LearnTronics
Band-Pass, Notch, and Q
Recipe #44 — band-pass, notch, and q
Resonance becomes even more useful when viewed as frequency selection. The same R, L, and C ideas can create a band-pass that favors a range or a notch that rejects a narrow range.
Band-pass: a series RLC current is greatest near resonance, so voltage across R peaks there. Notch: with a finite source resistance RS, a series LC branch to ground has very low impedance near resonance and diverts that narrow frequency range away from the output.
Q = f0 / BW where BW = f2 − f1
Q connects resonance to selectivity
A high-Q resonant circuit has a narrow bandwidth compared with its center frequency. A low-Q circuit responds over a wider range. Q is therefore a useful bridge between the time-domain idea of slow damping and the frequency-domain idea of narrow selectivity.
Band-pass and notch are building blocks, not just laboratory curves
Band-pass networks select wanted frequency ranges in radios, audio, instrumentation, and communications. Notch filters remove a narrow unwanted frequency, such as hum or an interfering tone. Active and digital filters can create similar response shapes without relying on inductors.
This closes the present capacitance/inductance run. The progression has moved from RC timing and oscillation, through magnetic energy and flyback, into LC resonance, radio tuning, crossovers, and frequency selectivity.