Digital Signal Processing Reference
In-Depth Information
L
R
G
C
Figure 8.4
Transmission line circuit model. Resistance in the conductor and losses in the dielectric
cause signal loss.
0.8
0.31
0.7
0.28
α
0.6
0.24
0.5
0.20
α
0.4
0.16
0.3
0.12
α
0.2
0.08
0.1
0.04
0.0
0.00
1 GHz
2 GHz
3 GHz
4 GHz
5 GHz
Frequency
Figure 8.5
Losses for a 5-mil-wide (0.13-mm), 50
half-ounce copper stripline on FR4 as com-
Ω
puted by Linpar [1]. Total loss in dB (
α
t
) is the sum of the dielectric (
α
d
) and conductor (
α
c
) losses.
losses. This comes about because at a given frequency the dielectric loss in stripline
traces is the same for all widths, but it falls slightly as the width of microstrips
increase.
8.4
How Do Impedance and Loop Resistance Affect Conductor Loss?
The conductor loss (
α
c
) in decibels (dB) is related to the AC loop resistance (
R
ac
,
introduced in Chapter 7) and the trace impedance (
Z
o
) as shown in (8.3), which is
valid for either stripline or microstrip traces:
4.3
×
R
α
=
ac
(8.3)
c
Z
o
Because of the way in which field lines become concentrated at sharp corners,
calculating
R
ac
by hand is not easy for rectangular traces. Fortunately, there are
