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Precise and stable ULF oscillator

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ricko_uk:
Hi,
what is the best way to generate a ULF oscillator with the following characteristics:
1) square wave output (or sine (then I can square it with a comparator)
2) user selectable frequency between 5 Hz and 7 Hz (can be tuned manually with a trim-pot or any other way)
3) as stable as possible
4) without using higher frequencies "counted down" or micros. This is important because I don't want to have harmonics which are not either the fundamental or odd harmonics (i.e. square wave at the fundamental frequency)

I have looked at some of the simple not-gate oscillators or similar ones but in various forums/web-pages they say their stability is poor.

Any suggestions/tips?

Thank you :)

unitedatoms:
Fairly accurate epilepsy inducer can be made with TLC555 chip. Look for capacitors with temperature coefficient stated in datasheets. The rest of components other than trimpot is not too critical.

schmitt trigger:
Hmmmmmm......

Would you kindly elaborate on your requirement #4 ?
I am not following the “no even harmonics” bit.

This is a serious question, I am not being sarcastic.

fourfathom:

--- Quote from: ricko_uk on January 08, 2020, 02:57:39 am ---4) without using higher frequencies "counted down" or micros. This is important because I don't want to have harmonics which are not either the fundamental or odd harmonics (i.e. square wave at the fundamental frequency)
--- End quote ---

Do you have a specification for the spectral purity of this ULF square wave?  You can get *extremely* close to perfect using simple digital generator designs.  Add an analog low-pass filter to an appropriate digital design and I defy you to measure *any* spurious output signals.  If you go digital, then you have the stability requirement solved.

T3sl4co1l:
Funny thing is, even a "magic sinewave" can have tons of transitions (it looks very square, or, well, pulsed at least) but can null arbitrarily many harmonics.  In the limit, it approximates PWM, which has no spectral content between BW and Fclk - BW, where BW is the signal bandwidth and Fclk is the switching (modulation) frequency.

Heh, hmm, I wonder if you can further tweak PWM to null an arbitrary amount of the lower sideband, thus extending the stop band from BW to Fclk.  Any SSB modulator should do, I think... adapted from PWM output, of course.  Wouldn't be very useful but would help push high frequency content up just a little bit further, allowing a slightly smaller filter.  Assuming one needed a class D amp to deliver wide bandwidth (like audio) that also must meet EMC requirements (which they never do, so..). ;)


Regarding stability and trimmability: pick one.  Trimmers are shite.

Tim

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