Optimal Tank (LC) Circuit

Discussion in 'The Projects Forum' started by enahs, Aug 11, 2014.

  1. enahs

    Thread Starter New Member

    Aug 11, 2014

    I need to design and build a tank (LC) circuit. Reading around the internet, and basing it off of previous ones I have, it is fairly easy.

    I however, really want to optimize the efficiency of the tank circuit for amplifying a specific frequency, specifically 8 MHz. I do want a fairly high Q (narrow frequency of amplification).

    When shopping for inductors at digi-key, there are specs I do not quite understand, such as "frequency self resonant". Do I want to use an inductor that is self resonant at 8 MHz?

    I want to send a decent amount of current through it, so should I just go with a inductor that has high capacity, or is excess capacity going to negatively impact the performance of the circuit?

    Is there in anyway I should account for the impedance of the device I will be coupling the tank circuit to, which might have an effect?
    KL7AJ likes this.
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    With air-core inductors you do not have to worry about core saturation due to high currents; size the wire based on current-carrying capacity. Here is a calculator for winding your own coil.

    The self-resonant frequency should be a factor of five to ten times higher than 8Mhz.

    Usually, if this is a tank circuit, then it would be configured as a Pi section low-pass filter which also matches the output impedance of the device to the load. The loaded Q of such a circuit is usually not more than 10, while the Q of the just the inductor by itself might be several hundred.

    There are many on-line calculators that show how to configure an output matching network...
    Last edited: Aug 11, 2014
  3. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    The ARRL Handbook has tons of good information on tank circuit design. Here are a few hints, however.

    1) You want to avoid any self-resonance point like the plague! These suck up huge amounts of energy.

    2) The L/C ratio is HIGHLY dependent on both the source and load impedance. HOWEVER, you're usually better off erring on the side of high-C, Low-L when used as a parallel tank. This is because the LOAD resistance appears as a parallel resistance, thus lowering the LOADED Q.

    3) There is a large difference between loaded and unloaded Q. The greater the ratio between unloaded and unloaded Q, the more efficient the circuit will be. Unloaded Q is determined by internal CIRCULATING resistance, while loaded Q is the "external" resistance.

  4. KL7AJ

    AAC Fanatic!

    Nov 4, 2008