Bandpass Bandwidth

Formula

Description

The bandwidth of a bandpass filter is simply the difference between the upper and lower -3dB cutoff frequencies. It defines the range of frequencies that pass through the filter with less than 3 dB of attenuation. A narrow bandwidth means the filter is highly selective (high Q factor), while a wide bandwidth passes a broader range of frequencies. The center frequency is the geometric mean of the cutoff frequencies: f0 = √(f1 × f2), and the quality factor is Q = f0 / BW. Bandpass filters are essential in radio receivers, spectrum analyzers, and communication systems.

Variables

• BW — Bandwidth (Hz)
• f1 — Lower -3dB cutoff frequency (Hz)
• f2 — Upper -3dB cutoff frequency (Hz)

Practical Notes

In communication systems, channel bandwidth determines the maximum data rate (Shannon's theorem). Voice channels use 3.1 kHz bandwidth (300-3400 Hz), AM radio uses 10 kHz, FM radio uses 200 kHz, and WiFi uses 20-160 MHz channels. Narrower bandwidth also means less noise power captured by the receiver.