Question on quarter wave circuit?

1. Dec 31, 2015

BHY-BK

Hi all, first post here.

I am studying 1/4 wave circuits and I've been wondering.

Can you have a LC circuit in which the reactances of the L and C produce resonance, while at the same time having the circuit be a 1/4 wavelength?

If so what would this circuits characteristics be?

2. Dec 31, 2015

Staff: Mentor

Welcome to the PF.

Can you post the Relevant Equations? Please show what you have been studying, and show the part that you are wondering about...

3. Jan 1, 2016

BHY-BK

OK,

You have a series LC circuit. The L value is 500uH while the C value is 10pF. So the resonant frequency is 2.25MHz. Now, lets say the electrical length of the inductor is exactly 1/4 wavelength at 2.25MHz. How would this circuit behave when driven at it's resonant frequency (2.25MHz) by an AC source?

4. Jan 1, 2016

tech99

I presume that the inductor is actually a piece of shorted transmission line having a length of 1/4 wavelength at 2.25 MHz. In such case it will behave as an open circuit and cannot look like 500uH.
If, however, the length of wire in the inductor is about 1/4 wavelength and the inductor looks like a shorted line 1/8 wavelength long, then it will have an inductive reactance numerically equal to its characteristic impedance. In this case the circuit appears the same as the LC series series circuit over a fairly wide range of frequencies centred on the resonant frequency.
500uH has a reactance of 707 ohms at 2.25 MHz, so the line would require a Z0 of 707 ohms.
If the circuit is driven by a low resistance generator in series with it, high voltages will appear across L and C.

Last edited by a moderator: Jan 1, 2016
5. Jan 1, 2016

BHY-BK

Tech99

Thanks for the explanation, It is appreciated.

What about the current? How would you determine current flow in this circuit?

6. Jan 1, 2016

tech99

A series resonant circuit has a resistance near zero, so the current will be mainly decided by the generator.

Last edited by a moderator: Jan 1, 2016