Question
Which of the following is most frequently used as a band-pass or notch filter in VHF and UHF transceivers?
Answer Options
- A) A Sallen-Key filter
- B) A helical filter
- C) A swinging choke filter
- D) A finite impulse response filter
Correct Answer: B
Explanation
At VHF and UHF frequencies, achieving the necessary high Q factor for selective filtering often requires special resonator structures because conventional low-frequency inductors and capacitors become lossy and parasitic. These specialized filter structures are necessary for things like duplexers and receiver front-ends.
A helical filter is a passive filter type most frequently used as a band-pass or notch filter in VHF and UHF transceivers. It consists of a series of spiral (helical) coils mounted inside a shielded metallic cavity. This structure provides the very high Q factor and tight mechanical stability needed to achieve sharp selectivity at high frequencies, often seen in the front-end filtering of many commercial and amateur VHF/UHF radios.
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