Colpitts oscillator !!! frequency mistake ?

Discussion in 'General Electronics Chat' started by majd-ghadab, Jul 11, 2013.

  1. majd-ghadab

    Thread Starter New Member

    Dec 25, 2012
    12
    0
    Hello everyone I have faced very strange thing that calpp oscillator (Colpitts oscillator) have different frequency from formulas and big different !
    I have used 100uH inductor which I measured its value 100% with inductance meter (and I bought it not made it)
    Any way the idea is I always got a frequency about 2.6 MHz and smaller depending on capacitors values , with tow 39pF capacitors and 100uH inductor I must get around 3.6 MHz but I get 2.6 MHz with any capacitors values even with 2pF in other words I have removed the tow capacitors from the oscillator and the circuit still oscillating with 2.6 MHz !!!!!!!!
    I can get higher frequency with different inductor only !
    You can see real pictures circuit with oscilloscope and the simulated on multisim witch its results is very close to theoretical formulas.
     
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  2. ramancini8

    Member

    Jul 18, 2012
    442
    118
    I haven't looked at your schematic, but my opinion is that the circuit is oscillating on a stray feedback path rather than the feedback path you intended.
     
  3. majd-ghadab

    Thread Starter New Member

    Dec 25, 2012
    12
    0
    i didn't precisely understand what do you mean but how to solve the problem
     
  4. w2aew

    Member

    Jan 3, 2012
    219
    64
    That inductor looks like a RF choke, and thus probably has a fair amount of parasitic parallel capacitance on its own, and is probably the main cause of the frequency shift. Also, I don't think you need the 2.2nF cap in series with the inductor - short it out.
     
    majd-ghadab likes this.
  5. majd-ghadab

    Thread Starter New Member

    Dec 25, 2012
    12
    0
    w2aew you are right i have not seen such thing named "parasitic parallel capacitance" i have changed the inductor type and the frequency is changing like magic so it must be the reason
    i must read more ,by the way i was broken cause i have completely lost a day without find out what is the problem and now i will go to sleep with full comfort
    thank you 1
     
  6. Shagas

    Active Member

    May 13, 2013
    802
    74
    A true future engineer :D
     
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