UNIT B.8 — SIGNALS

Signals: time domain and frequency domain

BRIDGE · EE Foundations Bridge · ~11 min

Community content — not verified by a licensed electrician. Lessons and questions here are written for study practice and pinned to NEC 2023. Code editions and local amendments vary by jurisdiction. Always confirm against your adopted code book and your licensing board's published exam outline before test day.

A signal viewed on an oscilloscope — value versus time — is the time domain. The frequency domain is the other lens: any repeating signal, however jagged, can be decomposed into a stack of pure sine waves at different frequencies (Fourier's theorem). The clean 60 Hz sine from Unit 1.8 is a signal with exactly one frequency component.

Real building power is dirtier. LED drivers, VFDs, and computer supplies draw current in gulps rather than smooth sines, adding *harmonics* — components at 3×, 5×, 7× the fundamental. Total Harmonic Distortion (THD) measures how much. Harmonics matter to the trade: triplen harmonics (3rd, 9th…) add up in the neutral instead of canceling, which is why heavily-loaded office circuits can cook a neutral that 'should' carry nothing.

The frequency domain is EE's superpower — filters, communications, motor drives, and audio all get designed there. Signals and Systems, the course that teaches it, is widely considered the most distinctively 'electrical engineering' subject in the whole degree.

Go deeper: edX Signals and Systems Parts 1–2 — Semester 2 of the Go Deeper pathway.

Time domain
Value vs. time
Frequency domain
Sine components
Triplen harmonics
Add in the neutral
Knowledge check

Quiz — 3 questions

1. Fourier's theorem says any repeating signal can be built from:

2. A perfect 60 Hz sine wave contains:

3. Triplen harmonics are a jobsite concern because they:

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