Digital Signal Processing Reference
In-Depth Information
100
Received
90
80
70
60
50
40
30
20
Lost
10
0
0
5
10
15
20
Transmission Line Loss (dB)
Figure 8.3
Amplitude (in percent) to decibel conversion. A 3-dB loss is seen to represent a signal
swing of 70.8% at the receiver.
Alpha (
α
) is used to indicate loss, with a subscript showing that it is either the
dielectric (
α
c
) part. As shown in (8.2), the total loss in dB is
the sum of the two and is represented by
α
d
) or the conductor (
α
t
.
ααα
=+
(8.2)
t
c
d
4.2,
LT
= 0.02), at low frequencies losses in
the conductor contribute more to the total loss than dielectric losses. However, the
two losses increase at different rates, so at some frequency losses in the dielectric
overtake and eventually contribute more to the total than the conductor loss. In
this example the crossover point occurs at about 1 GHz.
This type of analysis is used to decide if a laminate system having dielectric
constants and loss tangent values lower than typical FR4 should be considered. Us-
ing Figure 8.5 as a specific example, a laminate having very low loss tangent values
(which lowers the dielectric loss) is not warranted if most of the signal harmonics
are below roughly 1.5 GHz.
As Figure 8.5 shows for FR4 (
ε
r
=
8.3.1 Loss Differences Between Microstrip and Stripline Traces
The difference in loss of microstrips and striplines is evident in Figure 8.6. By pick-
ing one frequency (the 3-dB bandwidth of a 1-ns rise time pulse), the graph clearly
shows how the two physical parameters most easily adjusted by the signal integrity
engineer (trace width and impedance) influence loss.
The loss for narrow traces is higher in microstrips than stripline, but the situ-
ation reverses for wider traces, where stripline losses are higher than microstrip
