AC2 Fundamentals Unit 2 – Series Resonance
The selectivity of a circuit is determined by the bandwidth (B) of the circuit.
The bandwidth is determined by the upper and lower cutoff frequencies of the circuit
(B = f 2 - f 1 ).
The selectivity and bandwidth of a series RLC circuit depends on the circuit Q (Q = XL/R).
NEW TERMS AND WORDS
resonant frequency (fr) - the frequency at which the inductive and capacitive reactances in an
RLC circuit are equal.
resonance - the condition where the inductive and capacitive reactances in any RLC circuit are
equal and cancel one another.
tuning - varying the inductance or capacitance in an RLC circuit in order to set the resonant
frequency and select or reject specific signals.
filtering - the process of either passing or rejecting specific frequencies.
selectivity - the measure of the ability of a tuned circuit to pass selected frequencies or bands of
frequencies and reject all others.
bandwidth (B) - the range of frequencies that will be passed or rejected by a resonsant circuit;
the difference between the upper and lower cutoff frequencies.
cutoff frequencies - the frequencies above and below the resonant frequency of a tuned series
circuit where the current is 70.7% of, or 3 dB down from, its peak value; also known as the half
power points.
Q - the ratio of inductive reactance to resistance (Q = XL/R).