Question
What is considered an acceptable maximum IMD level for an idling PSK signal?
Answer Options
- A) +5 dB
- B) +10 dB
- C) +15 dB
- D) -30 dB
Correct Answer: D
Explanation
The PSK31 mode is known for its extremely narrow bandwidth, requiring very low levels of Intermodulation Distortion (IMD) to maintain spectral purity and avoid splatter on adjacent channels. Since PSK is a complex modulation scheme, an IMD test is used to measure the linearity of the SSB amplifier used to transmit the signal.
IMD is measured as the power level of the third-order intermodulation products relative to the power of the main signal (the carrier and sidebands). For weak-signal modes like PSK, a stringent standard is required. An acceptable maximum IMD level is considered to be -30 dB (i.e., the spurious products are 30 dB weaker than the main signal). This ensures the PSK signal is clean and causes minimal adjacent channel interference.
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