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We have planned to make 30 kHz pure sine wave using LM358, by the use of a Wien bridge oscillator, I have made a Wien bridge oscillator connection in LM358.

I have calculated the frequency by using the formula f=1/2*3.14*RC, where C=0.01 uF and R=530 Ω (std resistor 560 Ω is used in the circuit).

But there is more noise and no proper sine wave. Please help me to solve this problem.

I have attached the waveform measured by a DSO.

Supply voltage is +6V and -6V.

Waveform

enter image description here

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    \$\begingroup\$ Having updated the other one. you could ask for it to be re-opened. But I'd recommend drawing a proper schematic using the built in schematic editor. \$\endgroup\$ – Brian Drummond Mar 12 at 16:14
  • \$\begingroup\$ @sethupathy - Hi, I have voted to re-open your previous question since even though more information has been requested and would be helpful, you are at least trying to improve it (good!). But this is just a duplicate of that (now improved) question and duplicates are not allowed here, as they waste time and potentially can lose the effort and comments already invested in that question. Therefore I've voted to close this as a duplicate. Please don't duplicate your questions. \$\endgroup\$ – SamGibson Mar 12 at 17:15
  • \$\begingroup\$ I just built the circuit with 10k and LM324 (560 is too low IMO and Jonk's). It does work, but has trouble starting up. Frequency is 1.5 kHz with these parts. I often need to momentarily touch a 1k from the output to +6 V. \$\endgroup\$ – Mattman944 Mar 12 at 18:29
  • \$\begingroup\$ please check my answer post!!! \$\endgroup\$ – sethupathy Mar 13 at 11:05
  • \$\begingroup\$ @sethupathy - You said: "please check my answer post" What you wrote is not an "answer", because you are still asking for help in that "answer". As I told you in my previous comment, you should be adding new information as updates (edits to the question) to your previous question which has now been re-opened. As I said before, making this duplicate question (and now, not following the rules by writing new information in an "answer") was, and still is, a bad idea as those actions (duplication and "non-answers") break the basic principles of Stack Exchange. (Flagged for mod attention.) \$\endgroup\$ – SamGibson Mar 13 at 14:09
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The LM358 is so noisy and has so much distortion that it is not used for audio. Its datasheet does not even tell you how much noise it produces. Also, it has trouble producing frequencies above only 2kHz (poor slew rate) and it has hardly any gain at 30kHz. Use an audio single opamp like an OPA134 instead.

The second opamp in the LM358 is not disabled so it might be producing interference.

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    \$\begingroup\$ The LM358 ain't no great shakes, that's true. It isn't as bad as to generate the waveform shown in the question, though. There's something wrong besides a not especially high performance opamp. \$\endgroup\$ – JRE Mar 12 at 18:13
  • \$\begingroup\$ please check my answer post!!! \$\endgroup\$ – sethupathy Mar 13 at 11:04
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You need to properly tie off the inputs to the unused op amp in the 358, which could be injecting tons of noise into your power rails.

Your scope probe ground, or the loop it creates, could also be picking up noise relative to the power rails. Add bypass caps.

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  • \$\begingroup\$ please check my answer post!!! \$\endgroup\$ – sethupathy Mar 13 at 11:04
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A good place to start when you are designing a circuit that you are unfamiliar with is Wikipedia. They explain that the Wien Bridge Oscillator needs a non-linear element to force the feedback ratio to be exactly 2:1. An incandescent lamp is commonly used to accomplish this.

I found a very old chassis mount incandescent lamp that is probably about 5W @ 120 VAC. I measured the resistance at various low voltages (since I don't have a milliamp meter that will go below 1 mA, I put a resistor is series and measured the voltage across the resistor). The bulb needs to have a reasonable resistance (~300 - ~3k) at 1V.

One volt (DC or RMS) or slightly less is a good target voltage for the bulb. The output Vrms will be 3 times this and the peak will be 1.414 times that, so 1 volt RMS at the bulb will be 4.2V peak at the output. If your OpAmp output won't go rail-to-rail, your target bulb voltage may need to be a little less than 1V (or use higher than +/-6 OpAmp voltage). Now, pick your feedback resistor to be twice the resistance of the bulb at the target bulb voltage. For my bulb, the resistance at 1V is a nice even number (500 ohms), so the feedback resistor is 1k.

Good decoupling is important. I have a 100 uF and 0.1 uF ceramic on each rail to ground.

Next, I found a new issue. If the loop recovery time is near the thermal time constant of the bulb, you will create an amplitude modulator. I made a nice 6 Hz amplitude modulated sine at one point. If the oscillator frequency is higher or the OpAmp has a higher bandwidth it doesn't modulate. If you are making a product to sell, it will be difficult to prove that AM oscillations won't happen under all circumstances.

This is a finicky circuit, if you want to use this in a product, you should reconsider. Hewlett and Packard created the famous tube Wien bridge audio oscillator in the 1950's, but they were damn smart, and there are better ways to do this now.

schematic

simulate this circuit – Schematic created using CircuitLab

enter image description here

enter image description here

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