*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)  

To determine which filter is most frequently used as a band-pass or notch filter in VHF and UHF transceivers, let's analyze the options:

### Correct Answer: B - A helical filter

- **Why it is correct**: 
  - Helical filters are specifically designed for VHF and UHF applications. They utilize a helical resonator, which provides excellent selectivity and low insertion loss. This makes them ideal for filtering out unwanted frequencies while allowing desired signals to pass through, making them effective as band-pass or notch filters.

### Why the other choices are incorrect:

- **A: A Sallen-Key filter**:
  - While Sallen-Key filters are versatile and can be used for various filtering applications, they are typically implemented in lower frequency ranges and are more common in audio applications. They may not provide the same level of performance in VHF and UHF as helical filters.

- **C: A swinging choke filter**:
  - Swinging choke filters are not commonly used in VHF and UHF transceivers. They are more associated with power supply filtering and RF applications at lower frequencies. Their design does not lend itself well to the high-frequency performance required in VHF and UHF.

- **D: A finite impulse response (FIR) filter**:
  - FIR filters are digital filters used in signal processing. While they can be designed for various applications, they are not typically used in analog VHF and UHF transceivers. Their implementation requires digital signal processing capabilities, which are not standard in all transceivers.

### Summary:
- **Helical filters** are the preferred choice for VHF and UHF band-pass or notch filtering due to their high selectivity and low loss.
- Other options either do not perform well at these frequencies or are not commonly used in the context of analog transceivers.

This understanding will help you recognize the appropriate filter types for specific applications in your studies for the Extra Class exam.