*Question E7C08:
Which of the following is most frequently used as a band-pass or notch filter in VHF and UHF transceivers?*  

*Correct Answer is  B: A helical filter*  

### AI Response: (may contain errors)  

In VHF and UHF transceivers, filters are crucial for selecting desired signals and rejecting unwanted ones. Let's explore why a helical filter is the most frequently used option for this purpose:

### Correct Answer: B: A helical filter

1. **Why the correct answer is correct:**
   - **Helical filters** are commonly used in VHF and UHF applications because they offer high selectivity and can handle high frequencies effectively. They are compact and provide good performance in terms of bandwidth and insertion loss, making them ideal for these frequency ranges.

2. **Why the other choices are incorrect:**
   - **A: A Sallen-Key filter**: This is an active filter design typically used for low-frequency applications. It is not suitable for VHF and UHF frequencies due to its limitations in handling high frequencies and power levels.
   - **C: A swinging choke filter**: This type of filter is generally used in power supply applications to smooth out voltage variations. It is not designed for RF applications like VHF and UHF transceivers.
   - **D: A finite impulse response (FIR) filter**: FIR filters are digital filters used in signal processing. While they are versatile, they are not typically used as band-pass or notch filters in analog VHF and UHF transceivers.

### Summary

- **Helical filters** are preferred in VHF and UHF transceivers for their ability to handle high frequencies with good selectivity and compact size.
- Other filter types like Sallen-Key, swinging choke, and FIR filters are not suitable for these applications due to their design and frequency handling limitations.