Page 155 - Anatomy of a Robot

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140 CHAPTER FOUR
Use Long Rise-Time Signals
No signals inside the robot really change from low to high voltage instantaneously. If
they did, the emissions would be of unlimited frequency. As a practical matter, signals
rise over a certain time period; let’s call it T. When this happens, emissions centered at
a frequency of about 3/T predominate in the emissions spectrum. Generally, FCC reg-
ulations restrict higher-frequency emissions to lower values, so it makes sense to limit
the rise times of signals within the robot, which can be done in several ways:

First, we can use integrated circuits that have slower signal rise times. Use the chip
technology that is just fast enough for the robot, but not much faster.
Use lower clock frequencies. Although this does not guarantee slower rise times,
it often helps.
Attenuate the signals with filtering components. Electrical engineers often must
alter signals so they will not generate transients on a PCB. Transients on one sig-
nal can create errors on that signal or on other signals. As the transients are atten-
uated, so too are the high-frequency components of the transients. It’s a fine art
eliminating signal transients, shown at www.commsdesign.com/main/9802fe4
.htm and www.eedesign.com/editorial/1996/pcbdesign9605.html.

Grounding

Make sure that signals travel over a ground path that carries their return signal. All the
electrons sent down a signal trace balance the corresponding electrons returning in the
ground plane beneath the signal. If the ground plane has gaps in it, then the return elec-
trons must trace a different path back. This creates a loop of electrons moving about,
generating more interference. Avoid splitting ground planes in a PCB layout.

www.devicelink.com/mddi/archive/96/08/011.htm
www.cae.wisc.edu/~benedict/pcbpres.pdf

Filter the Power Supply

As logic circuitry switches signals from one voltage to another, the power supply strug-
gles to deliver current to each logic node. Since it takes time to move electrons, it makes
sense to store electrons in the places likely to need them most. Power supply capacitors
are designed to provide this power and to decrease the transients on the PCB. Consult
the following URLs to learn how to filter power supplies properly on a PCB.
www.icst.com/products/pdf//note07.pdf
www.quicklogic.com/images/pcb_de_1.pdf