July/August 2018 19
different in what is important.
Once a filter family is selected, the next order of
business is to determine how many “sections” of filtering
are needed to meet the performance requirement. The
number of sections is the filter’s order — the higher the
order, the more filtering is applied to the input signal.
Order describes the number of LC or RC pairs in a
passive filter, such as in Figure 1A which is a fifth-order
filter. Figure 5 shows the effect of cascading three filter
sections of Figure 1B’s active RC low-pass filter. You
can see the rolloff of the filter increasing as order is
In general, as order increases, performance can
more closely approach the ideal filter’s response. However,
filter complexity is increased, tuning is more critical,
and component variation and losses can have a more
pronounced effect on performance. We’ll cover order in
more detail next time.
Common Filter Applications
Let’s discuss a few applications for filters. Figure 6
is a block diagram showing where these filters are found
in typical ham radio transceivers and accessories. Signal-processing filters are more and more likely to be performed
by software (DSP), but there are still many analog filter
circuits in radio.
• Audio Filters: Along with the CW band-pass filter
mentioned above, audio filters are also used to get
rid of hum ( 50 or 60 Hz signals caused by magnetic
fields), buzz (caused by 120 Hz rectified AC and
ac neutral currents), and high-frequency hiss. FM
modulators and noise-reduction circuits also use preemphasis (high-pass) and de-emphasis (low-pass) filters
n FIGURE 5. The effect of increasing a simple RC filter’s
order from 1 to 3. Note the increasing steepness of the filter’s
rolloff at higher orders. (Graphic courtesy of Spinningspark
at Wikipedia, CC BY-SA 3.0, https://en.wikipedia.org/w/
n FIGURE 6. A transceiver block diagram showing typical
applications of filters for different functions. Filters
are used at frequencies from AC power through the
transmitted and received RF signals.