E8D09: IMD: What is considered an acceptable maximum

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

The correct answer is D: -30 dB. The correct answer is D: -30 dB. An acceptable maximum IMD level for an idling PSK signal is -30 dB or lower. This means the intermodulation distortion products should be at least 30 dB below the desired signal. For PSK signals, especially when idling (not actively transmitting data), IMD should be very low to avoid interference. -30 dB IMD means the distortion products are 30 dB below the signal level, which is acceptable for clean operation. Higher IMD levels (like +5 dB, +10 dB, or +15 dB) would be terrible - they would mean distortion products are stronger than or equal to the signal, which is completely unacceptable. For good signal quality, IMD should be well below the signal level (negative dB values).

Updated: Dec 9, 2025 | Source: 2024-2028 Question Pool | Topic: E8D

E8D09E8D

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

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Deep Dive: E8D09

The correct answer is D: -30 dB. An acceptable maximum IMD level for an idling PSK signal is -30 dB or lower. This means the intermodulation distortion products should be at least 30 dB below the desired signal. For PSK signals, especially when idling (not actively transmitting data), IMD should be very low to avoid interference. -30 dB IMD means the distortion products are 30 dB below the signal level, which is acceptable for clean operation. Higher IMD levels (like +5 dB, +10 dB, or +15 dB) would be terrible - they would mean distortion products are stronger than or equal to the signal, which is completely unacceptable. For good signal quality, IMD should be well below the signal level (negative dB values).

Why Other Answers Are Wrong

Option A: Incorrect. +5 dB IMD would mean distortion products are 5 dB stronger than the signal, which is terrible and completely unacceptable. IMD should be negative (below the signal). Option B: Incorrect. +10 dB IMD would mean distortion is 10 dB stronger than the signal, which is even worse. This is completely unacceptable. Option C: Incorrect. +15 dB IMD would mean distortion is 15 dB stronger than the signal, which is extremely bad. IMD must be negative (below signal level).

Exam Tip

PSK IMD maximum = -30 dB. Remember: Acceptable maximum IMD for an idling PSK signal is -30 dB (distortion 30 dB below signal). Positive IMD values are unacceptable.

Memory Aid

**P**SK **I**MD **M**ax = **-3**0 **d**B (think 'PIM = -30dB')

Real-World Example

You're testing your PSK signal and measure -35 dB IMD. This is good - the distortion products are 35 dB below your signal level, which is better than the -30 dB maximum. If you measured +5 dB IMD, that would be terrible - distortion stronger than your signal. For clean PSK operation, IMD should be -30 dB or lower.

Source & Coverage

Question Pool: 2024-2028 Question Pool

Subelement: E8D

Reference: FCC Part 97.307

Key Concepts

IMD PSK Intermodulation distortion Signal quality

Verified Content

Question from the official FCC Extra Class pool. Explanation reviewed by licensed amateur radio operators and mapped to the E8D topic.