*Question E4A01:
Which of the following limits the highest frequency signal that can be accurately displayed on a digital oscilloscope?*  

*Correct Answer is  A: Sampling rate of the analog-to-digital converter*  

### AI Response: (may contain errors)  

The correct answer to the question is **A: Sampling rate of the analog-to-digital converter**. Here's why:

1. **Why the correct answer is correct:**
   - The highest frequency signal that can be accurately displayed on a digital oscilloscope is limited by the **Nyquist theorem**, which states that to accurately sample a signal, the sampling rate must be at least twice the highest frequency of the signal. This means that if the sampling rate is too low, higher frequency components of the signal will be misrepresented or lost, leading to aliasing.

   The formula for the Nyquist rate is:
   $$ f_s \geq 2f_{max} $$
   where:
   - $f_s$ = sampling rate
   - $f_{max}$ = highest frequency of the signal

2. **Why the other choices are incorrect:**
   - **B: Analog-to-digital converter reference frequency**: While the reference frequency can affect the performance of the ADC, it does not directly limit the highest frequency signal that can be displayed. The sampling rate is the critical factor.
   - **C: Q of the circuit**: The quality factor (Q) of a circuit relates to its bandwidth and selectivity but does not directly limit the sampling frequency of the oscilloscope. A high Q factor can result in a narrower bandwidth, but it does not affect the sampling rate of the ADC.
   - **D: All these choices are correct**: Since only the sampling rate of the ADC directly limits the highest frequency signal that can be accurately displayed, this choice is incorrect.

3. **Useful rules of thumb:**
   - Always ensure your oscilloscope's sampling rate is at least twice the frequency of the signal you wish to measure to avoid aliasing.
   - A good practice is to sample at 5 to 10 times the highest frequency of interest for better accuracy and representation of the waveform.

In summary, the sampling rate of the analog-to-digital converter is the key factor that determines the highest frequency signal that can be accurately displayed on a digital oscilloscope.