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AGC in oscillator circuits

Mohamad Nizar Kezzo
Prodigy 175 points


I am getting myself familiar with AGC in oscillator circuits. I noticed that in most designs and application notes the comparing circuit is an integrator implemented using an op amp. My question is why the use of an integrator. Is it possible to use another configuration?  

  • 0
    TI__Mastermind 22825 points
    H Mohamad,
    The reason for having an integrator is that the gain control servo is centered around what the "average" value of a waveform is. The integrator acts to find the average of a waveform for comparison.

    Sometimes the averaging is done on the positive, or the negative peaks of the waveform. Alternatively, you could average the full wave rectified version of a waveform as well which considers both peaks.
    However, I think having the integrator, which also acts like a low pass filter is probably necessary because otherwise if the gain control element is fast enough, it would respond to the instantaneous waveform (which is not what you need).

    Regards,
    Hooman
  • 0 in reply to Hooman Hashemi
    Prodigy 175 points

    Hi Hooman, 

    Thanks for the informative answer. The main issue I see with integrators is the large DC gain. But averaging and LP filtering are of course desired. 

      *

    In the above oscillator schematic for example, let's assume the controller is an integrator. The oscillator reaches a steady state where the RMS is close to DC. I am assuming that there exists a steady state in which the output of RMS with respect to Vset is such that the output of the controller maintains the gain of stage 3 at a certain level that gives the same output from RMS over and over and the loop is closed and happily oscillating. Is that assumption good enough? Do I have to rely on simulations to validate that oscillation can happen or would you advise an analytic method (which I prefer)?The DC behavior of the integrator is making it hard to analyze, other configurations such as low pass filters and so on where easily analyzed and simulated.  

    Thank you,

    Mohamad Nizar 

    --------------------------------------

    * Schematic is from "Micro-Oven Based Temperature Compensation Systems for MEMS Oscillators", A dissertation submitted by James Salvia for his PhD to Stanford University

  • 0 in reply to Mohamad Nizar Kezzo
    TI__Mastermind 22825 points
    Hi Mohamad,
    I'm not really an expert on what you are asking, but I'll give you my opinion.

    I believe that as long as there is negative feedback (i.e. increasing the output swing results in the Controller output to move in a direction that reduces output swing), and the steady state conditions are compatible (i.e. the required oscillator output swing results in an attainable Controller output voltage), the loop will "settle" at steady state. The large DC gain of the Controller will act as the large "loop gain" required in an analog feedback network (e.g. op amp closed loop arrangement) to reduce the "error" (which is DC at the point of comparison to VSET) towards zero.

    The loop stability is another factor which I think you are referring to and that is an "AC" problem and not directly related to large DC loop gain of the setup, which is a desirable thing to have. I believe you are saying that you have a handle on that aspect of it?

    Regards,
    Hooman
  • 0 in reply to Hooman Hashemi
    Prodigy 175 points
    Yes I do agree that the large DC gain is a method to reduce the error. That is actually a really good way of seeing the AGC correction mechanism. Since the large gain is at DC, there is no risk of self oscillation within the loop, especially with a low pass filter as a stage in the loop. I believe these are good indicators of a stable loop.
    I haven't analytically investigated resonances at very high frequencies, my handle on finding such instabilities is simulations.
    Thank you very much for the help. It is really appreciated.

    Best,

    Mohamad Nizar