Precise and stable ULF oscillator - Page 1
Forum section: electronics / beginners

author: ricko_uk
author_rank: Super Contributor
timestamp: January 08, 2020, 02:57:39 am
content: 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 possible4) 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

author: unitedatoms
author_rank: Frequent Contributor
timestamp: January 08, 2020, 04:02:48 am
content: 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.

author: schmitt trigger
author_rank: Super Contributor
timestamp: January 08, 2020, 04:13:41 am
content: 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.

author: fourfathom
author_rank: Super Contributor
timestamp: January 08, 2020, 04:19:52 am
content: Quote from: ricko_uk on January 08, 2020, 02:57:39 amDo 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 08, 2020, 09:15:14 am
content: 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

author: Benta
author_rank: Super Contributor
timestamp: January 08, 2020, 05:50:37 pm
content: Quote from: schmitt trigger on January 08, 2020, 04:13:41 am+1 to this question. A divided-down crystal oscillator should provide a perfect square wave, depending on your design of the final stage.

author: edpalmer42
author_rank: Super Contributor
timestamp: January 08, 2020, 06:12:03 pm
content: Quote from: ricko_uk on January 08, 2020, 02:57:39 amYou have to do better than that. Do you need parts-per-thousand or parts-per-billion stability?QuoteSame problem as #3. What level of harmonics can you tolerate?

author: Kleinstein
author_rank: Super Contributor
timestamp: January 08, 2020, 06:37:55 pm
content: The easy way is a stable higher frequency and divide down. One can remove the clock coupling to a very high degree if really needed. If really needed the divider could be twice an odd number, so that there would be only the very high clock or odd harmonics.Without a divider the on/off ratio will not be very good - so even if not using a divider there will still be even harmonics. Chances are good just a simple divider would get the much better square wave, with less of the even harmonics.

author: AG6QR
author_rank: Frequent Contributor
timestamp: January 08, 2020, 06:38:12 pm
content: Quote from: edpalmer42 on January 08, 2020, 06:12:03 pmAgreed. Furthermore, what kind of stability are you talking about? Long-term drift, short-term phase noise/jitter, or are there requirements somewhere in the intermediate term?A GPS disciplined oscillator can have virtually no long-term drift, but it's impossible to completely eliminate short-term instability. Like in many other situations, spending more money lets you approach perfection more closely, but each step is much more expensive than the previous.

author: fourfathom
author_rank: Super Contributor
timestamp: January 08, 2020, 07:13:56 pm
content: The OP is looking at a freq range from 5 to 7 Hz, and contemplating analog tuning, so I doubt if the picosecond jitter of a GPS clock would be an issue. Give the OP a 10 MHz xtal, and follow that with a divider in the 700000 to 1000000 range (output = 14.28xxx to 10 Hz). Follow that by a div by 2 and he has a damn near perfect square wave with approximately 0.2 ppm frequency tuning resolution (accuracy depending on the xtal). You can probably do that with a dirt-cheap PIC internal counter.Need something different? We've got NCOs, fractional dividers, noise shaping, hybrid analog/digital PLLs, etc. The bag of tricks is deep.But given the vague requirements, I have no idea what level of sophistication or performance is actually called for, although I suspect not much.

author: atmfjstc
author_rank: Regular Contributor
timestamp: January 08, 2020, 07:28:03 pm
content: I'm more curious about the OP's application. What use could one have for a 7Hz square wave? Doubt it's for audio - it would sound like crap. Drive a LED or a solenoid? Then why worry about the harmonics?

author: unitedatoms
author_rank: Frequent Contributor
timestamp: January 08, 2020, 08:06:43 pm
content: 1. Miniature device for bridges demolition2. Standing wave actuator for liquid mercury tank for gold mining industry3. Acoustic emitter for subliminal fear in crowd control situations4. Ikea furniture flexibility demo setup

author: Circlotron
author_rank: Super Contributor
timestamp: January 08, 2020, 08:52:39 pm
content: https://en.m.wikipedia.org/wiki/Schumann_resonances