Limit Testing in Digital Filter Design

You might want to design lowpass filters with a passband ripple of 10 dB and stopband attenuation of 60 dB. You can use limit testing to make sure the frequency response of the filter always meets these specifications. The first step in this process is to specify the limits. You can specify a lower limit of -10 dB in the passband region and an upper limit of -60 dB in the stopband region, as shown in the following figure. After you specify these limits, you can run the actual test repeatedly to make sure that all the frequency responses of all the filters are designed to meet these specifications.