# Tuned Circuit Filter Quality Factor

### In this section

Resonant circuits consist of inductors and capacitors. It is therefore often necessary to look at the quality factor of an LC, i.e. inductor capacitor system.

It is necessary to be able determine the LC filter Q factor to assess the performance of these circuits.

Fortunately there are some simple formulae or equations that can be used to determine the LC filter quality factor.

## Q factor and LCR tuned circuits

One of the key features of an LC tuned circuit is that at resonance the inductive and capacitive reactances become equal. However dependent upon the type of tuned circuit, the effect is slightly different.

There are two basic types of tuned circuit:

• Parallel tuned circuit:   At resonance the impedance of a parallel tuned circuit peaks, decreasing either side of resonance. Below resonance the inductive reactance dominates and above resonance it becomes capacitive. As a result of its action any alternating or RF signal voltage placed across the circuit will peak at resonance.
• Series tuned circuit:   The series tuned circuit is very much the inverse of the parallel tuned circuit in that rather than showing a peak in impedance at resonance there is a minimum.

The equivalent circuit for a series tuned network is given below. In this, the resistance "R" is the equivalent series resistance for the inductor and capacitor:

A parallel tuned circuit is also affected by the resistance in the circuit.

.

In the case of the parallel tuned LC circuit, the Q factor is still an issue. Again there is resistance within both the inductor and capacitor. However as the inductor resistance dominates normally, it has been included in this leg for convenience.

## LC Q factor equations

When determining the Q of an LC tuned circuit it is necessary to determine whether the circuit is series or parallel tuned. The LC Q factor for a series tuned circuit is:

The LC Q factor for a parallel tuned circuit is:

<< Previous   |   Next >>

### Other popular tutorials

Miguel Fernandez | Avnet EMEA
Too good to be true - the cost of counterfeit electronics and how to avoid them
The issue of counterfeit electronic components is one that has troubles the electronics industry - using them can have some major issues, everything from being removed from a preferred suppliers list to a reduction in quality.
Online - Effective Vector Network Analyzer Measurements
How to make effective VNA measurements at RF and microwave frequencies

More training courses

LTE for Automotive Applications
Read the insight in this white paper from u-Blox about LTE for automotive applications. Discover all you need to know.

More whitepapers

##### Channels
Radio-Electronics.com is operated and owned by Adrio Communications Ltd and edited by Ian Poole. All information is © Adrio Communications Ltd and may not be copied except for individual personal use. This includes copying material in whatever form into website pages. While every effort is made to ensure the accuracy of the information on Radio-Electronics.com, no liability is accepted for any consequences of using it. This site uses cookies. By using this site, these terms including the use of cookies are accepted. More explanation can be found in our Privacy Policy