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I am trying to design a sine wave oscillator. The frequency is about 10 kHz (not critical), the distortion should be reasonable but not necessarily super good (say under 1%). However, the amplitude has to be stable, well defined and repeatable for different instances of the product. The total deviation from its nominal value (which is not critical at all, could be 100 mV as well as 2 V) has to be under 5 % (preferably better) across component tolerances, temperature etc. No trimming is allowed. The power supply would ideally be +5V but adding -5V would be acceptable.

This brings me to a Wien bridge oscillator (simplest) with AGC. The AGC would use a precise peak detector with op amps, I don't think a peak detector without op amps depending on a voltage drop across a diode would not be accurate enough.

Now, Jim Williams uses temperature compensation of the detector diode (e.g. Fig 47) in his application note

https://www.analog.com/media/en/technical-documentation/application-notes/an43f.pdf

enter image description here

Even this would not be precise enough for me I think. In any case, he uses something that looks like compensation for stability in the feedback of LT1006 and LT1115. I am puzzled how this stability compensation has been designed. And when I design my AGC with a different precision peak detector, how can I verify how stable the op amps are? I don't know how to verify the stability when the oscillator, by definition unstable, is included in the loop. Can Bode plots or something else be used, how?

Wrt AGC, why is a JFET usually used, is it because a bipolar transistor is unusable as it does not behave as a relatively constant resistance and this would increase the distortion? And how about a MOSFET?

Neil_UK
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Hyp
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  • Two comments: (1) The term "stability" can have different meanings: (a) stable bias point, (b) stability against unwanted oscillations, (c) stable oscillations (as given by the oscillation condition. (2) No - you cannot create a Bode plot because such transfer functions plots are allowed for linear circuits only. However, in your circuit are many non-linear parts- – LvW Apr 12 '23 at 10:28
  • @LvW I assume wanted oscillations fulfill the oscillation condition and are therefore stable, my main point was stability against unwanted oscillations. And I also referred to the amplitude being stable, i.e. not fluctuating much over time or temperatures. – Hyp Apr 12 '23 at 11:07
  • Did you consider using a tiny dedicated microcontroller and a DAC or R2R resistor network with a small R/C low pass filter? Very stable and much less components. – Jens Apr 12 '23 at 16:24

2 Answers2

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Periodic steady state and periodic stability simulations are what you are looking for.

If your simulator does not have this capability, you can

  1. introduce a step disturbance in the AGC loop and see how the loop settles in transient. The number of rings in the amplitude indicates the stability of the AGC loop.
  2. If you can linearize the system like what is done to check PLL stability, you can use normal bode plots to find the phase margin.
sai
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Hyp - I have the following question:

You have selected a relatively complex oscillator circuit (with three opamps). For which reason - dictated by which requirement? For example, you have mentioned the term "temperatur compensation". Which parameter of the output signal is so temperature sensitive that you need such a compensation ?

Did you review some other (simpler) oscillator topologies which also could meet your requirements? For example, a THD of max. 1% (as mentioned by you) is really not a severe problem.

LvW
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  • FYI, these would be better placed as additional comments, not an answer. – Tim Williams Apr 12 '23 at 13:34
  • Is this really a problem? Of course, I am able to reword my contribution so that it does not require a question mark. However, for which purpose? – LvW Apr 12 '23 at 13:39
  • See: https://electronics.stackexchange.com/help/deleted-answers – Tim Williams Apr 12 '23 at 13:57
  • @LvW I didn't select the oscillator in the picture, I used it to demonstrate what I meant by "compensation for stability". My topology would be a Wiener oscillator + precise peak detector for the AGC. I need a precise detector to have a precise amplitude. A simple diode-based detector would not be precise enough (diode voltage drop changes across diode production lots and is temperature dependent) and even Jim Williams's temp compensation is not enough. So instead I could use an op amp-based detector to get an accurate reading of the amplitude. But pls point out any errors I may be making. – Hyp Apr 12 '23 at 14:25