High-frequency switching circuits—such as switching power supplies,
power factor correction circuits, and adjustable-speed drives—create
high-frequency components that are not at multiples of line frequency.
For instance, a switching power supply operating at 75 kHz produces
high-frequency components at integer multiples of the fundamental 75 kHz
switching frequency, as shown in Figure 1.4. These frequency components
are sometimes termed “interharmonics” to differentiate them
from harmonics, which are multiples of the line frequency. Other worldwide
standards specify the amount of harmonic noise that can be injected
into a power line. IEC-1000-2-1 [1.40] defines interharmonics as follows: