*Question T7A05:
What is the name of a circuit that generates a signal at a specific frequency?*  

*Correct Answer is  D: Oscillator*  

### AI Response: (may contain errors)  

An oscillator is a circuit that generates a periodic signal, typically a sine wave or square wave, at a specific frequency. 

### Explanation of the Correct Answer:
- **Oscillator**: This is the correct answer because an oscillator is specifically designed to produce a continuous output signal at a predetermined frequency. It can be used in various applications, including radio transmitters, clocks, and signal generators.

### Explanation of Incorrect Choices:
- **Reactance modulator**: This is not a circuit that generates a signal at a specific frequency. Instead, it is used to modulate a signal by varying the reactance in the circuit, which affects the amplitude or phase of the signal but does not generate a frequency on its own.

- **Phase modulator**: Similar to the reactance modulator, a phase modulator alters the phase of an incoming signal based on the modulating signal. It does not generate a frequency; rather, it modifies an existing signal's phase.

- **Low-pass filter**: This circuit allows signals with a frequency lower than a certain cutoff frequency to pass through while attenuating higher frequencies. It does not generate a signal; instead, it processes existing signals.

### Summary:
- The correct answer is **Oscillator** because it generates a signal at a specific frequency.
- The other options either modify existing signals or filter frequencies, but do not generate a new signal.

### Useful Concept:
- An oscillator can be characterized by its frequency of oscillation, which can be determined by the components used in the circuit (like resistors, capacitors, and inductors). For example, in a simple LC oscillator, the frequency can be calculated using:

$$
f = \frac{1}{2\pi\sqrt{LC}}
$$

where:
- $f$ = frequency in hertz (Hz)
- $L$ = inductance in henries (H)
- $C$ = capacitance in farads (F)

This formula shows how the frequency of an oscillator is influenced by its inductance and capacitance.