Colpitts Oscillator on Breadboard help

Discussion in 'The Projects Forum' started by BLOBY, Mar 10, 2011.

  1. BLOBY

    Thread Starter Member

    Mar 10, 2011
    37
    1
    Hi,

    I tried this circuit on NI Multisim which is of a colpitts oscillator and the circuit oscillated..:)
    http://i53.tinypic.com/vpdgck.jpg

    I used similar circuit on breadboard and it doesnt seem to oscillate...

    I know that in this circuit the transistor is not well biased...

    I used ceramic capacitors for C1 and C2 (the tank circuit caps) of same value..

    I used a coil of 60 turns for the inductor L1 ( I used a wire with plastic coating and wound the coil.. I hope its allright.. and I calculated the value of inductance from Wheelers formula.. and also tried some other inductance calculating softwares and got it as 30uH)..

    I didnt have an oscilloscope so what I did was to connect a capacitor(0.47 uF) and a diode(silicon type) in series at the emitter terminal and tried to measure the dc voltage part of oscillations(hope im right.. ) but the multimeter didnt show any readings...

    Plz tell me in which area i could improve so that this circuit oscillates.. if this circuit oscillates, I will bias it properly next time with voltage divider bias method...

    I dont care much about the frequency of oscillations... I just want it to oscillate.. so also tell me how to detect the oscillations.. Also tell me whether the feedback of my circuit is proper or not..

    I know it is a big question posted by me.. thanks for help in advance.. :confused:
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    I calculate the resonant freq of your oscillator tank circuit as 178Khz. Protoboards don't handle high frequencies well, DVMs even less so. You need to bump that frequency way down if you want to experiment with primitive equipment.

    Have you ever checked into the sound card oscilloscope? It is in the AAC book, Volume 6. Be careful though, you can do damage to the sound card with high signal levels.

    PC Oscilloscope
     
  3. BLOBY

    Thread Starter Member

    Mar 10, 2011
    37
    1
    So cool.. I got to try it.. I got some questions though..

    Does voltage around 9 volts damage the sound card (Pc oscilloscope)

    and related to my oscillator... is the design good? should i add a capacitor near feedback path? and also is my self made inductor (with plasitic covering having 60 turns) good?
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    It looks like it might work, I don't know for sure.

    9V probably would damage the sound card. I'd put a couple of diodes back to back to clamp the signal. I have a project drawn up I've never finished...

    PC Oscope

    Basically the roll off for the PCB scope is around 15Khz, I'd think 10Khz. By being so low you will miss a lot of detail, but it is better than nothing.
     
  5. BLOBY

    Thread Starter Member

    Mar 10, 2011
    37
    1
    Oh.. u mean the frequency range of PC oscilloscope is about 15kHz?
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    Yep.......
     
  7. BLOBY

    Thread Starter Member

    Mar 10, 2011
    37
    1
    Thank you :)...
    Will try to implement that PC oscilloscope
    I want this oscillator to be used as a radio transmitter (after checking and improving this design)..
    I will work on this project and will post my progress...
     
  8. BLOBY

    Thread Starter Member

    Mar 10, 2011
    37
    1
    Ok.. I slightly modified my circuit near output part by replacing a diode(some random silicon diode) so that I could measure the DC part of the oscillating signal.. (the pic is in the attachment)..

    While simulating in multisim, I get a value of about 2.7 volts and in my breadboard circuit, the value is 1.5 volts....

    Does this mean that my circuit is oscillating? because I removed my (so called) 30uH inductor and replaced it with just a wire and the multimeter reading reduced to zero volts...

    Plz tell me if this diode experiment proves the existence of oscillations? :)

    I used the 2N2222 transistor which has a hfe of 140(in my breadboard circuit).. and I got the idea of this diode experiment from "RF probe" from this link.. http://www.io.com/~n5fc/rfprobe1.htm
     
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