Friday, July 30, 2010

I need a quick refresher on the meaning of Q. Can you help? ...

Q I need a quick refresher on the meaning of Q. Can you help?

A Hmmm…I believe Q was the name of an omnipotent alien who appeared occasionally on Star Trek: The Next Generation. His “meaning” wasn’t always clear! But if you’re talking about the ratio of an electronic component’s ability to store energy to the sum total of all of its energy losses, I can help. That’s Q in a nutshell and it is expressed mathematically as:


Q = figure of merit or quality
X = XL (inductive reactance) for inductors and XC (capacitive reactance) for capacitors (in ohms), and R = the sum of all resistances associated with energy losses in
the component (in ohms).

The Q of capacitors is ordinarily high. Good quality ceramic capacitors and mica capacitors may have Q values of 1200 or more. Small ceramic trimmer capacitors may have Q values too small to ignore in some applications. Microwave capacitors typically have poor Q values (10 or less at 10 GHz).

Inductors are subject to many kinds of electrical energy losses including wire resistance, core losses and skin effect. As a result of inherent losses, inductor Q rarely, if ever, approaches capacitor Q in a circuit where both components work together. 

From QST August 2000