20 July/August 2018
for good speech reproduction.
• Noise Reduction: The ability of DSP to make decisions
“on the fly” about what signals are noise makes
it possible to create “smart speakers” and noise
reduction (NR) algorithms that remove undesired
portions of the audio output of a receiver. Although
they introduce some audible “artifacts,” the result is
much cleaner and easier to understand.
• Notch Filters: A type of band-stop filter, notch filters
are designed to remove a very narrow range of signals
— often just a single-frequency tone caused by a
spurious signal or an over-the-air carrier. Analog notch
filters require manual adjustment but DSP notch filters
can automatically sense the presence of the interfering
signal and “notch it out” — even for several signals at
once by using current technology.
• Anti-aliasing: When digitizing analog signals, it is
necessary for the signals to contain
frequency components no higher
than one-half the sampling rate.
Higher frequency signals will appear
in the digitized data as low frequency
spurious signals through a process
known as aliasing. An anti-aliasing
filter is a low-pass filter that removes
the undesired high frequency signals.
Similarly, when a digital signal is
reconstituted as an analog signal, the
digital “stairstep” created in the output
waveform contains high frequency
signals that are removed by another
low-pass smoothing filter.
• Preselector: Tunable analog LC
band-pass filters at the antenna input
that attenuate strong “out-of-band”
signals that might overload a receiver,
causing it to produce spurious signals.
Preselectors are most often used on the HF amateur
bands which are shared with or adjacent to powerful
shortwave and military stations.
• Broadcast Reject: Instead of band-pass, “BC” reject
filters usually have a notch or band-stop response.
They are used in the vicinity of strong fixed-frequency
AM, FM, or TV stations. A similar filter can be used to
attenuate commercial paging or repeater signals.
• Harmonic-rejection Filter: FCC rules require the
spurious outputs from amateur transmitters to be at
least 43 dB (20,000x) less powerful than the intended
transmitter output. These low-pass filters are placed
between the output of an RF amplifier and the
antenna connection. The cutoff frequency of the filter
is selected according to the operating band of the
transceiver. External low-pass filters are also used to
provide additional attenuation of harmonics and other
spurious signals that might interfere with broadcast
TV reception. (With TV stations having moved to UHF
channels for DTV signals, TV interference is much less
of a problem these days.)
• AC Line Filter: Low-pass filters that attenuate RF
signals on the hot and neutral lines of an appliance’s
AC power connection. Line filters reduce both
on RF signals generated inside the appliance that
shouldn’t get out to be radiated and on RF signals
picked up on the power wiring that might disrupt the
appliance. These line filters usually do not filter the
equipment ground (a.k.a., the third-wire or green-wire
ground) conductor so additional filtering — such as
from a ferrite core — may be necessary. (See Ham’s
Wireless Workbench columns from the July 2015 and
September 2015 issues on ferrite and RFI for more
information about this kind of filtering.)
Do Filters Have to Be Circuits?
Analog filters with Ls, Cs, and Rs are very common but
they aren’t the only electronics that can perform filtering
functions. Indeed, there are no perfect circuits for which
frequency doesn’t matter, so technically, every circuit is a
filter! However, there are a few notable non-LCR filters you
should be aware of:
• Crystal and Mechanical Filters: Once the best
available passive filters for receiving RF signals, these
filters used quartz crystals and precisely machined
mechanical resonators. A crystal roofing filter is still
used in many receiver designs to improve strong
signals from interfering in crowded conditions.
• Stubs: Resonant sections of feed line can act as band-pass and band-reject filters. Books on transmission
Filter Design Resources
Many books have “canned” filter designs for various common
applications. If you are careful to use the same type of component and
circuit layout, you’ll probably get acceptable results. Beginners should
start with pre-designed filters. As you progress, you’ll be presented
with the need to “roll your own” filter. Along with the Tonne Software
programs we’ll be using in the next column, there are some tried-and-true resources for the experimenter. For serious readers, the various
editions of Arthur Williams’ Filter Design Handbook are widely available
as used copies.
If you’re looking for more of a “plug and chug” solution, the
venerable Active Filter Cookbook by Don Lancaster in its various
printings will provide design formulas and examples for filters in the
audio range. Finally, for RF filters, the ARRL Handbook’s chapter on
Analog and Digital Filtering provides theory and projects through
the microwave range with a long list of references and additional
supplemental information online.