Question E4C01:
What is an effect of excessive phase noise in an SDR receiver’s master clock oscillator?  

Choices:  
A: It limits the receiver’s ability to receive strong signals  
B: It can affect the receiver’s frequency calibration  
C: It decreases the receiver’s third-order intercept point  
D: It can combine with strong signals on nearby frequencies to generate interference    

Correct Answer is  D: It can combine with strong signals on nearby frequencies to generate interference  

Correct answer: D — It can combine with strong signals on nearby frequencies to generate interference.

1) Why D is correct
- Excessive phase noise in the SDR’s master clock (which drives the LO/NCO and ADC sample clock) spreads the LO energy into phase-noise sidebands.  
- A strong off-frequency signal then “reciprocally mixes” with this LO phase noise, raising the noise floor in your receive passband. You hear this as hiss or hash that masks weak signals near strong ones.

Useful relation (reciprocal mixing noise):
$$P_{rm} \approx P_{blocker} + L(\Delta f) + 10\log_{10}(B)$$
where:
- $P_{rm}$ = resulting noise power in the IF/audio bandwidth (dBm)
- $P_{blocker}$ = power of the nearby strong signal (dBm)
- $L(\Delta f)$ = LO/clock phase noise density at offset $\Delta f$ (dBc/Hz)
- $B$ = receiver noise bandwidth (Hz)

Example: If $P_{blocker} = -30$ dBm at 2 kHz offset, $B = 500$ Hz, and you want $P_{rm} \le -130$ dBm, then you need
$$L(2\,\text{kHz}) \le -130 - (-30) - 10\log_{10}(500) \approx -127\ \text{dBc/Hz}.$$

Also relevant for SDRs (clock jitter to SNR):
$$SNR_{j} \approx -20\log_{10}(2\pi f_{in}\sigma_t)$$
- $f_{in}$ = input tone frequency
- $\sigma_t$ = RMS clock jitter  
More jitter (phase noise) degrades SNR and close-in dynamic range.

2) Why the other choices are incorrect
- A: “It limits the receiver’s ability to receive strong signals”  
Strong-signal handling is mainly limited by front-end linearity, gain compression, and ADC headroom. Phase noise primarily hurts weak-signal reception near strong signals, not the ability to copy the strong ones themselves.

- B: “It can affect the receiver’s frequency calibration”  
Calibration is set by long-term frequency accuracy and stability (ppm, aging, temperature). Phase noise is short-term random phase fluctuation; it does not cause a systematic frequency offset.

- C: “It decreases the receiver’s third-order intercept point”  
IP3 measures nonlinear intermodulation performance. Phase noise is not a nonlinearity; it doesn’t change IP3, though it can limit close-in dynamic range via reciprocal mixing.

3) Key takeaways / rules of thumb
- Think “reciprocal mixing”: strong adjacent signals plus LO phase noise = raised noise floor in your passband.
- Lower $L(\Delta f)$ (better phase noise) at small offsets and lower ADC clock jitter both improve close-in dynamic range.
- Frequency accuracy (ppm) ≠ phase noise (dBc/Hz); nonlinearity (IP3) ≠ phase noise.