Filter Basics

A common type of filter, the band-pass,passes signals in a range of frequencies — the passband — while rejecting signals outside that range — the stop band. To pass signals from dc up to some cutoff frequency at which output power is halved (reduced by 3 dB) we would use a low-pass filter. To pass signals above a cutoff frequency (also called the “3-dB frequency”) we would use a high-pass filter. Similarly, to pass signals within a range of frequencies we would use a band-pass filter. To pass signals at all frequencies except those within a specified range requires a band-stop or notch filter. The region between the passband and the stop band is logically called the transition region. The attenuation for signal frequencies in the stop band far from the cutoff frequency is called the ultimate attenuation.


















Fig 11.4 illustrates the magnitude response of a high-pass filter. Signals above the 3 MHz cutoff frequency are passed with minimum attenuation while signals below that frequency are attenuated. Again, the degree of attenuation is dependent on several variables.

Fig 11.5 illustrates the magnitude response of a band-pass filter. Signals within the band-pass range (between the lower and upper cutoff frequencies) are passed with minimum attenuation while signals outside that range are attenuated. In this example the filter was designed with cutoff frequencies of 2 MHz and 4 MHz, for a passband width of 2 MHz.

Fig 11.6 illustrates the magnitude response of a band-stop filter. Signals within the bandstop range are attenuated while all other signals are passed with minimum attenuation. A notch filter is a type of band-stop filter with a narrow stop-band in which the attenuation is a maximum at a single frequency.


An ideal filter — a low-pass filter, for example — would pass all frequencies up to some point with no attenuation at all and totally reject everything beyond that point. This is known as a brick wall response because the filter’s passband and stop band are flat, meaning no attenuation, and the rolloff in the transition region is infinitely steep. The magnitude response of such a filter would be drawn as a flat line representing 0 dB attenuation up to the cutoff frequency that then abruptly changes at the cutoff frequency to a flat line at infinite attenuation throughout the stop band to infinite frequency.