Find LC by frequency

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

  1. Synaps3

    Thread Starter Member

    Jun 5, 2013
    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
    Something like this:

    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


    Feb 24, 2006
    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


    Nov 30, 2010
    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.
  5. t_n_k

    AAC Fanatic!

    Mar 6, 2009
  6. vk6zgo

    Active Member

    Jul 21, 2012
    Minimal at 1MHz,though!
  7. vk6zgo

    Active Member

    Jul 21, 2012
    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.