*Question E7C06:
What are the characteristics of an elliptical filter?*  

*Correct Answer is  C: Extremely sharp cutoff with one or more notches in the stop band*  

### AI Response: (may contain errors)  

An elliptical filter, also known as a Cauer or Zolotarev filter, is characterized by its ability to achieve an extremely sharp cutoff between the passband and the stopband. This sharp transition is accompanied by the presence of ripples in both the passband and the stopband, which can include one or more notches in the stopband.

### Explanation of the Correct Answer (C):
- **Extremely sharp cutoff**: Elliptical filters are designed to have a very steep transition from the passband to the stopband, which means they can effectively separate frequencies that are very close together.
- **One or more notches in the stop band**: These filters can have deep notches or dips in the stopband, which help in further attenuating unwanted frequencies.

### Why Other Choices Are Incorrect:
- **Choice A**: "Gradual passband rolloff with minimal stop-band ripple" describes a Butterworth filter, which is known for its smooth and gradual rolloff and lack of ripples.
- **Choice B**: "Extremely flat response over its pass band with gradually rounded stop-band corners" describes a Chebyshev Type I filter, which has a flat passband but does not have the sharp cutoff or notches characteristic of an elliptical filter.
- **Choice D**: "Gradual passband rolloff with extreme stop-band ripple" does not accurately describe any standard filter type. Elliptical filters have ripples in both the passband and stopband, but their rolloff is not gradual.

### Key Points:
- Elliptical filters are useful when a very sharp cutoff is needed, and some ripple in the passband and stopband is acceptable.
- The trade-off for the sharp cutoff is the presence of ripples, which can be controlled to some extent by the filter design parameters.

Understanding these characteristics will help you identify the correct filter type based on the desired frequency response.