*Question T7A03:
Which of the following is used to convert a signal from one frequency to another?*  

*Correct Answer is  B: Mixer*  

### AI Response: (may contain errors)  

To convert a signal from one frequency to another, the correct answer is:

**B: Mixer**

### Explanation of the Correct Answer:
- A **mixer** is a non-linear circuit that combines two signals, typically a radio frequency (RF) signal and a local oscillator (LO) signal. The output of the mixer contains signals at the sum and difference of the input frequencies. This process is essential in radio communications for frequency conversion, such as in superheterodyne receivers.

### Why the Other Choices are Incorrect:
- **A: Phase splitter**
  - A phase splitter is used to create two signals that are 180 degrees out of phase. It does not change the frequency of a signal.

- **C: Inverter**
  - An inverter is a circuit that flips the polarity of a signal (e.g., from positive to negative). It does not alter the frequency of the input signal.

- **D: Amplifier**
  - An amplifier increases the amplitude (power) of a signal but does not change its frequency. It simply makes the signal stronger.

### Useful Concepts:
- In a mixer, the output frequencies can be calculated as:
  - If the input frequencies are $f_{RF}$ (radio frequency) and $f_{LO}$ (local oscillator frequency), the output frequencies will be:
    - $f_{out1} = f_{RF} + f_{LO}$
    - $f_{out2} = f_{RF} - f_{LO}$

This illustrates how a mixer effectively converts signals from one frequency to another, making it a crucial component in many radio systems.