Digital Signal Processing Reference
In-Depth Information
Width (mm)
0.05
0.10
0.15
0.21
0.26
0.31
0.36
0.060
0.024
0.055
0.022
Stripline
0.050
0.020
50
Ω
Microstr
i
p
65
Ω
Microstr
i
p
0.045
0.018
0.040
0.016
2
4
6
8
10
12
14
Width (mils)
Figure 8.9
Dielectric loss of stripline is the same for all trace widths and impedances, but changes
for solder mask-covered microstrip. (Data for FR4 (
ε
r
=
4.2,
LT
=
0.02) from Linpar [1].)
8.5.3 What Can Be Done to Reduce Dielectric Losses?
To lower dielectric losses:
Use a low loss tangent laminate system.
•
Use a low dielectric constant laminate system.
•
Use wide, high-impedance traces when using microstrip since that lowers
ε
r_eff
.
•
8.6
Summary of Signal Loss and Distortion Characteristics
A truly lossless line is distortionless. Provided the line is properly terminated,
pulses sent down these transmission lines do not change shape.
•
Signal energy is lost to the conductor resistance and to the dielectric.
•
A lossy line distorts the pulse by attenuating the high-frequency harmonics
making up the pulse.
•
Loss reduces the amplitude and rounds the pulse.
•
Conductor losses increase as the square root of frequency, but dielectric
losses increase directly with frequency.
•
The crossover frequency where the dielectric loss outweighs the conductor
losses depends on the trace width, its thickness, and its impedance.
•
