Re: Sinewave oscillator without gain control.

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.