Question E4E12:
What causes interference received as a series of carriers at regular intervals across a wide frequency range?  

Choices:  
A: Switch-mode power supplies  
B: Radar transmitters  
C: Wireless security camera transmitters  
D: Electric fences    

Correct Answer is  A: Switch-mode power supplies  

Correct answer: A — Switch-mode power supplies

Why A is correct:
- Switch-mode power supplies (SMPS) switch at a fixed frequency $f_s$ (typically tens to hundreds of kHz). Their fast edges produce harmonics and modulation products that appear as a “comb” of evenly spaced carriers across a wide span.
- The spectral lines are typically at:
  $$f_n = n f_s \quad (n = 1, 2, 3, \dots)$$
  and often with mains-related sidebands:
  $$f = n f_s \pm m f_{line} \quad (m = 0, 1, 2, \dots;\ f_{line} = 50\ \text{or}\ 60\ \text{Hz})$$
- On a waterfall, this looks like regularly spaced “pickets” over a broad frequency range.

Why the others are incorrect:
- B: Radar transmitters
  - Usually microwave bands, often pulsed or swept (chirped). They produce broadband pulses or moving sweeps, not a dense set of evenly spaced carriers across HF/VHF bands.
- C: Wireless security camera transmitters
  - Operate on specific ISM bands (e.g., 900 MHz, 2.4 GHz, 5.8 GHz). You’d see one or a few carriers in those bands, not a wideband comb across many bands.
- D: Electric fences
  - Produce repetitive impulsive bursts (“tick-tick”) at low repetition rates (~1 Hz). In spectrum, this shows as broadband clicks or vertical stripes in time, not a uniformly spaced carrier comb.

Rules of thumb:
- Seeing a “picket fence” with spacing $\Delta f$ suggests an SMPS with $f_s \approx \Delta f$.
- Presence of small 50/60 Hz sidebands around each line further points to rectified-mains switchers.
- Common culprits: computer PSUs, LED drivers, chargers, TV/monitor supplies.