Question E8D09:
What is considered an acceptable maximum IMD level for an idling PSK signal?  

Choices:  
A: +5 dB  
B: +10 dB  
C: +15 dB  
D: -30 dB    

Correct Answer is  D: -30 dB  

For PSK (e.g., PSK31), intermodulation distortion (IMD) products must be well below the main signal to avoid splatter and interference. The accepted benchmark for an idling PSK signal is that IMD products be at or below −30 dB relative to the main tone.

Why D (−30 dB) is correct:
- IMD is measured relative to the desired signal: negative values mean the distortion is weaker than the main signal.
- −30 dB means the IMD power is 1/1000 of the main signal, which is generally considered the minimum acceptable cleanliness for PSK.
- This aligns with standard operating practice for linear digital modes to prevent adjacent-channel interference.

Why the others are incorrect:
- A (+5 dB), B (+10 dB), C (+15 dB): Positive dB values imply the IMD products are stronger than the desired signal, which would be unacceptably dirty. Even −20 dB would be marginal for PSK; +5 to +15 dB is far worse.

Useful formulas and rules of thumb:
- IMD in dB (power):  
  $$IMD_{dB} = 10 \log_{10}\left(\frac{P_{IMD}}{P_{signal}}\right)$$
  At −30 dB:  
  $$\frac{P_{IMD}}{P_{signal}} = 10^{-3} = 0.001$$
- If measuring voltages (e.g., spectrum amplitude):  
  $$IMD_{dB} = 20 \log_{10}\left(\frac{V_{IMD}}{V_{signal}}\right)$$
  At −30 dB:  
  $$\frac{V_{IMD}}{V_{signal}} \approx \frac{1}{31.6}$$
- Operating tips:
  - Keep ALC action minimal or none for PSK; avoid clipping or compression.
  - Reduce audio drive until IMD on monitoring software is ≤ −30 dB.
  - Use a clean, linear RF chain; do not overdrive the PA.