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oscillator help

Discussion in 'General Electronics Discussion' started by ericstanleysamar, Dec 13, 2015.

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  1. ericstanleysamar

    ericstanleysamar

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    Dec 13, 2015
    how to calculate values for oscillator circuit such as this? i need help for my project. i am very lost.
    step-by-step calculations needed, say, frequency will be 98Mhz, help

    ,
     

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  2. dorke

    dorke

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    Jun 20, 2015
    This is a C.B Colpittz oscillator,
    here
    you have what you need to calculate the values.

    But,
    that is the least of your worries... :rolleyes:
    For it to work ,you do need RF skills and equipment,
    Have you got any?
     
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  3. ericstanleysamar

    ericstanleysamar

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    Dec 13, 2015
    what do you mean skills, and equipment? what skills exactly is RF skills? and what equipment i need ? );
     
  4. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Eric, I believe you answered the question accurately.

    Chris
     
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  5. dorke

    dorke

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    Jun 20, 2015
    Eric,
    "what skills exactly are RF skills?"
    In short, something you acquire with " Blood,sweat and tears" :confused:
    over a long period of time.

    I don't intend to discourage you,but you should know what you are facing.
    If you have a "digital background",
    there would be a long way for you to enter the "RF world" and survive...


    "what equipment I need ?"

    For starters for that 98Mhz osc. analog circuit:

    1)100MHz (preferably 150MHz) scop and probe(s)
    2)100Mhz (preferably 150MHz) "sensitive frequency counter".
    3)Very "clean" power supply.
    4)RF quality Capacitor/Inductors "sets".
    5)Bare PCB for "dead-bug" assembly.
     
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  6. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    Besides what dork stated I'd like to add that one does not make their first oscillator 98MHz. One starts their RF journey far more conservatively .. like 1MHz or lower.

    Chris
     
  7. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    It's much easier to make a 98MHz osc because you can pick it up on FM.

    You really need a cap across the coil to make a reliable osc.

    See
    FM Transmitters and Spy Ccts for circuits
     
  8. duke37

    duke37

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    Jan 9, 2011
    The effective capacitor across th coil is the one from collector to emitter. It is not how I would do it.
    The components at VHF are never simple. Capacitors have inductance and inductors have capacitance. Wires have both.
    Meters are available to measure capacitance and inductance but these only measure at low frequencies so they could be very different at the operating frequency. Experience is needed to get a good result reasonably quickly. Six turns of a 6mm diameter coil and a 30pF capacitor may be somewhere near and could be checked with a grid dip meter. Connecting to a transistor will change things due to the added capacitance.
    To get a circuit on the right frequency, start with the calculations and experiment from there. All connections should be short, a 'bread board' is of little use, better to use the dead bug method where there is a ground plane handy.
     
  9. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    "The effective capacitor across the coil is the one from collector to emitter."
    Where do you get this from?????
    This is ENTIRELY INCORRECT.
    The cap from collector to emitter is the FEEDBACK capacitor.
     
  10. LvW

    LvW

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    Apr 12, 2014
    Is this a contradiction? Of course, there is a cap in parallel to the inductor.
    As a consequence, we have a tuned circuit, which determines the oscillator frequency.
    And a part of the voltage across the tank is fed back to the emitter.
    The transistor operates as a common-base amplifier - and, hence, we have positive feedback.
     
  11. BobK

    BobK

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    Jan 5, 2010
    The two capacitors are across the inductor with the battery in series.

    Bob
     
  12. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    "Of course, there is a cap in parallel to the inductor.
    As a consequence, we have a tuned circuit, which determines the oscillator frequency."
    You say this because you don't understand capacitor values.
    Once you insert capacitor values you will see how wrong you are.
     
  13. duke37

    duke37

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    Jan 9, 2011
    Colin, you are fond of CAPITALS.:)

    I attach a copy of the circuit with the capacitors labeled.
    The tuned circuit consists of L1 with C1, C2 and C3 in series connected across it.
    Typically, C1 and C3 will be 1nF and C2 may be 20pF so the total capacitance will be about 19pF.

    As I said this will be the effective capacitance and will determine the frequency.
    This configuration involves the HT line in the tuned circuit which is why I said that this is not the way I would do it.
    I would put a capacitor across the coil to determine the frequency and a separate feedback capacitor to the emitter with no C3.

    This is where some high frequency skills are required.
     

    Attached Files:

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  14. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    If you are saying C in the tank circuit is 20p, the frequency of oscillation will be higher than 200MHz.
    The feedback capacitor should be no more than 5p so your figures are way off suitability.
    Simply build you circuit and see it does not work.
     
  15. duke37

    duke37

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    Jan 9, 2011
    Colin
    Do you agree that C2 is the primary tuning capacitor?
    20pF is within the range of the tuning capacitors for VHF/FM radios. Just chose an inductance to suit.

    It is not my circuit so I have no intention of optimising the capacitors to try to get it to work. The HT line is live and will need to be fed through a choke or high value resistance In fact, I said it was not the way I would do it.

    Trevor
     
  16. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    The second problem with your circuit is this:
    The feedback capacitor is designed to turn the transistor OFF.
    Its effect is severely limited by pushing against 1,000p.
    What effect do you think 20p will have against 1,000p???
    A transistor can be turned ON by holding the emitter firm and driving the base.
    Or holding the base firm and driving the emitter.
    What is the point in holding both the base and emitter firm ????
     
    Last edited: Dec 16, 2015
  17. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    I'm loving this!
    Lord, please let this run for at least 4 pages.

    Your humble sinner,
    Chris
     
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  18. duke37

    duke37

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    Jan 9, 2011
    Careful Chris or I will set Martin on to you.:)

    The transistor DC conditions are set by the base voltage and the emitter resistance. Driving either the base or the emitter via a capacitor will vary the collector current to give it an AC component. The phase will differ in the two methods of drive. To suggest that one method of drive turns the transistor on and the other turns it off is incorrect, both methods do the same thing.

    The tuned circuit will have a Q value of maybe 100, in other words, the circulating current will be much higher than the input current, this will be built up over several cycles. The output from the resonant circuit can be obtained by tapping the inductance or capacitance to get a low output impedance i.e.low voltage, high current. This is used to drive the emitter of the transistor and the same AC current is produced at high impedance at the collector. The trick is to get a positive loop gain to maintain the oscillation.

    With a certain current through the tuned circuit and a series combination of 20p and 1000p, then the voltage across the 1n will be one fiftieth of that across the 20p. The transistor will need to amplify this voltage sufficiently to maintain oscillation. The emitter is not held 'firm', the capacitor is not infinite. Getting the components right can be difficult at VHF since the components are far from ideal with inductors having capacitance and capacitors having inductance and connections having both. There is quite an art in getting a VHF circuit to work as intended. I do not think you would have much of a chance using a 'breadboard', you could use the dead bug or Manhatten style with minimum lead length. A PCB is helps to give consistent results.

    Trevor
     
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  19. LvW

    LvW

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    Apr 12, 2014
    I think, it is not appropriate for this circuit to think in "ON" and "OFF" terms. The circuit produces a sinusoidal output!
     
  20. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    The collapsing magnetic field of the inductor makes the collector-end of the coil more positive and this is transferred to the emitter to turn the transistor OFF.
    Where is the capacitor across the coil that sets the rate of collapse?
     
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