When Signal Integrity Matters
Douglas Brooks
I recently served on a panel where I was asked, At what In our industry, (circuit board design) we have
point does signal integrity become a problem? The intent of come to equate signal integrity with high speed
the question was to ask at what frequency did signal integrity only in recent years. That is because up until now
become an issue for board designers, but as stated, the ques- the circuit board has been a purely passive device
tion was more general. And that got me thinking about how with virtually no circuit impact (unless you were
broad the term signal integrity really is, even though we among the very few designers putting RF or micro-
have come to think of it terms of more narrowly defined high- wave circuits on the board.) But in recent years, the
speed issue. board itself has begun to cause S/N degradation.
A not-really-facetious answer to the question is that we Here s why.
have a signal integrity problem whenever the signal begins to Frequency components (as opposed to the fre-
lose its integrity! And this is not related to frequency. Two of quencies themselves) have steadily increased
the more obvious and common ways a signal can lose integrity through the years. Consider what we typically view
are when it becomes distorted or when the signal-to-noise ra- as a square-wave clock or data signal (Figure 1). If
tio (S/N) begins to degrade. we disassemble a square wave we find that it
Signal distortion typically means that the waveform of really is the complex sum of an infinite series of
interest begins to change shape. The degree that this can hap- sine (really cosine) waveforms. The formula for this
pen before it becomes a problem depends very much on the series is given in the caption to the figure. Each
application. Digital signals, which we typically think of as term in the series represents a higher frequency
being rectangularly shaped pulses, usually carry one bit of (harmonic). To perfectly represent a square wave,
information per clock cycle. They can often withstand a fair our circuit must faithfully pass all these frequency
amount of distortion without obscuring the bit-state they are components (harmonics) without any additional
in. Analog signals, on the other hand, such as we find in video distortion or phase shift. Figure 2 shows the degree
and audio systems, can be very sensitive to distortion. A of degradation caused by limiting the system band-
change in the waveform will often be seen or heard. Your width to just the first few harmonics.
home hifi system, for example, probably has a spec for har- Clock and signal square waves are never
monic distortion, which relates to the purity of the audio perfect. They are really characterized by a rise
signal as it is processed. time. (Footnote 1) It can be shown that the highest
I my lifetime, home entertainment harmonic distortion frequency component we usually need to be con-
specs have improved greatly, and today s hifi and home enter- cerned with (in a practical sense) on our board can
tainment systems are, for all practical purposes, distortion be approximated by 1/(3*Tr), where Tr is the rise
free. The most common way to make a distortion-free system time of the pulse. Thus, to reproduce a one nanosec-
is to design it so that the gain is absolutely linear over the fre-
quency range of interest.
S/N issues come into play whenever an unwanted noise
becomes detectable and/or interferes with the signal we are
concerned with. For example, if you hear a 60 Hz hum (which
is, of course, a very low frequency) from your home hifi sys-
tem, you have a S/N or a signal integrity problem! And signal
integrity issues and solutions are not new. People who have
spent part of their careers designing power or electromagnetic
switching systems have understood the importance of decoup-
ling and separation of power supply and grounding systems
for decades. Problems associated with reflections, and their
transmission line solutions, have also been around for years.
Radio frequency engineers (even with low AM transmitting Figure 1
frequencies below one MHz) have needed to understand trans- The formula for a square wave is given by the infinite series
mission line techniques since the broadcasting industry began. Y=cos(wt)-cos(3wt)/3+cos(5wt)/5-cos(7wt)/7 & (etc.)
Signal integrity issues at the circuit design level have been where w = 2*Pi*f, f is the cyclical frequency, t is time, and
around for a long, long time. Pi is the constant 3.14159
This article appeared in Printed Circuit Design, a CMP Media publication, September, 2001
© 2001 CMP Media, Inc. © 2001 UltraCAD Design, Inc. http://www.ultracad.com
ond rise time pulse typically requires a system band-
H=3
width of something over 300 MHz.
Back to our boards. Let s use inductance for our H=5
illustration. Traces have always had inductance. That
H=15
is important for new board designers to understand.
The fact that there is inductance associated with the
trace is not new. The voltage generated across an
inductor can be approximated by V = L*di/ Tr, where
L is, in this case, the inductance of the trace, di is the
change in current being switched, and Tr is the rise
(fall) time associated with the switching current. V
gets larger as (among other things) Tr gets smaller. V
can be considered a noise voltage. Thus, the S/N
Figure 2
ratio gets worse as V gets bigger (or, as Tr gets
The curves look more like square waves as the number
smaller!). Therefore, signal integrity gets worse
of harmonics in the series (H) increases.
(because the S/N ratio degrades) as Tr gets smaller
(i.e. rise time gets faster.)
This illustration used inductance. We can use
stray trace capacitance to illustrate exactly the same
point. There is stray (parasitic) inductance and ca-
pacitance all over our boards.
So, back to the initial question: At what point
does signal integrity become a problem. For board
designers, the answer is: When the rise time de-
creases to the point where the parasitic inductances
and capacitances on the board begin to result in noise
signals that become troublesome. And when is that?
Well, it depends, of course, on the circuit specifics,
so it is very difficult to generalize on a specific value.
But for the most part, this can happen in the range of
2 nanoseconds or so, and faster. These rise times are
sometimes (but not always) associated with high
frequencies. Therefore, signal integrity issues on our
boards have become associated with high frequency
signals on our boards. But it is important to recog-
nize, first, that signal integrity issues are not neces-
sarily related to frequencies and rise times, and sec-
ondly, when they are it is usually rise time that is the
culprit, not frequency.
Footnotes:
1. Rise time is usually defined as the time required to transition between the 10% and 90%, or sometimes 20% and
80%, amplitude points on the waveform. Fall time is similarly defined, and is equally as important.
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