Question
Which of the following methods measures intermodulation distortion in an SSB transmitter?
Answer Options
- A) Modulate the transmitter using two RF signals having non-harmonically related frequencies and observe the RF output with a spectrum analyzer
- B) Modulate the transmitter using two AF signals having non-harmonically related frequencies and observe the RF output with a spectrum analyzer
- C) Modulate the transmitter using two AF signals having harmonically related frequencies and observe the RF output with a peak reading wattmeter
- D) Modulate the transmitter using two RF signals having harmonically related frequencies and observe the RF output with a logic analyzer
Correct Answer: B
Explanation
Intermodulation distortion (IMD) is a measure of non-linearity in an amplifier or transmitter, often the main cause of splatter or adjacent channel interference for an SSB signal. To measure IMD, a standardized test signal is used to excite the transmitter, and the output is analyzed in the frequency domain.
The correct method is to modulate the transmitter using two AF (audio frequency) signals having non-harmonically related frequencies (typically 700 Hz and 1900 Hz). The resulting RF output is then observed with a spectrum analyzer. The presence of spurious sidebands next to the main SSB signal indicates IMD, and the level of the third-order products (IMD3) relative to the main tone is the standard figure of merit.
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