How To Specify >> Basic Filter Responses  
This section will help you understand important definitions and guide you from the simple filter shape concept, to pertinent information needed in specifying your filter requirements.
 
 
The Butterworth response (maximally flat) is characterized by the fact that is has no ripple in the passband or stopband. The attenuation is approximately 6 dB per octave per section. Butterworth filters are usually normalized for an attenuation of 3 dB at the cutoff frequency.   Basic Filter Response - ButterWorth
   
ButterWorth
 
 
 
 
Basic Filter Response - Chebyshev   The Chebyshev response is characterized by the presence of ripple in the passband and no ripple in the stopband. The amount of ripple can be controlled, and is directly proportional to the standing wave ration and the reflection coefficient. The cutoff frequency is specified at an attenuation equal to the passband ripple. The Chebyshev response is more selective than the Butterworth response at the expense of the insertion loss and greater group delay.
Chebyshev
    
 
 
 
 
The Cauer (Elliptic) response is characterized by the presence of ripple in the passband and in the stopband. The amount of ripple in the passband and in the stopband can be controlled and as in the Chebyshev is proportional to the standing wave ratio and the reflection coefficient. Cauer (Elliptic) filters are more selective than the Chebyshev but exhibit more group delay variation in the passband.   Basic Filter Response - The Cauer (Elliptic)
   
The Cauer (Elliptic)
 
 
  When group delay response is of primary concern, a Gaussian or Bessel response is recommended. They both exhibit linear phase characteristic at the expense of selectivity. Like the Butterworth there is no ripple in the passband or the stopband.