Question E7F10:
What aspect of receiver analog-to-digital conversion determines the maximum receive bandwidth of a direct-sampling software defined radio (SDR)?  

Choices:  
A: Sample rate  
B: Sample width in bits  
C: Integral non-linearity  
D: Differential non-linearity    

Correct Answer is  A: Sample rate  

To avoid aliasing, a real (single-channel) ADC can only represent frequencies up to its Nyquist frequency. Thus, the sample rate sets the maximum instantaneous bandwidth you can capture.

1) Why the correct answer (Sample rate) is correct
- The Nyquist limit is $f_N = \dfrac{f_s}{2}$, where $f_s$ is the sample rate.
- The maximum instantaneous receive bandwidth of a direct-sampling SDR is therefore limited to about $f_s/2$ (for real sampling).
- In practice, you also allow guard bands for filtering, so usable bandwidth is slightly less than $f_s/2$.

2) Why the other choices are incorrect
- Sample width in bits: Determines dynamic range and quantization noise floor, not bandwidth. Quantization SNR ≈ $6.02N + 1.76$ dB.
- Integral non-linearity (INL): A linearity/distortion spec affecting spurs and accuracy, not bandwidth limit.
- Differential non-linearity (DNL): Step-size uniformity; affects distortion/missing codes, not bandwidth.

3) Useful formulas and rules of thumb
- Nyquist frequency: $$f_N = \frac{f_s}{2}$$
- Max instantaneous bandwidth (real sampling): $$B_{\max} \approx \frac{f_s}{2}$$
- Quantization SNR (ideal N-bit ADC): $$\mathrm{SNR_{quant}} \approx 6.02N + 1.76\ \text{dB}$$

Key point: Bandwidth is governed by how fast you sample (sample rate), not how many bits or the linearity specs.