LC oscillator component values

Discussion in 'General Electronics Chat' started by froten_140, Jan 20, 2010.

  1. froten_140

    Thread Starter Active Member

    Jun 4, 2006
    hello all,

    when designing LC oscillators, is there a rule of thump on how to select the L & C values? How I can select these values for a specific frquency?

    for example:

    440pF & 0.5UH will reasonate at 10.7MHz
    220pF & 1UH will reasonate at 10.7MHz
    110pF & 2UH will reasonate at 10.7MHz
    55pF & 4UH will reasonate at 10.7MHz

    so which values are the best?

    any help is very appreciated
    Last edited: Jan 20, 2010
  2. hgmjr

    Retired Moderator

    Jan 28, 2005
    One thing to consider in chosing the LC combination is that the circuitry used to implement the oscillator will have inherent capacitance as well as inductance. It would therefore be prudent to use a combination in which neither the capacitor nor the inductor are small in value. That way the circuit is likely to operate more consistently with changes in the various components over time and temperature since time and temperature are likely to change these inherent inductances and capacitances.

  3. SgtWookie


    Jul 17, 2007
    If you want to get a decent Q in the inductor, you will have to chose your toroid material carefully.

    If you use an Amidon T22-6 toroid with 14 turns of AWG28 magnet wire wound as evenly as possible, you will have an inductor with a Q of about 170 and around 1uH; parasitic capacitance will be pretty low. You can adjust the inductance somewhat higher by squeezing the windings together somewhat, but this reduces the Q of the inductor.

    The T22 part relates to the dimensions of the toroid. The -6 is the type of material.
    That particular toroid can be ordered from this page:
  4. KL7AJ

    AAC Fanatic!

    Nov 4, 2008

    As you have discovered, there are an infinite number of L/C combinations that can give you any given resonant frequency. The issue when designing oscillators is the loaded Q, which is greatly dependent on the load presented to the tank circuit. Each oscillator type is different...sometimes radically so.

    The best primer on the whole matter is older versions of the ARRL Handbook. (pre-1980 or so).