16 July/August 2018
THE HAM‘S WIRELESS WORKBENCH n BY WARD SILVER N0AX
What is a Filter?
Looking up “filter” generates a lot of entries! Along
with the electrical definitions, there are adaptive filters
and image filters and so forth, as well. The general idea,
though, is to operate on an input signal (or data) so that
the output is modified based on some pre-defined quality
of the input signal. With respect to electrical signals, a filter
is a circuit or function with an effect that varies with the
input signal’s frequency, wavelength, or phase. All three of
those characteristics are basically the same thing: a regular
variation in time.
Analog and Digital Filters
As with just about everything else in ham radio, filters
can be analog or digital. The common passive (unpowered)
analog filter consists of familiar inductor-capacitor (LC) or
resistor-capacitor (RC) circuits. If some kind of amplifier is
used, the circuit becomes an active filter. Most active filters
use op-amps in a variety of feedback circuits. Figure 1
shows an example of each type of filter circuit.
A digital filter requires analog signals to be digitized,
creating a stream of digital data representing the original
signal. Once digitized, any mathematical operation can
be performed on the data. If the result is to be viewed or
listened to by a human, the digital data is converted back
to an analog signal. Processing and converter electronics
are now inexpensive enough to implement that digital
filters (and many other signal-processing functions) are
taking over from analog in ham radio. This column will
stick with the traditional analog circuits, but the general
definitions and specifications for how the filter behaves
apply to all filters.
Common Filter Types
Nearly all electrical filters (assume we are talking about
electrical filters from this point on) are described and
specified by their effect on the amplitude or magnitude of
signals having different frequencies. (The phase response
is also important in many applications.) This is the filter’s
frequency response and because it
describes signal amplitude, it is a
magnitude response. Several types of
Stop, Block, and Roll(off)
casual observer might think that wireless systems consist
primarily of filters connected by the occasional bit of circuit!
Block diagrams of transceivers often include as many filters as any
other function. This is true at the system level, just as it is at the
circuit level — and many circuits behave in a filter-like way, whether
intended to be a filter or not! That makes understanding filter basics
important for wireless success.
n FIGURE 1. Filter A is a passive high-pass LC filter used for rejecting AM
broadcast band signals. Filter B is an active low-pass op-amp filter typically
used for audio signals.