Oscillator Startup Issues

Discussion in 'The Projects Forum' started by ChateauduChillon, Nov 5, 2014.

  1. ChateauduChillon

    Thread Starter New Member

    Jan 28, 2014
    24
    0
    Hi All,

    I'm having some issues with an oscillator startup. I'm using a 2 op-amp quadrature oscillator like this one:
    http://www.changpuak.ch/electronics/images/Oscillators-1-4.png

    oscillation frequency is around 30kHz.

    For R3 on the diagram I have a JFET controlled variable resistance, where JFET gate is driven by an integral controller. the controller has the output of a peak detector which is measuring the peak of one output and a reference peak level to compare it to.

    If I make the time constant of the peak detector low enough that the peak detector is discharging significantly per oscillation period, and the controller time constant much slower, the circuit starts up just fine and oscillates stably. However, when the peak detector output changes this much on a per period basis, the system doesn't correctly regulate the oscillator amplitude as it drifts over temperature, as the controller essentially responds to the average of the peak detector output as it sawtooths up and down. I need to slow down the response of the peak detector such that it is constant over many periods of oscillation in order for this behavior to be fixed. However, no matter what I do, I can't seem to get the oscillator to startup on powerup correctly if i try to slow down the peak detector response enough. When I slow down the peak detector like this, the circuit will oscillate between dead and sitting on the rails at low frequency (10-100s Hz range), no matter how slow I make the controller response.

    I've tried a few ways (adding different resistor networks to/around jfet load) with no success.

    Any advice is appreciated!
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,009
    3,233
    Post your complete schematic including the JFet controller.
    Hard to give good advice without seeing the circuit. ;)
     
  3. ChateauduChillon

    Thread Starter New Member

    Jan 28, 2014
    24
    0
    will do.

    Also, I have alleviated the startup issues with a weird modification, which I will include in the schematic. But 2nd harmonic distortion still plagues me.
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,009
    3,233
    How much distortion can you tolerate?
     
  5. ChateauduChillon

    Thread Starter New Member

    Jan 28, 2014
    24
    0
    In my original configuration, I had the second harmonic down from the principal by 55dBu. This was potentially tolerable but still a bit high. With my new configuration it is up to 45dBu down. My goal is 60dB down solidly. 3rd harmonic and on aren't much of an issue for my application, just the second.

    I guess I should change the title of the thread now, to managing 2nd harmonic distortion in JFET controlled oscillators :)
     
  6. ChateauduChillon

    Thread Starter New Member

    Jan 28, 2014
    24
    0
    a little tweaking and I've gotten it down to 50... still not good enough!

    right now my R3 consists of 3.9k -- JFET -- 3.9k series combination with a 680 ohm resistor across the JFET . . . tried linearization resistors, which destabilize my oscillator :(
     
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