esonant circuits are used to respond selectively to signals of a given frequency while discriminating against signals of different frequencies. If the response of the circuit is more narrowly peaked around the chosen frequency, we say that the circuit has higher "selectivity". A "quality factor" Q, is a measure of that selectivity, and we speak of a circuit having a "high Q" if it is more narrowly selective.

 The selectivity of the tuning must be high enough to discriminate strongly against stations above and below in carrier frequency.The selectivity of a circuit is dependent upon the amount of resistance in the circuit.