Digital Signal Processing Reference
In-Depth Information
1.0
0.39
0.9
0.35
R
a
= 2
μ
R
a
= 1
μ
m
0.8
0.31
Smooth
0.7
0.28
0.6
0.24
0.5
0.20
0.4
0.16
0.3
0.12
0.2
0.08
0.1
0.04
0.0
0.00
1 GHz
2 GHz
3 GHz
4 GHz
5 GHz
Frequency
Figure 8.8
Conductor loss of a 5-mil-wide, 50
Ω
, half-ounce solder mask covered microstrip with
and without surface roughness. A 60° tooth angle is applied to both trace and return plane on
adjacent surfaces. Resistance increases slightly for smaller angles. (Data from the Simbeor 3D elec-
tromagnetic fi eld solver [3].)
When the traces are controlled impedance and the board is a fixed thickness,
select a laminate system with a lower dielectric constant. The Problems show
how this can sometimes improve conductor loss.
•
Specify high-impedance traces. This increases the distance to the return plane
(especially for wide traces), lowering the loop resistance.
•
Use copper with low surface roughness.
•
traces are required for high-speed signaling, which can make it
impractical to lower conductor losses by signaling with higher impedance traces.
Note that because of the skin effect, much of the high-frequency current flows
near the trace surface, so increasing the copper thickness will not significantly re-
duce high-frequency conductor losses.
Often 50
Ω
8.5 Understanding Dielectric Losses
In Chapter 7 we saw how conductance (the circuit element used to represent dielec-
tric loss in many circuit simulators) is affected by the impedance and trace width
for microstrips and striplines. Here we examine dielectric losses directly, without
using conductance.
The amount of signal energy lost to the dielectric is affected by frequency, the
dielectric constant, and the loss tangent as shown in (8.4), which is slightly modi-
fied from that appearing in [4, 5].
