BANDWIDTH
Definition
Probably no word is used more often in the world of RF than the word bandwidth. Now that you know what frequency is, it's not too difficult to understand the term bandwidth. Bandwidth is a way of describing a range of frequencies. It equals the difference between the highest frequency and the lowest frequency of the device or application, and therefore two frequencies are required to define a bandwidth. For instance, if a particular device can accommodate all frequencies between 75 MHz and 125 MHz, it has a bandwidth of 50 MHz (125 MHz-75 MHz).
Sometimes bandwidth is expressed in terms of a percentage. In this case, the bandwidth is simply divided by the average of the upper and lower frequencies. A simple example will explain everything (see Example 2-2).
Example 2-2
Calculating percentage bandwidth.
If a device can accommodate all frequencies between 75 MHz and 125 MHz, what is its percentage bandwidth?
First, you calculate the actual bandwidth. As noted above, the bandwidth for this example is 50 MHz (125 MHz-75 MHz).
Next, you calculate the average of the two frequencies. In this case it is 100 MHz (125 MHz + 75 MHz) รท 2.
Finally, you divide the bandwidth by the average frequency and multiply by 100%.
A device which operates from 75 MHz to 125 MHz has a 50% bandwidth.
Octaves and Decades
There are two other important descriptors of bandwidth: octave and decade. Octave and decade come from the world of logarithms in which octave means twice as big and decade means ten times as big. If the upper frequency of a device is twice as big as the lower frequency, then the device has an octave bandwidth. For instance, a device which operates from 100 MHz to 200 MHz has an octave bandwidth. The same is true for a device whose lower frequency is 1.2 GHz and upper frequency is 2.4 GHz. To really confuse you, if the lower frequency of a device is 100 MHz and the upper frequency is 300 MHz, the device has a two octave bandwidth. Any device which has a bandwidth greater than one octave is said to have a multioctave bandwidth (as if it could be anything else).
If the upper frequency of a device is ten times the lower frequency, then the device has a decade bandwidth. An example of a decade bandwidth is a device which operates from 100 MHz to 1000 MHz (1 GHz).