What is the frequency above which a low-pass filter’s output power is less than half the input power?
The correct answer is C: Cutoff frequency. The frequency above which a low-pass filter's output power is less than half the input power is the cutoff frequency. Cutoff frequency is where power drops to 50% (-3 dB point). For amateur radio operators, this is a fundamental filter parameter. Understanding this helps when designing filters.
Exam Tip
Low-pass filter half-power frequency = cutoff frequency. Think 'C'utoff 'F'requency = 'C'utoff at 'F'ifty percent (-3 dB). Cutoff frequency is where power drops to 50% (-3 dB point). Not notch frequency, not Neper, not rolloff - just cutoff frequency.
Memory Aid
"Low-pass filter half-power frequency = cutoff frequency. Think 'C'utoff 'F'requency = 'C'utoff at 'F'ifty percent. Cutoff frequency is where power drops to 50% (-3 dB point). Standard filter parameter."
Real-World Application
A low-pass filter: At the cutoff frequency, output power is 50% of input power (-3 dB). Above cutoff, power drops further. The cutoff frequency defines where the filter starts to significantly attenuate signals. This is a fundamental filter parameter - the -3 dB point.
Key Concepts
Why Other Options Are Wrong
Option A (Notch frequency): Incorrect. Notch frequency is where a notch filter has maximum rejection, not the cutoff of a low-pass filter. Notch frequency is different.
Option B (Neper frequency): Incorrect. Neper frequency isn't a standard filter term - cutoff frequency is the standard term. Neper isn't used for this.
Option D (Rolloff frequency): Incorrect. Rolloff frequency isn't a standard term - cutoff frequency is where rolloff begins, but cutoff is the specific term. Rolloff isn't the term.
题目解析
The correct answer is C: Cutoff frequency. The frequency above which a low-pass filter's output power is less than half the input power is the cutoff frequency. Cutoff frequency is where power drops to 50% (-3 dB point). For amateur radio operators, this is a fundamental filter parameter. Understanding this helps when designing filters.
考试技巧
Low-pass filter half-power frequency = cutoff frequency. Think 'C'utoff 'F'requency = 'C'utoff at 'F'ifty percent (-3 dB). Cutoff frequency is where power drops to 50% (-3 dB point). Not notch frequency, not Neper, not rolloff - just cutoff frequency.
记忆口诀
Low-pass filter half-power frequency = cutoff frequency. Think 'C'utoff 'F'requency = 'C'utoff at 'F'ifty percent. Cutoff frequency is where power drops to 50% (-3 dB point). Standard filter parameter.
实际应用示例
A low-pass filter: At the cutoff frequency, output power is 50% of input power (-3 dB). Above cutoff, power drops further. The cutoff frequency defines where the filter starts to significantly attenuate signals. This is a fundamental filter parameter - the -3 dB point.
错误选项分析
Option A (Notch frequency): Incorrect. Notch frequency is where a notch filter has maximum rejection, not the cutoff of a low-pass filter. Notch frequency is different. Option B (Neper frequency): Incorrect. Neper frequency isn't a standard filter term - cutoff frequency is the standard term. Neper isn't used for this. Option D (Rolloff frequency): Incorrect. Rolloff frequency isn't a standard term - cutoff frequency is where rolloff begins, but cutoff is the specific term. Rolloff isn't the term.
知识点
Low-pass filter, Cutoff frequency, Half power point, -3 dB point
Verified Content
Question from official FCC General Class question pool. Explanation reviewed by licensed amateur radio operators.