*Question G7C12:
What is the frequency above which a low-pass filter’s output power is less than half the input power?*  

*Correct Answer is  C: Cutoff frequency*  

### AI Response: (may contain errors)  

The correct answer to the question is **Cutoff frequency**. Here's why:

1. **Why the correct answer is correct:**
   - The cutoff frequency of a low-pass filter is defined as the frequency at which the output power drops to half of the input power. This corresponds to a decrease in voltage to approximately 70.7% of the input voltage, since power is proportional to the square of the voltage. Mathematically, this is represented as:
   $$
   P_{out} = \frac{P_{in}}{2}
   $$
   At the cutoff frequency, the filter begins to attenuate higher frequencies, allowing only lower frequencies to pass through effectively.

2. **Why the other choices are incorrect:**
   - **Notch frequency:** This refers to a specific frequency where the filter attenuates signals significantly, but it does not define the point where output power is half of the input power.
   - **Neper frequency:** This term is not commonly used in the context of low-pass filters. It relates to the natural logarithm of the ratio of output to input power, but it does not specifically indicate the half-power point.
   - **Rolloff frequency:** This term describes the rate at which the filter attenuates signals beyond the cutoff frequency, but it does not define the specific frequency where the output power is half of the input power.

3. **Useful formulas or rules of thumb:**
   - The cutoff frequency ($f_c$) for a simple RC low-pass filter can be calculated using the formula:
   $$
   f_c = \frac{1}{2 \pi R C}
   $$
   where:
   - $R$ = resistance in ohms
   - $C$ = capacitance in farads

Understanding the concept of cutoff frequency is crucial for working with filters in radio communications, as it helps in designing circuits that effectively manage signal frequencies.