Question E4E06:
What type of electrical interference can be caused by computer network equipment?  

Choices:  
A: A loud AC hum in the audio output of your station’s receiver  
B: A clicking noise at intervals of a few seconds  
C: The appearance of unstable modulated or unmodulated signals at specific frequencies  
D: A whining-type noise that continually pulses off and on    

Correct Answer is  C: The appearance of unstable modulated or unmodulated signals at specific frequencies  

Correct answer: C — The appearance of unstable modulated or unmodulated signals at specific frequencies

Why C is correct:
- Computer network gear (Ethernet switches/routers, NICs, Wi‑Fi APs) and their switch‑mode power supplies use high‑frequency clocks and digital signaling. These create discrete spectral lines at their clock rates and harmonics.
- As network traffic varies, these spurs can turn on/off or change amplitude, so they appear “unstable” and may be modulated by data bursts.
- Common examples:
  - 100BASE‑TX uses a 125 MHz symbol clock; harmonics and mixes can land in HF/VHF.
  - Switch‑mode PSUs switch around 50 kHz–1 MHz, with harmonics across HF.
  - Power‑line networking (BPL/PLC) produces many narrow carriers across HF at specific spacings.

Useful rules of thumb:
- Harmonics from clocks:  
  $$f_n = n\,f_{clk} \quad (n = 1,2,3,\dots)$$
- Mixing products between multiple clocks:  
  $$f = \lvert m f_1 \pm n f_2 \rvert \quad (m,n \in \mathbb{Z}^+)$$
- Expect discrete lines, often evenly spaced, that may flicker or vary with data load.

Why the other choices are incorrect:
- A: A loud AC hum in the audio output
  - 50/60 Hz hum (and its 120 Hz ripple) is usually from ground loops, poor audio shielding, or bad linear power‑supply filtering—not typical of network equipment RFI.
- B: Clicking noise at intervals of a few seconds
  - Intermittent clicks every few seconds are characteristic of devices like electric fence chargers, thermostats, or relay controls, not typical LAN equipment.
- D: Whining-type noise that continually pulses off and on
  - A “whine” is often associated with alternators, motor controllers, or some switching supplies under changing load. Network gear more commonly produces discrete spurs rather than a periodic on/off whining tone pattern.

Tip for mitigation:
- Use ferrite chokes on Ethernet and DC leads, keep UTP away from antennas/feeds, try shielded Ethernet (STP), and ensure good bonding/grounding to reduce common-mode radiation from network devices.