Wien Bridge Oscillator Problems

Discussion in 'The Projects Forum' started by gmcwilliams, Dec 12, 2006.

  1. gmcwilliams

    Thread Starter Member

    Dec 12, 2006
    10
    0
    Hi,

    I am attempting to create a resistance controlled oscillator to use as a low power transmitter at 300 kHz. I am attempting to use a wien-bridge oscillator with a diode limiter. I am using a LM318N high speed op amp.

    I have designed the circuit to oscillate at 300kHz using:

    2*pi*f = 1/RC

    I am using a high precision, low tolerance 2 pF capacitor with a 270k 5% tolerance 1/4 W power rating. I am using a Pioneer 1N914A diode.

    When I simulate in PSpice using an LF411 I am able to achieve oscillations above 200kHz. Usually ~ 225kHz, I can't get higher but I am assuming that this is a limit of the model of the LF411. THe LM318N has a high slew rate and a gain bandwidth of 15 MHz.

    When I created this circuit and actually went to test it I was oscillating at > 3MHz and was unable to get above 200 mVpeak.

    I was wondering if there were common known problems about this circuit that might be causing this or if I need to excite the circuit in some way. Is there a way to upload my PSpice schematic so that you could view my circuit?

    Any help would be greatly appreciated. Thanks in advance.

    Greg McWilliams
     
  2. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    Too many unknowns. Can you post a circuit?
     
  3. gmcwilliams

    Thread Starter Member

    Dec 12, 2006
    10
    0
  4. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    I'm not familiar with this particular circuit but your choice of 2pF seems a bit low in this context. I would imagine the strays in the wiring would add quite a few pFs to those values.
     
  5. gmcwilliams

    Thread Starter Member

    Dec 12, 2006
    10
    0
    That sounds like a good idea. I will redesign this tomorrow with a higher capacitance; the only problem is that I am limited in the types of caps I have. If I start getting to high I start to have an extremely small resistance. < 200.

    Would starting at ~82pF sufficiently insulate myself from this problem?
     
  6. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    That would be much better.
     
  7. gmcwilliams

    Thread Starter Member

    Dec 12, 2006
    10
    0
    Ok so today I redesigned using 100pF as the beginning of the design criteria.

    I solved for a resistance of 5.3kOhms and went about constructing the circuit again. In one of my text books some of the practical considerations for actually building an oscillator is to make sure that your gain bandwidth product greater than 43 * Fo. Fo = 300kHz so I have achieved this threshold. It also says to make sure your slew rate is high enough to achieve your desired results. I believe the slew rate is high enough.

    With these new resistors and capacitances I create an oscillation that is ~4 Vp-p but the period is only .3 us. This period is 10 times too small for my applications.

    Additionally the lower half of the sinesodial wave is very clean but the top has a large dip in it right at the peak so that it looks like an "M." Any tips or things I should try next?

    I am going to keep the capacitors in and start changing resistor values to see what happens. If I change resistor values and I don't see an improvement in frequency then I will really start to be confused.
     
  8. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    I've just written you a reply. Went to 'preview post' and up came a screen that said 'You cannot view this page'. So I lost the lot!!

    I'll write it again tomorrow.
     
  9. gmcwilliams

    Thread Starter Member

    Dec 12, 2006
    10
    0
    Thanks for all the help. I got it working with the correct design values!! w00t!

    I found my problem to be that I had another generator turned on that was to close and it was effecting the start up of the oscillation. After I turned it off I was able to get a perfect oscilation.
     
  10. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    Glad you got it working.
     
Loading...