Question E8A08:
Why are direct or flash conversion analog-to-digital converters used for a software defined radio?  

Choices:  
A: Very low power consumption decreases frequency drift  
B: Immunity to out-of-sequence coding reduces spurious responses  
C: Very high speed allows digitizing high frequencies  
D: All these choices are correct    

Correct Answer is  C: Very high speed allows digitizing high frequencies  

Correct answer: C — Very high speed allows digitizing high frequencies

Why C is correct:
- Direct (flash) ADCs convert the input in a single clock step using many parallel comparators, so they achieve extremely high sampling rates (often GSPS). This lets an SDR digitize RF or IF signals directly without analog mixing.
- By the sampling theorem, to directly digitize a highest input frequency $f_{max}$ in baseband you need:
  $$f_s \ge 2 f_{max}$$
- Even with bandpass (undersampling) techniques, the required rate must still exceed twice the signal bandwidth $B$:
  $$f_s \ge 2B$$
  High sampling speed is therefore the key advantage for SDR front ends.

Why the other choices are incorrect:
- A: “Very low power consumption decreases frequency drift”
  - Flash ADCs are not low power. They use many comparators in parallel — about $2^N - 1$ for $N$ bits — which increases power and heat. Frequency drift is not the limiting factor addressed by ADC choice.
- B: “Immunity to out-of-sequence coding reduces spurious responses”
  - Flash ADCs can suffer “bubble” (out-of-sequence) errors in their thermometer-to-binary encoding and are not inherently immune. Digital correction may help, but this is not why they’re used in SDRs.
- D: “All these choices are correct”
  - False because A and B are false.

Useful rules of thumb:
- Nyquist (baseband): $f_s \ge 2 f_{max}$
- Bandpass sampling (undersampling): $f_s \ge 2B$ with proper bandpass filtering
- Flash ADC comparator count: $2^N - 1$
- Ideal ADC SNR from resolution: $SNR \approx 6.02N + 1.76$ dB

Key takeaway: Flash ADCs are used in SDRs primarily for their very high sampling speed, enabling direct RF/IF digitization.