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April 1969 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
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The April 1969 issue of
Electronics World magazine included a large number of articles on the topic
of filter design and use. Included were titles such as "LC Filters," "Practical
Operating Limits for Filters," "Filters for Microwave," "Crystal Filters," and
others which were contributed by experts in the field from industry and academia. Check
the
table of contents to see which ones have been posted here. This piece on the
use of ferrite beads for blocking RF signals is more of a sidebar note than an article,
but it's still worthy of inclusion. Ferrite beads and toroids are still widely
employed for stopping radio frequency interference (RFI) on everything from AC power
cords to millimeter-wave circuit boards.
Beads Block Noise
Typical ferrite bead and its equivalent series circuit.
Block spurious noise without d.c. losses? It can be done with a ferrite bead
says the Electronic Components Division of Stackpole Carbon Co. They claim a ferrite
bead is one of the simplest and least expensive methods of obtaining r.f. decoupling,
shielding, and parasitic suppression without sacrificing low-frequency power or
signal level.
Unlike conventional r.f. chokes, ferrite beads are compact; they do not couple
to stray capacitance to introduce detuning or spurious oscillations. In addition,
their impedance varies from quite low at low frequencies to quite high at noise
frequencies. What else makes them different? Well, they need not be grounded, but
grounding isn't detrimental to performance if they should touch the chassis.
Ferrite beads are available in a variety of sizes, from 0.038 inch o.d. and 0.150
inch long to 0.120 inch o.d. and 0.300 inches long. They come with either a single
hole or multiple holes through their length, through which the current-carrying
conductor passes. Some beads are being made with leads to which wires can be soldered.
Here's how they work. As noise current flows through a conductor (passing through
a ferrite bead), it creates a magnetic field. As the field passes through the bead,
the permeability of the bead at the noise frequency (r. f.) causes the impedance
of the bead to rise rapidly, creating an effective r.f. choke. The higher the frequency,
the higher the impedance and the greater the attenuation. Meanwhile, low-frequency
current passes through the bead unimpeded. Several beads can be strung together
for increased efficiency.
Posted August 2, 2024
(updated from original
post on 12/15/2017)
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