Weird Colpitts readings

Discussion in 'General Electronics Chat' started by Shagas, Jun 26, 2014.

  1. Shagas

    Thread Starter Active Member

    May 13, 2013
    802
    74
    Hello

    So I've put together a Colpitts oscillator to try out my new scope at higher frequencies and I've come across something odd.

    I'm powering the circuit with +- 9V and for some reason the scope measures
    about 42 Volts pk-pk across ground and the collector (The attenuations are 10x probe and 10x software) . First I thought that my chinese scope was out of its mind but after plugging in a resistor-diode-capacitor through the collector/ground I confirmed with my DMM that there is about 27 volts across the cap so the scope can't by lying (right?).
    By putting a metal object through the coil at variable distances (to vary the inductance) of the coil I found at that certain frequencies the collector - gnd voltage even goes up to 48 V pk-pk.

    Can someone explain what is going on?

    Enclosed are some pictures.

    I adjusted the cap values and inductor to give me about 21Mhz
    Sometimes when I attach the probe to the collector my speaker (it's about 40 cm away) starts whining.
    Is the interference bad for the speaker? It only seems to buzz at a certain frequency so I'm guessing it's going to be something with resonance.

    LC-Colpitts-osc-01.gif
    27DMM.jpg
    42V.jpg

    *Ps , I know I've misspelled the word 'Weird'
     
    Last edited: Jun 26, 2014
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    the higher rf voltages on the collector are due to resonance of the lc tank in the collector lead. normal.
     
  3. Shagas

    Thread Starter Active Member

    May 13, 2013
    802
    74
    Yes I guessed that much , but could you care to explain in abit more detail how that happens? Is it from the collapses of the inductor field that the higher voltages come to be ?
     
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    study dthe effects of a paralell resonant circuit. the impedance goes way up at resonance.
     
    Shagas likes this.
  5. dougc314

    Member

    Dec 20, 2013
    38
    11
    You have implemented a tuned step up auto transformer, with capacitive taps, (C2 and C3) The step up ratio is C3/C2. Thats because the transformer ratio is the impedance to ground to the ratio of the impedance back to the collector, which is (1/2/pi/F/C3)/(1/2/pi/F/C2) and simplifies to C3/C2. Running the oscillator off of +/- 9 volts allows for a large emitter swing, which gets stepped up. Eventually either the base-emmiter gets cutoff, or Vce goes to zero and that limits how big the voltage gets.

    Bringing the metal close must be changing the feedback ratio, but its not clear to me how.

    As for the speaker I can only assume that its connected to a audio source. If so, then whats happening then the RF is getting into the active electronics and in someway creating an audible tone. I don't think simple rectification would do it.
     
  6. Shagas

    Thread Starter Active Member

    May 13, 2013
    802
    74
    Thanks for the answers. Yes the speaker Is connected to an audio source.
    I'll check out what you said about the auto transformer
     
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