AC2 Fundamentals Unit 2 – Series Resonance
Exercise 2 – Q and Bandwidth of a Series RLC Circuit
EXERCISE OBJECTIVE
When you have completed this exercise, you will be able to calculate the bandwidth and Q of a
series RLC circuit by using standard formulas. You will verify your results with an oscilloscope.
DISCUSSION
- Resonant RLC circuits are often used for tuning and filtering input signals.
- Selectivity, in series RLC circuits, is the ability to produce a high current (IRESON) at
resonance and high impedance at all other frequencies. - Highly selective circuits are responsive to a limited range of frequencies.
- The bandwidth (B) of a resonant circuit determines the selectivity.
- Bandwidth is determined by the upper and lower cutoff frequencies of the circuit.
B = f 2 − f 1 - Upper and lower cutoff frequencies occur where circuit current is 3 dB down from the
maximum current (IRESON). - Resonant circuit selectivity is characterized by a factor called Q. The Q of a circuit is
calculated from this equation: Q = XL/R - Circuits with high Q values have a high selectivity. Q values have a wide range, a Q below
10 is very low, while a Q larger than 250 is very high. - The higher the Q the smaller the bandwidth. B = fr/Q
- Q of the resonant circuit can determine the voltage across L or C using this relationship:
VC or VL = Q x VGEN