Post by lutzvw on Apr 27, 2010 6:40:56 GMT -5
Comments and corrections to tutorial 26: Filters
*Page 3 and page 4
1.) Quote: Conversely, an active filter is one that has an amplification function built into it. The filter of figure 3 is a passive filter as there is no gain in the pass-band.
Correction: An active filter can also have a gain of „1“ or even less. On the other hand, a filter with an amplification built in is not necessaroly an active filter. Example: Two RC or two RLC branches with a buffer in between are still passive filters, since the amplifier does not determine the filter response. In an active filter the amplifier takes over the task of the inductor.
Active filters are inductorless and are able to produce conjugate-complex poles.
2.) The phase response of Figure 3 is false. At w =1 rad/s the phase must be –135 deg and reaches a maximum of –270 deg. only.
3.) Quote: For this filter, which happens to be a Butterworth filter of order 9, the phase is linear in the pass-band......which is a characteristic of Butterworth filters.
Correction: The order of the shown transfer function is 3 (instead of 9). The phase of a Butterworth filter is, of course, NOT linear. No filter has a linear phase response. Only the Bessel characteristic is nearly linear in the first region of the pass-band.
*Page 6
Quote: The group delay shows up a function of the linearity of the filter. The filters that offer a linear phase response are generally characterized as linear filters.
Ultimately, we use group delay as a measure of the non-linearity of the system...
Correction: This violates the rules of system theory! A linear phase response (which does not exist!) has nothing to do with the linearity of a system! System linearity is separately defined elsewhere!
*Page 10
Quote: The primary principle of an analog filter is that it is based on a RLC circuit, and is inherently a non-linear device...
Correction: No, that is NOT the primary principle of an analog filter. There are only some filter topologies which are based on passive RLC structures. Mostly, active filters have their own specific topology.
The formulation "..inherently a non-linear device" is completely false (and catastrophic!!!).
Quote: Butterworth filters....so in the pass-band they have nearly linear phase response...
Correction: See comment above (pages 3 and 4)
*Page 12
Question: What is Chebyshev Type II ? Do you mean „inverse Chebyshev“ with ripple in the stop band? This is NOT an allpole filter!
*Page 3 and page 4
1.) Quote: Conversely, an active filter is one that has an amplification function built into it. The filter of figure 3 is a passive filter as there is no gain in the pass-band.
Correction: An active filter can also have a gain of „1“ or even less. On the other hand, a filter with an amplification built in is not necessaroly an active filter. Example: Two RC or two RLC branches with a buffer in between are still passive filters, since the amplifier does not determine the filter response. In an active filter the amplifier takes over the task of the inductor.
Active filters are inductorless and are able to produce conjugate-complex poles.
2.) The phase response of Figure 3 is false. At w =1 rad/s the phase must be –135 deg and reaches a maximum of –270 deg. only.
3.) Quote: For this filter, which happens to be a Butterworth filter of order 9, the phase is linear in the pass-band......which is a characteristic of Butterworth filters.
Correction: The order of the shown transfer function is 3 (instead of 9). The phase of a Butterworth filter is, of course, NOT linear. No filter has a linear phase response. Only the Bessel characteristic is nearly linear in the first region of the pass-band.
*Page 6
Quote: The group delay shows up a function of the linearity of the filter. The filters that offer a linear phase response are generally characterized as linear filters.
Ultimately, we use group delay as a measure of the non-linearity of the system...
Correction: This violates the rules of system theory! A linear phase response (which does not exist!) has nothing to do with the linearity of a system! System linearity is separately defined elsewhere!
*Page 10
Quote: The primary principle of an analog filter is that it is based on a RLC circuit, and is inherently a non-linear device...
Correction: No, that is NOT the primary principle of an analog filter. There are only some filter topologies which are based on passive RLC structures. Mostly, active filters have their own specific topology.
The formulation "..inherently a non-linear device" is completely false (and catastrophic!!!).
Quote: Butterworth filters....so in the pass-band they have nearly linear phase response...
Correction: See comment above (pages 3 and 4)
*Page 12
Question: What is Chebyshev Type II ? Do you mean „inverse Chebyshev“ with ripple in the stop band? This is NOT an allpole filter!