Can't get MRF136 to oscillate

Discussion in 'General Electronics Chat' started by Spottymaldoon, Dec 4, 2015.

  1. Spottymaldoon

    Thread Starter Member

    Dec 4, 2015
    51
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    I'm trying to persuade the attached circuit to oscillate (ultimately at 100MHz but any frequency will do for now).
    https://drive.google.com/file/d/0B1ZlKa1Knz2cQTA0TUlxMEpKQWc/view?usp=sharing
    The MRF136 is supposed to be operating as a Colpitts oscillator but everything is just DC - I am applying a drain voltage of 20 and a bias of 5-7 volts to the gate and I'm measuring 150mA going through the device but however I tune the two capacitors, no oscillation.

    Advice please!
     
    Last edited: Dec 4, 2015
  2. RichardO

    Well-Known Member

    May 4, 2013
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    What does your circuit layout look like? At these frequencies it can make a _lot_ of difference.
     
  3. Spottymaldoon

    Thread Starter Member

    Dec 4, 2015
    51
    1
  4. DickCappels

    Moderator

    Aug 21, 2008
    2,656
    632
    That is a beautiful layout and printed circuit board.

    It appears as though you are hoping an electrolytic capacitor will suffice as a bypass capacitor. Not a good idea because the equivalent series inductance in the hundreds of MHz makes electrolytics "invisible". I don't know whether that is what is keeping your oscillator from oscillating, but putting a good quality ceramic capacitor or two across the electrolytic will assure that you really do have a decoupling capacitor.
     
  5. bertus

    Administrator

    Apr 5, 2008
    15,648
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    Hello,

    As Dick said, the electrolitic capacitor will not help in the place you put it.
    There is a long line between C7 and C9, wich will act as an inductor.
    Also you did not have the intended inductor placed at C9, assuming TP6 is the power point.
    A couple of ceramic capacitors near C7 will help to decouple the circuit.

    Have a look at the following PDF for RF prototyping:
    rf_proto-1.pdf

    Bertus
     
  6. Spottymaldoon

    Thread Starter Member

    Dec 4, 2015
    51
    1
    Thanks for the kind words and very sensible advice which I have immediately implemented - please see revised arrangement with "your" capacitor inserted at ">":

    https://drive.google.com/file/d/0B1ZlKa1Knz2cdWtJUFFGRlFVQ0k/view?usp=sharing

    I also moved the two trimmers to the reverse of the board and cleaned away as much flux as I could.

    Unfortunately, there is still no oscillation ...

    Thanks for responding Bertus - I made the changes suggested by Dick and also cleaned up the board arrangement somewhat. I shall leave no stone un-turned and your advice is appreciated - I will go and get the choke you pointed out as missing and try again. Thanks also for the documentation - that is very specific and to-the-point for me and for this project - I am reading it.

    My ideal frequency here is 100MHz but right now I'll take anything except DC!
     
    Last edited by a moderator: Dec 5, 2015
  7. Spottymaldoon

    Thread Starter Member

    Dec 4, 2015
    51
    1
    Inductor now in place but still nothing. Very puzzling!
     
  8. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    I am getting confused with the partnumbering in the schematic and PCB.
    The partnumbering does NOT match.

    Also the soldering on the transistor tabs is very little on your PCB picture.
    Have a look at this PCB how the RF transistor is soldered:

    20082010331_1024x768.jpg

    Bertus
     
  9. Spottymaldoon

    Thread Starter Member

    Dec 4, 2015
    51
    1
  10. Spottymaldoon

    Thread Starter Member

    Dec 4, 2015
    51
    1
    Sure, I used only a little solder because that is an expensive transistor and I might have to junk the circuit! If the circuit works I shall not spare the solder, just like that other fella.
     
  11. Spottymaldoon

    Thread Starter Member

    Dec 4, 2015
    51
    1
    I discovered two things. (1) my inductor is around 6nH which, with a maximum capacitance of perhaps 10pF will give a frequency that is around 650MHz (way higher than I want) - I miscalculated the inductance and have now replaced this with a 330nH inductance. However, still no oscillation.

    And (2) I took a look at the transistor.

    While there is almost a linear variation of Isd with Vgs (Vds = 20V) with a cutoff at Vgs=4V and a maximum of 205mA for Vgs=8V - there is virtually no change of current with Vds. In other words, if I set Vgs=8V, Isd remains almost constant at 200mA for all Vds >10V, regardless of the voltage applied to the rail.

    I have treated this expensive transistor with kid gloves but, unless I am doing something really dumb (which is all too possible) I believe I have a bad transistor here.

    I'd be grateful for any confirmation - or otherwise.
     
  12. RichardO

    Well-Known Member

    May 4, 2013
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    The MRF136 has a huge amount of capacitance. It may be fine for a power amp where it is driven by a low impedance source but you will have a hard time making a stable oscillator in the 100 MHz range. The transistor's capacitance will overwhelm the capacitance in the tuned circuit.

    upload_2015-12-8_14-35-6.png
     
  13. Spottymaldoon

    Thread Starter Member

    Dec 4, 2015
    51
    1
    RichardO said:
    The MRF136 has a huge amount of capacitance. It may be fine for a power amp where it is driven by a low impedance source but you will have a hard time making a stable oscillator in the 100 MHz range. The transistor's capacitance will overwhelm the capacitance in the tuned circuit.

    Richard - thanks for responding. Your explanation fits what I was seeing and I have accordingly dumped that idea. Since the demise of the 2N3375 I haven't been able to find an equivalent and so lashed out to try a power FET.
     
    Last edited by a moderator: Dec 18, 2015
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