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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 19:28:02 -0000 (UTC)
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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. 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.