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From: Cursitor Doom <cd999666@notformail.com>
Newsgroups: sci.electronics.design
Subject: Re: Random thoughts on sinewave oscillators
Date: Fri, 18 Oct 2024 21:10:23 -0000 (UTC)
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On Fri, 18 Oct 2024 15:59:09 -0400, Edward Rawde wrote:

> "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.

Without doubt, it's the trickiest aspect of the design. Definitely do-able 
though. Let us know how you get on.

> 
>  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.
>>