The frequency of periodic signals is explained as well as the units used.
In electronics, signals that repeat in time are called periodic and are characterized by their frequencies. The duration of one cycle is called the period. The frequency of the signal is defined as the number of cycles per unit time. The Hertz is the SI unit defined as the number of cycles (or periods) per second.
If T is the period of the signal in question, then the frequency f is given by
f = 1 / T
For example a signal whose cycle has a duration of 5 seconds, has a frequency f = 1 / 5 = 0.2 Hertz
In electronics however, the duration of one cycle (a period) is very short and is given in microseconds (10^{6}seconds) and even nanosecond (10^{9}seconds). A quick calculation gives the corresponding frequencies as 1 / 10^{6}seconds = 10^{6}Hertz which is called 1 Megahertz = 1 MHz and 1 / 10^{9}seconds = 10^{9}Hertz which is called 1 Gigahertz = 1 GHz
.
Fig. 1  Period of Signals
Below is a table of higher frequencies with their names and values.
Name 
Symbol 
Value 
Kilohertz 
kHz 
10^{3} Hz 
Megahertz 
MHz 
10^{6} Hz 
Gigahertz 
GHz 
10^{9} Hz 
