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From: "Edward Rawde" <invalid@invalid.invalid>
Newsgroups: sci.electronics.design
Subject: Re: Random thoughts on sinewave oscillators
Date: Fri, 18 Oct 2024 15:59:09 -0400
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"Cursitor Doom" <cd999666@notformail.com> wrote in message news:veucs2$3cmo3$9@dont-email.me...
> On Fri, 18 Oct 2024 14:20:48 -0400, Edward Rawde wrote:
>
>> "Cursitor Doom" <cd999666@notformail.com> wrote in message
>> news:veu7kt$3cmo3$8@dont-email.me...
>>> On Fri, 18 Oct 2024 13:47:02 -0400, Edward Rawde wrote:
>>>
>>>> "Cursitor Doom" <cd999666@notformail.com> wrote in message
>>>> news:veu45s$3cmo3$5@dont-email.me...
>>>>> On Fri, 18 Oct 2024 11:25:19 -0400, Edward Rawde wrote:
>>>>>
>>>>>> "piglet" <erichpwagner@hotmail.com> wrote in message
>>>>>> news:vetde5$38sbk$1@dont-email.me...
>>>>>>> Edward Rawde <invalid@invalid.invalid> wrote:
>>>>>>>>
>>>>>>>> The circuit below produces a reasonable looking sinewave but the
>>>>>>>> rise time still seems to be slower than the fall time. It may be
>>>>>>>> that the amplifier in use is not ideal for this.
>>>>>>>>
>>>>>>>>
>>>>>>> Could that just be second harmonic distortion? You could test the
>>>>>>> amplifier by uncoupling the Wien network and injecting test inputs.
>>>>>>>
>>>>>>> Elsewhere I think your amplitude control problems could be simply
>>>>>>> due to too much gain.
>>>>>>
>>>>>> Perhaps, but I've not so far been able to get the circuit I posted
>>>>>> in response to Bill to produce a sine wave no matter what I do with
>>>>>> the control loop gain.
>>>>>> It either grows to clipping or dies.
>>>>>
>>>>> That's the main purpose behind having a thermistor or filament bulb
>>>>> in the f/b path.
>>>>
>>>> Sure, but why use thermistors or filaments if you don't have to?
>>>> Filaments don't last forever, particularly not if you drop your
>>>> equipment, and filaments make me think of something like a 5U4.
>>>> I forget when I last saw a filament. House lamps don't even have them
>>>> now.
>>>>
>>>> I'm trying to make a low cost oscillator which produces the cleanest
>>>> 1K Hz sinewave I can get, using only resistors, capacitors and
>>>> semiconductors.
>>>>
>>>> There are plenty of examples online, but some of them don't seem to
>>>> simulate.
>>>> Whether that's because they do work in reality but not in simulation
>>>> is hard to say.
>>>
>>> A real-world oscillator needs some kind of stimulus to start up.
>>
>> I don't seem to be having startup problems, startup is fine but then it
>> goes up to clipping or dies to nothing.
>> I don't find this surprising and I'm wondering whether a real circuit
>> would do the same.
>
> Yes, it's just what they do without some kind of dynamic gain control. The
> overall loop gain needs to be 1.

Yes. That's why I have a control loop which in theory should do that.

Any calculator will show that repeated multiplication of a number slightly greater than 1 increases without limit.
In practice an amplifier will limit at or near the supply rail.
And if the number it slightly less than 1 it will reduce to 0.

A Wien bridge has an overall voltage gain of 1/3 so the circuit needs to be held at a gain of 3.

As long as it starts up then the gain control loop should be able to hold the gain at whatever is needed for a specific output 
level.
I'm still working on that.

 Fixed resistors might give you that for a
> few seconds, but when something warms up, you're either damped or through
> the rails.
>
>>
>>> This could be a voltage 'shock' at switch-on or just inherent noise in
>>> the circuitry. JL informs me LTSpice doesn't have such a stimulus
>>> unless you provide it yourself. I suspect that's the main reason you
>>> will find oscillators difficult to get started in simulation.
>