Guide

Frequencies from mains power to WiFi

Frequency states how often an event repeats per second. In engineering it spans many orders of magnitude, from mains power at 50 hertz to CPU clocks in the gigahertz range.

Hertz and the prefixes

One hertz is one event per second. The prefixes follow the decimal system: one kilohertz is 1000 Hz, one megahertz 1,000,000 Hz, one gigahertz 1,000,000,000 Hz. For frequencies the prefixes are unambiguously decimal; there is no binary variant as with byte quantities.

Mains frequency and its consequences

In Europe mains frequency is 50 Hz, in North America 60 Hz. Practical differences follow: synchronous motors run about 20 % faster at 60 Hz, and mains-driven clocks count the frequency to measure time. Electrical devices with synchronous motors and such clocks are therefore not freely interchangeable between the two regions.

Hertz and revolutions per minute

Rotational speed is often stated in revolutions per minute rather than hertz. The relation is fixed: 1 Hz is 60 rpm. Mains frequency of 50 Hz therefore equals 3000 rpm, which is the speed of a synchronous motor with one pole pair. Hertz and rpm describe physically the same thing, just with a different time unit.

Frequency, wavelength and bandwidth

For electromagnetic waves frequency and wavelength are linked through the speed of light: wavelength is the speed of light divided by frequency. So 2.4 GHz corresponds to a wavelength of about 12.5 centimetres. Bandwidth, by contrast, is the difference between two frequencies and is stated in the same unit — a bandwidth of 20 MHz is not a frequency value but a range.

Matching conversions

Matching tables

Unit family: Frequency

Sources