Wein bridge with varying frequency

Discussion in 'The Projects Forum' started by akis02, Aug 4, 2014.

  1. akis02

    Thread Starter Member

    Apr 30, 2011
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    I have a wein bridge which works very well. I want to vary its frequency based on a square pulse every 1.5 seconds generated by an 555 (or perhaps an MCU).

    The way to vary its frequency is to modify the "R" of the RC or the "C" of the RC.

    But it has to be said, in order for it to oscillate at all, the two RCs must be almost perfect matches, even better than 1%. I actually pull components out of the draw and measure them and pair them, else it will not oscillate.

    So we cannot willy-nilly mess with the Rs or the Cs and expect it to work.

    To modify the "R" I thought of an LDR, actually two LDRs driven by the same LED. However LDRs have a varying resistance of many MOhm down to a few hundreds Ohms. I think it would be impossible to attempt to use two LDRs and expect them to either stay within a sensible range or to track each other to 1% or better. But I have a ton of LDRs so I may just try it.

    Another idea was to mess with the Cs. For that the only thing I found was "varicaps" or "varactors". So I have bought a few to try out. Never used them before.

    I am looking for ideas. Here is a schematic of the Wein bridge oscillator as it is and with the varactors in place which works on simulation. Simulation of course has perfect components so it does not prove anything.
     
  2. BobTPH

    Active Member

    Jun 5, 2013
    785
    114
    How much does the frequency need to vary? A Wien bridge is not the best circuit for variable frequency. A phase shift oscillator would allow you to vary it by varying a single resistor, over a range of maybe 2 or 3 to 1.

    Bob
     
  3. akis02

    Thread Starter Member

    Apr 30, 2011
    57
    3
    +/- 10% that 's all.

    A reason I use the Wein-bridge oscillator as in the schematic is its stability wtr temperature, the amplitude is really very stable.

    I have never made a phase shift oscillator before, is that the one where you have 3 RCs in series something or other?
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Hewlett and Packard had no trouble varying a Wein Bridge Oscillator (HP200A) over a 10:1 frequency range...

    A Wein Bridge is a phase shift oscillator. It oscillates at the one frequency where the phase shift through the network is zero...
     
  5. AlphaDesign888

    New Member

    Jul 27, 2014
    189
    10
    I thought an wein bridge had to have an grain of wheat bulb in the feedback path?

    The schematics posted look all over the place.
     
  6. akis02

    Thread Starter Member

    Apr 30, 2011
    57
    3
    @AlphaDesign88
    The wein bridge needs an amplification just over 3 to be able to oscillate, but as soon as it does it then hits the rails so you get a square wave rather than a sine. It also depends on the capabilities of the op-amp when it hits the rails how it will behave after.

    But the idea is to limit it somehow so it does not hit the rails. The original design used a temperature positive coefficient resistor (aka light bulb) so that as the signal output increased so did the current in the bulb, the heat, the resistance increased and this applied the brakes and in the end it all equalised nicely without clipping.

    In my design, it is not at all "all over the place", actually it is extremely sophisticated and as simple as possible. Not only I keep the wave form from clipping, but I also have tremendous temperature stability - my usual test is to blow a hair dryer on it until it reaches 80C and on the way there the waveform remains constant. I challenge anyone to produce such temperature stability from so few components.


    @MikeML: yes but he used a dual gang, variable air capacitor. I want to use a voltage if possible.


    Varactors arrived just now. I will try to test it on the breadboard somehow. I doubt it will work.
     
  7. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,348
    Hello,

    Perhaps a strange idea, but why not put a switched large capacitor parallel to the small one.
    You could swtich it with a PWM signal using a bilateral switch like the 4066.
    The PWM frequency should be 50 to 100 times higher as the highest wanted frequency.

    Bertus
     
    Last edited: Aug 5, 2014
  8. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    look up vco ( voltage controlled oscilator) designs.
     
  9. BobTPH

    Active Member

    Jun 5, 2013
    785
    114
    Or better yet, use DDS.

    Bob
     
  10. akis02

    Thread Starter Member

    Apr 30, 2011
    57
    3
    The varicaps have arrived but they are smaller than 0.5 mm in size, and I cannot use them. 40 new varicaps to a good home to whoever wants.

    I am now experimenting with a jFET VCR in parallel with the Rs of the RC of the wein bridge. Works on simulation at least.
     
  11. RichardO

    Well-Known Member

    May 4, 2013
    1,238
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    I would love to have the varicaps for an RF generator I am designing. Unfortunately, I am a wee bit more than a bicycle ride away. :D

    About using J-FET's in the Wein bridge oscillator. How are you going to deal with the FET that uses the floating power supply?

    If you still wanted to try to use those varicaps then you should reconsider the phase shift oscillator. The big advantage of the phase shift oscillator is that all of the all of the timing caps are connected to ground so it is easy to bias the varicaps.

    Another possibility is to use an oscillator that uses integrators, such as a quadrature oscillator. The integrator resistor is referenced to ground -- actually virtual ground -- which makes driving the FET fairly easy.

    Thinking of driving FET's reminded me of The National Semiconductor Linear Applications Handbook. It has a huge number of circuits, many of them using J-FET's. I would wager there is an example of just what you need.

    I found a PDF of the 1994 edition on the Web. Be warned that it is 60 MB in size:
    https://archive.org/details/NationalSemiconductorLinearApplicationsHandbook1994

    I downloaded the PDF version but it is a scan so it can't be searched. Maybe one of the text versions would be searchable...
     
  12. akis02

    Thread Starter Member

    Apr 30, 2011
    57
    3
    Cannot use the varicaps under any conditions since they are smaller than 0.5mm in size. I tried to solder lugs on one and it was impossible, the surface area is so small there is almost no mechanical strength on the solder joint and the lugs fall off.

    I have implemented floating voltage by means of large gate resistors. Here is the schematic of the simulation, which works. But, yes let's not hold our breath for simulation, I need to breadboard it fist (soon).

    The advantages of the Wein bridge which I have designed as in the first schematic are:
    1) extremely steady amplitude wtr supply rails
    2) extremely steady amplitude wrt temperature
    3) very good frequency stability wrt rail supply and temperature
    4) sine wave output

    There is distortion whenever we use a jFET VCR unlike, say, using an LDR. I have a jFET matcher and hope to match the jFETs. Maybe I am dreaming. When I tried the LDRs they were non-linearly related, that means they had different curves of LED drive vs resistance. If I can match the LDRs they would be much preferable to jFETs due to 0% distortion added.
     
  13. RichardO

    Well-Known Member

    May 4, 2013
    1,238
    385
    Just a thought... Have you considered opto-couplers using FET's? Sorry, a part number does not come to mind.
     
  14. akis02

    Thread Starter Member

    Apr 30, 2011
    57
    3
    The problem here is matching. A quick search on the net shows this is a 50 year old problem or older, seeking to make a VCR which is (a) linear (b) easy to match (c) floating (d) bi-directional ... and so on.

    My best bet would be my own LED filed diagonally, glued on two LDRs which I have previously tried to match at 2-3 light points, hoping that glueing it on will result in same amount of light for both.
     
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