chassis across the system’s rated operating temperature range.
A.6
Indirect Probing Test Method
The indirect probing test method is applicable for most devices that support 100BASE-T. Since probe capacitance can load the reference crystal and affect the measured frequency, the preferred method is to use the indirect probing test method when possible.
Almost all Intel LAN silicon that support 1000BASE-T Ethernet can provide a buffered 125 MHz clock, which can be used for indirect probing of the transmitter reference clock. The buffered 125
MHz clock will be a 5X multiple of the crystal circuit’s reference frequency (
).
Different LAN devices may require different register settings, to enable the buffered 125 MHz reference frequency. Please obtain the settings or instructions that are appropriate for the LAN controller you are using.
Figure 2. Indirect Probing Method
LAN Silicon IEEE Test Out +
2-pin header
P6246 or similar high impedance probe with less than 1 pF
LAN Silicon IEEE Test Out -
50 ohm Coaxial
Cable
Ch.1
Ch.2
input input
Tektronix
1103
Probe
Power Supply
50 ohm input
125.00047
Tektronix CMC251 or a similar capability
Frequency Counter
Application Note (AP-419)
9
Intel Ethernet Controllers Timing Device Selection Guide
A.7
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 Figure 2 .
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. An example can be found in
“GigConf.exe Register Settings for 82541GI(EI) and 82547GI(EI) Devices” .
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.
FrequencyAccuracy ppm
)
=
(
x
–
y
)
y
⁄
1000000
) where x = Average measured frequency in Hertz and y = Ideal reference frequency in Hertz
Example 1.
Given: The measured averaged center frequency is 124.99942 MHz (or 124,999,420 Hertz).
)
=
(
124999420
(
– 125000000
)
)
= – 4.64
ppm
10
Application Note (AP-419)
Intel Ethernet Controllers Timing Device Selection Guide
Example 2.
Given: The measured averaged center frequency is 125.00087 MHz (or 125,000,870 Hertz).
)
=
(
125000870
(
– 125000000
)
)
= 6.96
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.8
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.
Note:
Direct probing is not recommended for the 82541GI(EI) and 82547GI(EI) LAN silicon.
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
Application Note (AP-419)
11
Intel Ethernet Controllers Timing Device Selection Guide
Figure 3. Direct Probing Method
A.9
Direct 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 Figure 3 .
3. Using the appropriate controls for your model of high resolution digital counter, make sure it can display ~25.0000 MHz with at least four decimal places frequency resolution.
4. Ensure the LAN circuits are powered.
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
25.0000 MHz reference frequency.
12
Application Note (AP-419)
Intel Ethernet Controllers Timing Device Selection Guide
)
=
(
x
–
y
)
y
⁄
1000000
) where x = Average measured frequency in Hertz and y = Ideal reference frequency in Hertz
Example 3.
Given: The measured averaged center frequency is 24.99963 MHz (or 24,999,630 Hertz).
)
=
(
(
24999630 – 25000000
)
)
= – 14.8
ppm
Example 4.
Given: The measured averaged center frequency is 25.00027 MHz (or 25,000,270 Hertz).
)
=
(
25000270
(
– 25000000
)
)
= 10.8
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.
Application Note (AP-419)
13

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Key features
- High-performance networking
- Support for multiple Ethernet standards
- Auto-negotiation
- Flow control
- Jumbo frame support