Colpitts Oscillator Problem: Frequency won't stay put

Discussion in 'Wireless & RF Design' started by 64C113M Abe., Sep 13, 2016.

  1. 64C113M Abe.

    Thread Starter New Member

    Sep 13, 2016
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    Hello all,

    I have successfully made a shortwave transmitter using a crystal oscillator, now I want to try making a transmitter using a colpitts oscillator.

    I made a video of the problem, but screwed up editing the video. You can still hear the carrier wave moving across the frequency.



    The circuit I used was from this website:
    http://www.electronics-tutorials.ws/oscillator/colpitts.html

    What I first thought was the problem was that supply voltage was fluctuating, so I ran it through a 7805 voltage regulator, problem was still there.

    I suspecting that the RF generated by the circuit is feeding back into the circuit, but I have no way of knowing this.
     
  2. 64C113M Abe.

    Thread Starter New Member

    Sep 13, 2016
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    Well as an update I have noticed a change in stability when I use different transistors but it still is suffering from frequency "hunting"

    I'm thinking about putting in a constant current source at the emitter, but I feel this might not have any effect.
     
  3. SLK001

    Well-Known Member

    Nov 29, 2011
    810
    224
    Maybe the frequency is being pulled by the transmitter. You need to isolate the outputs of oscillators from everything, as everything can and will detune your tuned circuits. I don't see any schematics, but maybe you need a buffer amp b/t the osc and xmitter.
     
  4. upand_at_them

    Active Member

    May 15, 2010
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    Perhaps add a second stage to isolate and drive the antenna.
     
  5. BR-549

    Well-Known Member

    Sep 22, 2013
    1,981
    388
    Colpitts frequency drift.


    "
    • 1. Isolate the oscillator from succeeding stages with a well designed buffer stage followed by a stage of amplification. Large signals can often then be reduced by a 3 or 6 dB attenuator which also has the benefit of presenting a well defined load impedance to the amplifier. If the stage is feeding a mixer, as is most often the case, then another benefit is the mixer (you are using double balanced mixers?), also see a source impedance of 50 ohms.

    • 2. Ensure the mechanical stability of your oscillator is such that mechanical vibration can have no effect on components, especially those frequency determining components.

    • 3. Supply the oscillator with a clean well regulated supply. If using varactor tuning, doubly ensure the tuning DC voltage is as clean as possible, a few hundred micro volts of noise can be imposed on the oscillator signal. Use back to back diodes for the variable element. Air variables are hard to come by although they offer far superior Q figures. DC tuning tends to be more versatile.

    • 4. Minimize circuit changes from ambient variations by using NPO capacitors, polystyrene are dearer but excellent, silvered mica in my opinion are not what many people believe and are highly over rated.

    • 5. The inductor should be air wound on a coil form with a configuration to maximize Qu. If you must use a toroid, where possible try to use the 6 type as it offers the best Q. Sometimes, for other reasons you might have to use a slug tuned form.

    • 6. Parallel a number of smaller value NPO capacitors rather than using one large one in frequency determining components. For trimmers try and use an air variable. Keep an eye out for small value N750, N1500 capacitors, < 15 pF, when available and are found to be dirt cheap. These are sometimes useful in taming drift in an oscillator.

    • 7. Bipolar or FETS for active device seems to be a matter of personal preference and I've seen some ferocious arguments over that one. Consensus seems to come down in favour of FETS. Me, I'm a bipolar man because FETS hate me pure and simple.
    "
    I would recommend an Hartley with a fet.
     
    DickCappels likes this.
  6. RichardO

    Well-Known Member

    May 4, 2013
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    I would add an item 0 to the @BR-549 list:
    0. Bypass, bypass, bypass. Use both large and small value ceramic capacitors for power supply bypassing. Something like 0.1 uf, 1000 pf and 100 pf all in parallel. The smallest cap should be closest to the oscillator circuit.
     
  7. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
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    Quite a large proportion of radio hams have websites, and you can usually find those by googling their call letters.

    Its a very valuable resource that's well worth tapping into.

    You can collect callsigns to try from various ham radio books and magazines - there's a pretty good stack on americanradiohistory.com

    There shouldn't be much difficulty finding articles on VFO design.
     
  8. bertus

    Administrator

    Apr 5, 2008
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  9. ian field

    Distinguished Member

    Oct 27, 2012
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  10. bertus

    Administrator

    Apr 5, 2008
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    DickCappels likes this.
  11. radiohead

    Active Member

    May 28, 2009
    474
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    The Colpitts Oscillator is a poor choice for frequency stability in my opinion. Can you post a schematic of the video transmitter you intend to use? It will make circuit analytics easier for those of us trying to help you.
     
  12. 64C113M Abe.

    Thread Starter New Member

    Sep 13, 2016
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    0
    Thank you all for your replies, I have done some research on my own and it lead me to a video series by Stefan0719



    It was made clear stated that you use a buffer to get frequency stability.

    As it stands for now I've dropped all work for on the colpitts oscillator in favor of crystal oscillators for my radio transmitter projects.

    Though in the future I would like to have something that has a variable frequency, the end goal is be able to build a homebrew VHF/UHF transmitter.
     
  13. upand_at_them

    Active Member

    May 15, 2010
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    I'd like to point out that just because the Colpitts is less stable than a crystal doesn't mean you shouldn't use or experiment with it. There's much to be learned from all kinds of circuits.
     
  14. ian field

    Distinguished Member

    Oct 27, 2012
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    Indeed - you can learn a lot about "flogging a dead horse" by choosing the wrong oscillator for an application.
     
  15. radiohead

    Active Member

    May 28, 2009
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    Indeed it would be wise to study the different types of oscillators (with and without crystals) in order to make an educated decision on the right type of oscillator to use for a given application... data, voice, etc
     
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