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

Choices:  
A: A Sallen-Key filter  
B: A helical filter  
C: A swinging choke filter  
D: A finite impulse response filter    

Correct Answer is  B: A helical filter  

Correct answer: B — A helical filter

Why B is correct:
- Helical filters use one or more helical resonators (quarter-wave coils in shielded cans) that provide very high unloaded Q and sharp selectivity at VHF/UHF.
- They are compact compared to cavity filters, have low loss, and are easy to integrate in transceivers for front-end band-pass or notch filtering.
- Typical frequency range: roughly 100 MHz to >1 GHz, ideal for VHF/UHF radios.

Why the others are incorrect:
- A: Sallen-Key filter
  - An active RC topology using op-amps. At VHF/UHF, op-amps have insufficient gain-bandwidth, RC parasitics dominate, and Q is too low for sharp RF selectivity.
- C: Swinging choke filter
  - A power-supply filter choke whose inductance varies with current to improve regulation. It’s for DC power filtering, not RF band-pass/notch filtering.
- D: Finite impulse response (FIR) filter
  - A digital filter used in DSP. While SDRs may use FIR in IF/baseband, VHF/UHF front ends still need analog preselection; direct FIR at RF would require extremely high sample rates.

Useful formulas and rules of thumb:
- Filter selectivity and Q:
  $$Q = \frac{f_0}{\text{BW}_{3\,\text{dB}}}$$
  Higher $Q$ gives narrower bandwidth; helical resonators commonly achieve $Q \sim 200$–$1000$ in VHF/UHF.
- Quarter-wave resonance (rough approximation for a helical/cavity resonator):
  $$f_0 \approx \frac{c}{4\,L_{\text{eff}}\sqrt{\varepsilon_{\text{eff}}}}$$
  where $L_{\text{eff}}$ is the effective resonator length and $\varepsilon_{\text{eff}}$ is the effective dielectric constant.
- Practical rule: For VHF/UHF analog front ends, use high-Q distributed elements (helical, cavity, or microstrip) rather than RC/active filters. Digital FIR is used after downconversion.