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Very slow quadrature generator
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joeqsmith:
3Hz is the upper but what is the lower?  What soft of frequency resolution is needed?  Do you need to sweep, or is the frequency fixed for the most part after it is adjusted?   Any idea how clean the signals need to be?  Drive levels? 

I wonder if a 2 channel Arb would be a better choice.   Basically, this is the direction I would go.   Two DACs, an FPGA, oscillator or tied to the GPS, maybe just some dip switches to set the frequency. 
BrianHG:
I would use a cheap voltage output audio I2S stereo 24 bit DAC with a small 16bit or 32bit DSPic, using the PICs ADC if you want to sync to an external source reference sine wave.  Search on this forum, we have already discussed making a true perfect sine and cosine waveform using only integer math available in any microcontroller with a 16 bit adder and 16 bit multiplier with access to the top additional 16 bits.  Using a 24 bit stereo audio DAC running at 96Khz, you have no excuse about synthesizing a waveform which should roast anything under the sun well in-between 1Hz and 3KHz with only 2 ICs, 3.3v supply, for less than 4$ in parts.  The circuit should perform well up to 20KHz.

However, you need to be able to program for microcontrollers in 'C'.

Perfect sine generator example using all integer math: (It will need adjustments for 3Hz and 24 bit amplitude)
https://www.eevblog.com/forum/microcontrollers/code-used-for-sine-table/msg1108123/#msg1108123
You wont need to store this table, just generate it on the fly...
SilverSolder:

An Arduino can easily do this - use two PWM channels and change the PWM duty cycle to slowly create the sine waves in the two channels.

Put a simple RC filter on each channel to smooth out the PWM to a nice, clean, slowly changing voltage to drive the scope.
Didgitalpunk:

--- Quote from: mark03 on January 17, 2019, 04:18:38 pm ---At those frequencies, forget the DDS look-up tables.  Just have your atmega calculate floating-point sin() and cos() directly using the C math library!  The only reason look-up tables are used in a DDS is because the full-fledged sin/cos calculation (using taylor series or whatnot) is too slow at "normal" frequencies.  If your part doesn't have two D/A converters you could use two PWM outputs instead and RC-filter the result, depending how accurate this needs to be.

--- End quote ---

I... I don't know why I didn't think of this myself! I did just that with a simple RC filter (with R = 2.2k and C= 6.8F, beefy little foil cap!) and behold! a pretty good looking circle! I'll have to improve the signal a bit more, but it's good enough for now. Might try with a proper DAC, or an active filter.

Now, the point of all this.
I've been the proud owner since my granpa passed away a couple years ago of quite a bit of old gear, including and old heathkit O-12 oscilloscope lookalike (no heathkit markings on the PCBs and the metalwork looks like it was done by hand) which happens to have a long persistence 6" round CRT in it. After turning the lights in the room it's stored in, the tube continues to glow for a good 10 minutes.

since that 0-12 clone is missing more than a few parts, and doesn't have a case, I figured I had to make something with that CRT, and I though that making a radar PPI display lookalike was more than appropriate. and here we are.

The first step was generating a circle, the next is modulating it with a fast saw tooth to make a line rotate instead of just a point, and after that it's making the whole support circuitry for the CRT.
Ian.M:
A PPI display isn't much use unless you have something to plot on it, and most worthwhile data sources on the internet (e.g. weather radar) have already been converted to a rasterised format.  Building a rotary scan radar or sonar system from scratch, is almost certainly well outside your capabilities, or you wouldn't be here asking about generating low Hz frequency quadrature sinewaves

OTOH if you take a fast enough MCU board with three reasonably fast DACs (min 8 bits for X, Y and 6 for intensity) you could build a very nice vector graphics terminal.

Another scope CRT based project that's well worth building is a semiconductor curve tracer.   Depending on the tube persistence, you may be able to display more than the usual eight curves for the stepped parameter.
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