PMC-Sierra, Inc.
STANDARD PRODUCT
PM5945 -UTP5
PMC-940202 ISSUE 2. APRIL 7, 1995
APP_SAPI_UTP5
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APPENDIX F: LAYOUT NOTES
Background
The SAPI_UTP5 board is a 4-layer board that has both throughole and
surfacemount components with the number of surfacemount component being
dominant. Protel’s Autotrax version 2.0 for DOS is used to layout the board. The
schematics is done using OrCAD.
Trace Impedance Control
To reduce signal degradation due to reflection and radiation, the impedance of the
traces that carry high speed signals such as transmitted and received data should
be treated as transmission lines and terminated with matching impedance. Since
the UTP5 cables have a characteristic impedance of 50 Ohms, the traces connecting
the RJ45 connector and line side of both the TX and RX transformer should be 100
Ohms. The traces between the drop side of the TX transformer (U21) and the
TWISTER (U22) should also be 100 Ohms as the line side impedance is also reflect
to the drop side of the TX transformer. The RX transformer is a 1:1.4 step up
2
transformer. The line impedance seen by the dropside is thus 1.4 x 50 Ohm = 100
Ohm. Therefore 100 Ohm transmission lines are used. All other high speed
differential signals also use 100 Ohm transmission lines.
Line Side
Drop side
1:1
TX
50 Ohm
Transmission
Line
100 Ohm
Transmission
Line
RX
1:1.4
The low speed signals use 8 to 10 mil traces. Power and ground traces are all as
wide as the pads they connect to, up to a maximum width of 24 mil.
The traces are modeled as microstrip transmission lines. The calculation of the
trace width is calculated using the formula
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PMC [ PMC-SIERRA, INC ]
