Question E4D02:
Which of the following describes problems caused by poor dynamic range in a receiver?  

Choices:  
A: Spurious signals caused by cross modulation and desensitization from strong adjacent signals  
B: Oscillator instability requiring frequent retuning and loss of ability to recover the opposite sideband  
C: Poor weak signal reception caused by insufficient local oscillator injection  
D: Oscillator instability and severe audio distortion of all but the strongest received signals    

Correct Answer is  A: Spurious signals caused by cross modulation and desensitization from strong adjacent signals  

Correct answer: A — Spurious signals caused by cross modulation and desensitization from strong adjacent signals

1) Why A is correct
- Receiver dynamic range is the span between the weakest signals you can hear (noise floor/MDS) and the strongest signals you can tolerate without overload.
- When dynamic range is poor, strong nearby signals drive the front end or mixer into nonlinearity, causing:
  - Cross-modulation: the modulation from a strong signal appears on the desired weak signal.
  - Intermodulation spurs: products like $2f_1 - f_2$ and $2f_2 - f_1$ show up in-band.
  - Desensitization (blocking): the strong signal forces AGC or overloads stages, effectively raising the noise floor and masking weak signals.
- These are exactly the symptoms described in A.

2) Why the other choices are incorrect
- B: “Oscillator instability requiring frequent retuning and loss of ability to recover the opposite sideband”
  - Oscillator instability (frequency drift/phase noise) is not a dynamic range issue. “Opposite sideband” recovery refers to demodulator/oscillator issues, not strong-signal handling.
- C: “Poor weak signal reception caused by insufficient local oscillator injection”
  - Insufficient LO drive reduces mixer conversion gain/sensitivity, but that’s a sensitivity/mixer-drive problem, not dynamic range (simultaneous weak/strong signal handling).
- D: “Oscillator instability and severe audio distortion of all but the strongest received signals”
  - Again points to oscillator instability and generic distortion, not the specific overload/IMD/desense behavior tied to poor dynamic range.

3) Useful formulas and rules of thumb
- Minimum discernible signal (MDS):
  $$\text{MDS (dBm)} \approx -174 + NF + 10\log_{10}(B)$$
  where $NF$ is noise figure (dB), $B$ is bandwidth in Hz.
- Intermodulation dynamic range (third-order approximation):
  $$DR_{IMD3} \approx \tfrac{2}{3}\,(IP3 - N_{floor}) \quad \text{(in dB)}$$
  where $IP3$ is the third-order intercept (dBm) and $N_{floor}$ is the noise floor (dBm) in the measurement bandwidth.
- Blocking dynamic range (simplified):
  $$DR_{block} \approx P_{block} - \text{MDS}$$
  where $P_{block}$ is the level of a strong off-channel signal that causes 1 dB gain compression or specified desense.
- Intermod product frequencies (third order):
  $$f_{IMD} = 2f_1 - f_2 \quad \text{and} \quad f_{IMD} = 2f_2 - f_1$$

Rules of thumb:
- Higher $IP3$ → better strong-signal handling; a 10 dB rise in $IP3$ yields about 6–7 dB better IMD3 dynamic range.
- Narrower bandwidth lowers the noise floor by 3 dB for each halving of bandwidth, improving the ability to copy weak signals in the presence of strong ones.