Getting the wrong freq....

Discussion in 'Wireless & RF Design' started by MilK, Mar 7, 2010.

  1. MilK

    Thread Starter Member

    Mar 1, 2008

    Would like some help...
    i am building an osc for Experimental purpose. For my design, i have use a colpitts oscillator.

    My required freq is 700~800khz, but my measure freq is only about 800hz~3khz.

    I've use the following components:

    Cvdd = 10uF
    CB = 1nF
    R1 = 21k ohm
    R2 = 12k ohm
    RE1 = 1-10k ohm Potentiometer
    RE2 = 470 ohm
    L1 = 1.5uH
    C1 = 0.1uF
    C2 = 33nF
    Q1 = 2N2222A Bipolar Transistor

    Voltage measure at Base of Q1 is 4.4v, at Collector is 3.8v. Therefore the transistor is working.

    Does anyone know why i am getting the wrong freq?
    I've build it on a breadboard. Some practical issue that i have overlooked?

    i've tried changing the L1,C1, C2 but it seems to get the same result.
    Anyone have any clue?

    Thx in adv.
  2. bertus


    Apr 5, 2008
  3. MilK

    Thread Starter Member

    Mar 1, 2008
    Yap... can't seems to find any link to my problem..
    i am using the same eqn listed on the page to find my freq.
  4. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    The top of L1 should be connected back to the bottom of C2 via something like a 1uF ceramic or plastic film cap.

    As shown, the main tuned circuit, whic consists of L1 in parallel with C1+C2, is 'broken' by having the electrolytic decoupling cap in series with it.

    Electrolytics are great for bulk power storage and audio stuff, but useless at RF.
    You need to bypass the power circuit with a cap that has very low impedance at the frequency you are working with.

    Keep all connections absolutely as short as possible!
    MilK likes this.