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Very slow quadrature generator

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Didgitalpunk:
Hi,
I'm trying to find a simple way to generate a sine wave and its quadrature.
The problem I'm running into is doing it at very low frequencies, up to 3Hz MAX.

The whole point of this is to draw a perfect (or as close as it can be) circle on a scope in X-Y mode, as part of a larger project.
I don't want to use a function generator (or my sound card, like I've seen suggested on some websites) to generate these signals. It has to be standalone.

Has anyone ever done something like this?
does anyone have any pointers?

Thanks in advance!

hamster_nz:
So all you want is something to generate sin() and cos() outputs at 3Hz, at any easy voltage range and any price, as long as it is battery powered?

Me?, I would use the 12 bit ADCs on an $7 ESP32 module.. Would just need to tweak the example design's code a little...

Ian.M:
Doing that without digital techniques and without mechanical moving part is *difficult* even at 3Hz and gets proportionally harder as you reduce the frequency.  If you want to go down that road, you need to investigate the circuits and techniques used for trig function generation in electronic analog computers as it will be much easier to generate a slow linear ramp representing angle and calculate sine and cosine from it rather than trying to generate them directly to acceptable tolerances.  However I should caution you that its fairly close to being a dead art, with most of the progress in the field being over half a century ago, so much of what is online is either from retro hobbyists or is behind academic paywalls.

Is there any reason you couldn't work at say 100Hz?  That would make it possible to get good results from various OPAMP ring oscillator circuits.   

Dave:
A DDS algorithm would be simple enough to implement on pretty much any microcontroller, considering how low the frequency is. A pair of 12-bit DACs would be more than sufficient to draw a nice smooth line, whether it's an analog or digital oscilloscope.

Didgitalpunk:

--- Quote from: hamster_nz on January 17, 2019, 08:32:33 am ---So all you want is something to generate sin() and cos() outputs at 3Hz, at any easy voltage range and any price, as long as it is battery powered?

Me?, I would use the 12 bit ADCs on an $7 ESP32 module.. Would just need to tweak the example design's code a little...

--- End quote ---

yes this sums it up, although it doesn't have to be battery powered at all. The end project will be plugged in mains power anyways.
I've never used or programmed an ESP32 though, the only chips I know how to program are atmega328's via the arduino environment (yeah, it's sad, I'm aware).


--- Quote from: Dave on January 17, 2019, 11:51:24 am ---A DDS algorithm would be simple enough to implement on pretty much any microcontroller

--- End quote ---

what's a DDS algorithm? some kind of lookup table?


--- Quote from: Ian.M on January 17, 2019, 10:45:25 am ---Doing that without digital techniques and without mechanical moving part is *difficult* even at 3Hz[...]
Is there any reason you couldn't work at say 100Hz?  That would make it possible to get good results from various OPAMP ring oscillator circuits.   

--- End quote ---

I wouldn't mid mechanical moving parts to generate the two signals, but moving parts are probably more expensive than a sub 10 dollar micro and a handful of parts. I did try to use a resolver to get my two signals since there's a native 90deg phase shift due to the arrangement of the coils, it doesn't work at 3Hz.

I need it to be 3Hz because those two outputs will be modulated by a fast saw-tooth so that what appears on the scope screen is basically a sweeping line *not unlike a radar PPI scope*. If you go too fast, what you get is just a disc. |O

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