Question E4C08:
An SDR receiver is overloaded when input signals exceed what level?  

Choices:  
A: One-half of the maximum sample rate  
B: One-half of the maximum sampling buffer size  
C: The maximum count value of the analog-to-digital converter  
D: The reference voltage of the analog-to-digital converter    

Correct Answer is  D: The reference voltage of the analog-to-digital converter  

To overload an SDR’s ADC, you must exceed its analog full-scale input range, which is set by the ADC’s reference voltage.

Why D is correct
- The ADC’s full-scale input is defined by its reference voltage, $V_{ref}$ (or the full-scale range $V_{FS}$ derived from it). If the input amplitude exceeds this level, the converter saturates (clips), causing distortion and spurious responses.
- In short: overload occurs when the analog input exceeds the ADC’s full-scale level set by $V_{ref}$.

Useful statement:
$$\text{Overload if } |v_{in}(t)| > V_{FS} \approx V_{ref}$$

Why the other choices are incorrect
- A: One-half of the maximum sample rate — This is a frequency (Nyquist limit), not an amplitude level. It relates to aliasing, not overload.
- B: One-half of the maximum sampling buffer size — This is a memory size/count of samples, not an input voltage level.
- C: The maximum count value of the ADC — This is a digital code limit (e.g., $2^N-1$). It is the result of clipping, not the analog threshold. The analog threshold is set by $V_{ref}$.

Helpful formulas/rules of thumb
- Full-scale mapping (signed ADC):
  $$\text{Codes} \in [-2^{N-1},\, 2^{N-1}-1] \quad \leftrightarrow \quad v_{in} \in [-V_{FS},\, +V_{FS}]$$
- Ideal ADC SNR:
  $$\text{SNR}_{\max} \approx 6.02N + 1.76\ \text{dB}$$
- Operating practice: keep peaks a little below full-scale (e.g., −1 to −3 dBFS), and average levels 10–15 dB below full-scale to allow headroom and avoid clipping with real signals having crest factor.