Amplitude, period, frequency, and RMS value

Period and frequency

The period T is the duration, in seconds, after which the signal repeats identically. The frequency f, in hertz (Hz), indicates the number of oscillations per second. These two quantities are related by:

f = 1/T

Example: if T = 0.02 s (20 ms), then f = 1/0.02 = 50 Hz, which corresponds exactly to the French electrical grid.

Amplitude (maximum value)

The amplitude Umax is the highest value reached by the voltage. For the French mains supply, Umax is about 325 V.

The RMS value

However, this is not the value used on a daily basis: we speak of the RMS value Ueff, the one that would produce the same thermal effect as a direct current of the same value. For a sinusoidal signal:

Ueff = Umax / sqrt(2)

Quantity	Symbol	Value (France mains)
Amplitude	Umax	about 325 V
RMS value	Ueff	230 V
Frequency	f	50 Hz
Period	T	20 ms

Common pitfall

The most common pitfall is confusing the value displayed by measuring devices (multimeter, power company) with the amplitude. When we say "230 V", it is always the RMS value, never the maximum value.