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From: "Edward Rawde" <invalid@invalid.invalid>
Newsgroups: sci.electronics.design
Subject: Re: Sinewave oscillator without gain control.
Date: Fri, 24 Jan 2025 14:14:36 -0500
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"Edward Rawde" <invalid@invalid.invalid> wrote in message news:vn0nco$1l9o$1@nnrp.usenet.blueworldhosting.com...
> "john larkin" <JL@gct.com> wrote in message news:jei7pjt2pc35osipchs2891gqtavk25a7k@4ax.com...
>> On Thu, 23 Jan 2025 19:14:24 -0500, "Edward Rawde"
>> <invalid@invalid.invalid> wrote:
>>
>>>Before you can make a 1kHz sinewave oscillator with constant output level and better than 120dB harmonic distortion you'll need 
>>>an
>>>oscillator which does better than that when it passes through the required output level.
>>>The circuit below was simulated in LTSpice 24.1.0 with all updates.
>>>There are reasons why I'm not sure I'd trust version 17.x of LTSpice to give the most accurate results but I haven't tested this
>>>circuit in any other version.
>>>After starting the simulation, wait until it has simulated about 7 seconds and then stop the simulation. Close the annoying log
>>>window which 24.1.0 will produce.
>>>You can now select a sample of about 100ms when it passes through 0dB (just under 2V) and run an FFT on it with Blackman-Harris
>>>window.
>>>This will show that this circuit is approaching 120dB down on harmonics.
>>>So if you want to do better than that then you'll need an oscillator circuit which has better performance to begin with. You can
>>>then add gain control for constant level.
>>>I've not so far found an oscillator circuit in LTSpice with better harmonic distortion performance than this one.
>>>You can, of course, try to use filtering to reduce the unwanted harmonics. Removing the first four harmonics produced by this
>>>circuit will get it approaching 130dB but don't ask me how that would be measured in reality or what it would cost to do so even 
>>>if
>>>it can be measured.
>>
>> Open LT Spice and make a 1 KHz sinewave voltage source. Run for 10
>> seconds and FFT. The 3rd harmonic is down 48 dB.
>>
>> So Spicing low distortion oscillators might not be an exercize in
>> reality.
>>
>
> The following circuit was simulated in LTSpice 24.1.0 ...