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
0.13
0.051
50
Ω
0.12
0.047
0.11
0.043
60
Ω
0.10
0.039
70
Ω
0.09
0.035
0.08
0.031
0.07
0.028
0.06
0.024
2
3
4
5
6
7
8
9
10
Width (mils)
Figure 8.6
Total signal loss from Linpar [1] at 350 MHz for half-ounce rectangular stripline (solid
curve) and microstrip (dotted curve) on FR4.
ways to accurately calculate microstrip and stripline loss without first calculating
R
ac
, and we will see how in Chapter 17. The purpose of introducing (8.3) here is to
show how the loop resistance and impedance interrelate to affect conductor loss.
Equation (8.3) shows how increasing the trace impedance lowers the conduc-
tor loss and how increasing the AC loop resistance increases the conductor loss.
This makes sense; long, narrow, thin, heavily undercut traces should have a higher
resistance (and so conductor loss) than short, wide, thick, perfectly rectangular
traces (which have a lower resistance).
Chapter 7 showed how the skin effect causes the loop resistance to increase as
the square root of frequency. From (8.3) this means the conductor loss will also.
Equation (8.3) has another implication. For a given resistance (and therefore
trace width), high-impedance traces will have a lower conductor loss than lower-
impedance traces. For example, a 65
Ω
, 5-mil-wide trace will have a lower conduc-
tor loss than a 50
, 5-mil-wide trace.
The Problems show how to use (8.3) and
R
ac
to estimate the conductor loss.
Ω
8.4.1 How Does Surface Roughness Increase Conductor Loss?
Manufacturers roughen one or both sides of the copper foil sheet so it better ad-
heres to the epoxy substrate material (see Chapter 5). An example of surface rough-
ness is shown in Figure 8.7.
Chapter 7 showed how the skin effect causes high-frequency currents to flow
on the trace surface, increasing its resistance. Any irregularities or imperfections
present on the copper surface forces the highest frequency currents to flow through
the peaks and valleys of the rough surface, further raising the resistance to those
