50 DESIGN007 MAGAZINE I OCTOBER 2019
The plot on the left uses the absolute value
of the series reactive impedance on the hori-
zontal logarithmic scale. The plot on the right
uses the same data with the reactive imped-
ance normalized to the reference impedance.
The plots show two lines: return loss (the dB
value of the S
11
input reflection coefficient) on
the left axis and the insertion loss (the dB val-
ue of the S
21
transmission coefficient) on the
right axis.
Finally, Figure 8 shows similar plots when we
use lossy circuits. We model it with a lossless
transmission line and a series resistor. To allow
for easy comparison, the organization of the
two plots is exactly the same as in Figure 7. The
lines look very similar in the two figures, but
we have to notice that the insertion loss lines
in Figure 7 are much steeper. Numerically, this
tells us that when we deal with lossless, purely
reactive circuits; in other words, when we have
only reactive reflection loss, the loss of signal
magnitude diminishes very sharply as we re
-
duce the reflection magnitude, and even mod-
erate or medium reflections will result in rela-
tively small loss of signal strength at the output.
In contrast, when we have dissipative losses,
the signal strength on the output will be much
less, and even relatively small losses will re-
sult in a noticeable loss of signal magnitude at
the output.
As a final note, we need to keep in mind that
while single reactive discontinuities (e.g., con-
nector launches, vias, antipads, etc.) will re-
sult in minuscule signal loss, when we have
a periodic structure with evenly spaced mul-
tiple discontinuities, even small reflection will
result in significant signal loss at frequencies
where the small reflections all add up
[2 & 3]
.
Meanwhile, although dissipative losses result
in higher up-front signal loss, this loss of sig-
nal strength will be much less sensitive to the
parameter variations of multiple cascaded seg-
ments.
DESIGN007
References
1. Eben Kunz, Jae Young Choi, Vijay Kunda, Laura Kocu-
binski, Ying Li, Jason Miller, Gustavo Blando, and Istvan
Novak, "Sources and Compensation of Skew in Single-
Ended and Differential Interconnects," DesignCon 2014.
2. Gustavo Blando, Jason Miller, Istvan Novak, Jim
DeLap, and Cheryl Preston, "Attenuation in PCB Traces
Due to Periodic Discontinuities," DesignCon 2006.
3. Jason R. Miller, Gustavo J. Blando, and Istvan Novak,
"Additional Trace Losses Due to Glass-Weave Periodic
Loading," DesignCon 2010.
Istvan Novak is the principal
signal and power integrity
engineer at Samtec with over 30
years of experience in high-speed
digital, RF, and analog circuit and
system design. He is a Life Fellow
of the IEEE, author of two books
on power integrity, and an instructor of signal and
power integrity courses. He also provides a website
that focuses on SI and PI techniques. To read past
columns or contact Novak, click here.
Figure 8: Calculated return loss (RL) and insertion loss (IL) of circuits similar to shown in Figure 4.
