Variable frequency Colpitts oscillator

Discussion in 'The Projects Forum' started by GaryM, Nov 28, 2009.

  1. GaryM

    Thread Starter Member

    Nov 28, 2009
    37
    0
    Hello evryone,

    Im currently doing an electronics project and in this project I was given a colpitts oscillator which operates at 100Khz. But I need to modify this circuit so that it can give sine waves with variable frequencies. The inductor coil is constant and I can only change either of the 2 capacitors to get variable frequencies.

    So I decided to fix one of the capacitors at 150nF and replace the other capacitor with a Varactor or a variable capacitor. The capacitance of the varactor is changed by the use of a potentiometer as shown in the LTspice schematic attached.

    However, the circuit has stopped to oscillate now. Im not sure where i went wrong. Any suggestions are greatly appreciated :)
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    You need to save your file in .png or .gif format for other people to view it. I'm one of the graphical types around here, and was helping someone on another forum with a variation of the problem. Do you understand how a Colpitts inverts the signal?
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    It's an LTSpice file.

    You left D2 sitting all by it's lonesome way over on the right side. That causes a "Singular matrix" error when you try to run the simulation.

    Why don't you revert to an earlier version that you saved?
     
  4. GaryM

    Thread Starter Member

    Nov 28, 2009
    37
    0
    Hi all,

    I have attached a bmp of the schematic i am using. Sorry about the unconnected diode before..it was a mistake...

    I feel that there is something wrong with the way i connected the varactor. But im not sure what.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Your BMP was too large, so it didn't attach.

    .PNG format files are the best type to use.
    1) In LTSpice, press Ctrl+PrintScreen or Alt+PrintScreen to copy the current screen image to a buffer.
    2) Run MS Paint.
    3) Press Ctrl+V or select Edit/Paste to put the screen image into Paint.
    4) Crop the image to suit.
    5) Save as a .PNG type file.
    6) On the Board, click the "Go Advanced" button below the text box, then click the "Manage Attachments" button.
     
  6. GaryM

    Thread Starter Member

    Nov 28, 2009
    37
    0
    Sori.. Nw i have it in PNG format
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Have you tried returning the circuit to the point where it was working?

    We don't know what your original circuit looked like for certain.
     
  8. GaryM

    Thread Starter Member

    Nov 28, 2009
    37
    0
    Hi, I have attached my original circuit. I have kept the capacitor C1 fixed and I replaced the capacitor C3 with the varactor and the potentiometer that you can see in the variable frequency colpitts oscillator schematic that i posted previously.

    This circuit works fine and gives a sine wave of 45Khz but I am trying to get sine waves with different frequencies from this circuit and not just a single frequency and so i decided to use the varactor. But the circuit has stopped oscillating :(
     
  9. Wendy

    Moderator

    Mar 24, 2008
    20,766
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    Hmmm, I do not believe that can be classed as a Colpitts. A Colpitts depends on the feed back being inverted using the principal shown below, then feeding into an inverting amplifier such as a common emitter. Could be wrong, but that is my interpretation of how a Colpitts works.

    [​IMG]
     
  10. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,040
    287
    Hi Gary:

    Probably the best info on VFO design is to be found in older ARRL Handbooks, from the 50s and 60s.

    One thing to be careful of with a Colpitts is that the feedback is proportional to the RATIO of the two capacitors. Any successful design will alter BOTH capacitors so that the ratio between their values is always fixed. Any departure from this principle will result in an oscillator that quits at some frequencies, or has excessive feedback (and distortion) at others.

    The L/C ratio of a Colpitts is also important...and it's a hard choice to make if the oscillator is going to cover a large frequency range. Generally, you want a fairly low L/C ratio, especially if your oscillator uses a low impedance device like a BJT. A mosfet or vacuum tube has much less loading on the oscillator.

    This is one of the most frustrating and rewarding design projects in all of electronics!

    eric
     
  11. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    The Varicap version of the two schematics has a flaw. There should be a DC blocking cap there to isolate the DC Emitter voltage from the Varicap's cathode, along with its DC control voltage circuit. A 1000pF should work. Also, 1Meg for R5 seems a bit high too.
     
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