Bandwidth is the difference between the upper and lower frequencies in a continuous band of frequencies. It is typically measured in hertz, and depending on context, may specifically refer to passband bandwidth or baseband bandwidth. Passband bandwidth is the difference between the upper and lower cutoff frequencies of, for example, a band-pass filter, a communication channel, or a signal spectrum. Baseband bandwidth applies to a low-pass filter or baseband signal; the bandwidth is equal to its upper cutoff frequency.
Percentage bandwidth is a simple calculation, and gives a normalized measure of how much frequency variation a system or component can handle. As you go higher in frequency, the absolute bandwidth as a part will naturally increase, while its percent bandwidth will decrease.
Percent bandwidth is a less meaningful measure in wideband applications. A percent bandwidth of 100% corresponds to a ratio bandwidth of 3:1. All higher ratios up to infinity are compressed into the range 100–200%.
If you know the center frequency and the bandwidth, the percent bandwidth is:
Here “BW” is the absolute bandwidth and FC is the center frequency. Thus a filter with 1 GHz passband centered at 10 GHz will have 10% bandwidth, while a filter with 10 GHz bandwidth at 100 GHz will have the same 10%.
Alternatively, you can calculate BW% from two corner frequencies FL and FH:
Where FH is the upper (higher) frequency and FL is the lower frequency of the passband. This is simple math, it is derived from the center frequency being the arithmetic average between the upper and lower corner frequencies:
Here’s an alternative calculation from Daniel:
or more consistent with the use of percent sign: