Digital Signal Processing Reference
In-Depth Information
Width (mm)
0.05
0.08
0.10
0.13
0.15
0.18
0.20
0.23
0.25
1.40
0.55
50
Ω
1.27
0.50
1.14
0.45
60
Ω
70
Ω
0.40
1.02
50
Ω
60
Ω
70
Ω
0.89
0.35
0.76
0.30
0.64
0.25
0.51
0.20
0.38
0.15
2
3
4
5
6
7
8
9
10
Width (mils)
Figure 7.6
Loop resistance as calculated by Linpar [8] at 350 MHz for perfectly rectangular 0.65-mil
(0.017-mm)-thick stripline (solid curve) and solder mask-covered microstrip (dotted curve).
100
Ω
50
Ω
Current extends out
beyond conductor edges
Figure 7.7
Cross-section of two microstrips showing the current in the ground plane gathering
underneath the traces at high frequency. The proximity effect cause more spreading for traces of the
same width that are a greater distance from the return.
Confining the return current to a small channel in the vicinity of the trace raises
the loop resistance. Conversely, the resistance will be lower if the current can spread
out along a larger section of the return plane (as it does for traces higher above
the plane). The net effect is that, for a given dielectric constant and trace width,
the loop resistance is lower for high-impedance traces at any given high frequency.
