# How does Q of a resonator vary with frequency?

Discussion in 'General Electronics Chat' started by zero_coke, Jun 6, 2010.

1. ### zero_coke Thread Starter Active Member

Apr 22, 2009
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How can I maximize the Q of my resonating LC circuit? How can I know when it is at its maximum? What type of reading should I embark on?

2. ### Bychon Member

Mar 12, 2010
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The Q of an LC circuit is lowered by waste of power. The first place to look is the resistance of the coil. Low resistance = low power wasted. The next place is the absorbtion of the capacitor. Polywhatever-film caps are very good at having low absorbtion losses.

As 4 measuring, when you stimulate an LC circuit with its resonant frequency, any current that flows out of the generator is waste energy. You can measure Q by setting up a graph of tank impedance versus frequency.

3. ### zero_coke Thread Starter Active Member

Apr 22, 2009
294
1
Thanks for the great information! And as for the LC circuit you mentioned, does it matter if it is in parallel or series LC circuit? I did some simulations and the current that comes out of the generator is extremely high in my series LC circuit, and in my parallel LC circuit simulation the current output from the generator is very very small. In designing an efficient resonator circuit, we would need to make a parallel LC circuit as to draw the least current from the supply generator right? Am I correct?

Oh and can you tell me why any current drawn from the generator is waste energy? How is it waste energy?

Last edited: Jun 6, 2010
4. ### Wendy Moderator

Mar 24, 2008
20,772
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There are two different kinds of LC circuits, parallel and series. Parallel blocks the resonant frequency, allowing all others to pass, while series pass the resonant freq while blocking all others.

There is no getting around the math for resonance. Personally I still find the square root of -1 mind blowing (AKA imaginary numbers, also shown as j when doing the math). For something that was taught as couldn't exist it sure is used a lot, which makes me suspect middle school math is lacking.

As any good text book would have, here is the AAC take on LC resonance.

Chapter 6: RESONANCE

There are other aspects of resonance that is interesting, but you are best served if you take this in small bites. It is the bare beginning of understanding filters, low pass, band pass, and high pass, a very deep subject indeed.

5. ### Bychon Member

Mar 12, 2010
469
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The Q doesn't change if you change whether the L and C are in series or parallel.

I once put a parallel LC circuit from 120V house power to ground, and almost no current flowed! Then I put an LED in series with the LC connection and the current from the outlet went up by exactly enough to power the LED! What it amounted to was a 60 Hz to 20 ma converter.

Why is current flow from the generator waste current? Because the theoretical perfect tank circuit (at resonance) draws zero current. Any current that flows must be the difference between perfect and what you built.

6. ### zero_coke Thread Starter Active Member

Apr 22, 2009
294
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Bychon: Thanks for the input on this topic. So I am wondering how to maximize the Q of my LC circuit. How can I go about do this? I can't do anything about my coil's resistance except decrease the length of the wire, and I'll go buy a polypropylene film capacitor since it has low loss, now what? Is there anything else that could be done?

7. ### Bychon Member

Mar 12, 2010
469
41
I just told you how to maximize the Q of an LC tank circuit. Low resistance coil and low absorbtion capacitor, and you can lower resistance without having less millihenries. Use larger diameter wire and make the coil as compact as possible.

Is there anything else that could be done? Yes. Decide, "what is the point of making a high Q resonant tank circuit?" Are you going to make a larger circuit with it?

8. ### zero_coke Thread Starter Active Member

Apr 22, 2009
294
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Thank you for the reply once more. I am just building a wireless power model, and I plan on sending power 2-3 feet away with two LC circuits inductively coupled through resonance.

What is your advice Bychon on this topic? Am I in the right direction in making two parallel LC circuits as my primary sender and secondary receiver or should I build two series LC circuits? What would be better? Since Q does not depend on the series or parallel setup of this LC circuit, I would only imagine that it only depends on frequency and resistance in the circuit from the sources you mentioned (capacitor loss, inductor losses), right?

9. ### Bychon Member

Mar 12, 2010
469
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Transmitting power 2 or 3 feet? There is a really bad equasion about magnetic fields and distance. That means, it's hard to get serious power to travel through the air. And that's about all I know on that subject.

Good luck.

10. ### zero_coke Thread Starter Active Member

Apr 22, 2009
294
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Ah, well thank you for all your time and input into this topic. Much appreciated!