Extra Class question of the day: Filter types and applications

Different types of filters have different characteristics. For example, a Chebyshev filter is a filter type described as having ripple in the passband and a sharp cutoff. (E7C05)On the other hand, the distinguishing features of an elliptical filter are extremely sharp cutoff with one or more notches in the stop band. (E7C06)

Filters have both amplitude and phase-response characteristics. In some applications, both are important. Digital modes, for example, are most affected by non-linear phase response in a receiver IF filter. (E7C14)

The Chebyshev filter was named for Pafnuty Chebyshev, whose mathematical work led to the development of these filters. Sometimes filters are named for their circuit topoology. Pi is the common name for a filter network which is equivalent to two L networks connected back-to-back with the inductors in series and the capacitors in shunt at the input and output. (E7C11) When you look at the circuit diagram for a filter of this type, you’ll see that it looks like the Greek letter pi.

Often, you’ll choose a filter type for a particular application. For example, to attenuate an interfering carrier signal while receiving an SSB transmission, you would use a notch filter. (E7C07)

Today, many of these filters are implemented using digital signal processing. The kind of digital signal processing audio filter might be used to remove unwanted noise from a received SSB signal is an adaptive filter. (E7C08) The type of digital signal processing filter might be used to generate an SSB signal is a Hilbert-transform filter. (E7C09)

Some filters are used almost exclusively in a particular application. A cavity filter, for example, would be the best choice for use in a 2 meter repeater duplexer. (E7C10)


  1. question E7C11 asks the name of a filter with plural inductors and capacitors, then says the answer is a filter with a single inductor. There is something the matter with that.

    • Dan KB6NU says:

      What the question is saying is that the pi filter acts as if the circuit were made up of two L networks, not that it actually has two L networks connected together. As shown in Figure E7C-2, the pi filter only has one inductor.

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