Question
What causes interference received as a series of carriers at regular intervals across a wide frequency range?
Answer Options
- A) Switch-mode power supplies
- B) Radar transmitters
- C) Wireless security camera transmitters
- D) Electric fences
Correct Answer: A
Explanation
Interference that appears as a regularly spaced series of carriers across a wide range of frequencies is a classic signature of a device using a fixed-frequency oscillator, where the oscillator’s harmonics are being radiated. This pattern is characteristic of power supply technology that uses high-speed switching.
The cause of this interference pattern is typically Switch-mode power supplies (SMPS). These supplies operate by rapidly switching DC or rectified AC power at a fixed high frequency (often 50 kHz to several MHz). The square wave output of the switching circuit is rich in harmonics, and these harmonics radiate from the unit and its cables, appearing as regularly spaced ‘birdies’ (spurious carriers) across the HF and sometimes VHF spectrum.
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