# Find LC by frequency

Discussion in 'Wireless & RF Design' started by Synaps3, Jan 22, 2014.

1. ### Synaps3 Thread Starter Member

Jun 5, 2013
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How do I find the best L and C to use to construct a resonant LC circuit at a certain frequency when C must be less than 800pf? (Like 1 MHz for example)

Also, lets say I add length of wire in the LC circuit. Will that change the resonant frequency? How can that be factored in?

2. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,500
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Something like this: http://www.1728.org/resfreq.htm

I am not into radio stuff, but I was reading another thread about audio and poster there said that length of wire will add inductance. How much? I am not sure, but I think that sort of thing is part of the datasheet for the wire. I hope someone here can double check me.

3. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
Your problem, as stated does not have a unique solution. To create a well formed problem statement I would start by choosing a capacitor of any convenient value. There are two reasons for this: first capacitors come in standard values with a pretty coarse granularity, second inductors are normally wound to the half-turn and provide a much finer control of the final value.

If the inductor is not conveniently realizable, then you need to choose a different capacitor. Wash, rinse repeat until you get a combination that works.

4. ### #12 Expert

Nov 30, 2010
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7,310
In the tiny amount of study I did on this, it was immediately obvious that connecting the inductor and the capacitor with 6 inch alligator leads was absurd. So, yes, the lead length can quickly become an important factor.

Mar 6, 2009
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6. ### vk6zgo Active Member

Jul 21, 2012
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Minimal at 1MHz,though!

7. ### vk6zgo Active Member

Jul 21, 2012
677
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It largely depends on whether you are designing with ideal components or real ones.

In the former case,any combination that satisfies f=1/(2∏√LC) will work,but with the latter,you are (mainly) limited
in the value of L,by the inherent resistance of the wire making up the coil.

The more circuit "R",the lower the quality factor,or "Q" of the circuit.

C has some resistive losses,but they are very much smaller.

Also,"C" cannot be made too small,due to inherent stray capacitance in the circuit,which if comparable or larger than the desired capacitance will change the resonant frequency significantly.

Stray inductance also exists,(as in,I add length of wire"---), but will be too small to have much effect at 1 MHz.