AVR Freaks

Uggh, the author was Jesper and it was DDS rather than DTMF. Sorry for the false hint.

 

Jim

 

If you insist on filtered PWM, that is the cost you face. There is another scheme, however, that uses a sample/hold

 

https://www.edn.com/design/analo...

 

By the way, the conventional DDS algorithm can use any analog conversion structure. I would probably use a purchased DAC. I tried to look at the schematic at your github reference, and could find no reason for decreasing amplitude at low frequencies. Can you post the schematic that you are using?

 

Jim

That is what puzzles me. If it is a low-pass filter, the corner should be above 20Hz and there should not be much variation in amplitude.

 

However, now I realize, that when the frequency gets really low, a voltmeter, itself, will fail to respond properly. And, the lowpass filter will no longer function as it did at 20Hz. The original poster can try the experiment of increasing the size of the capacitor in the filter. It won't work as well at 20Hz but it should work better below 1Hz. If this is the case, then its stuck in the "basic physics of things". That is the cost of using PWM! The sample/hold scheme I linked above could be a solution.

 

Jim

Same issue with a Wien-bridge oscillator (first bridge half sets the frequency by RC, second bridge half for amplitude, 0.01% or less distortion, 0.001% distortion by four op amps instead of one)

The Wien-bridge oscillator amplitude issue has simple solutions (by incandescent lamp, nJFET, or opto-isolator)

My guess for making a Wien-bridge oscillator variable frequency is to replace the two equal R in the frequency control loop by a dual nJFET; the transconductance is inversely proportional to temperature so proportional control of the two V_gs by temperature (assumes low dT/dt, setting the operating point would need some thought)

 

2 equal R :

The Art of Electronics 3rd Edition | by Horowitz and Hill

[subject index, page 23 top middle, "oscillator, Wien bridge", page 437, "Figure 7.20 Wien-bridge low-distortion oscillators. ..."

 

One R :

AN-6604 JFET Circuit Applications (ON Semiconductor)

[page 6]

Figure 21. Wein Bridge Sine-Wave Oscillator

 

PWM-input DAC though would consider its accuracy (edit : and noise)

DAC are common in XMEGA with reduced precision DAC in some late model AVR.

Check the errata for these as there may be defects (excessive noise therefore LPF, temperature?)

 

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PWM to Voltage Output D/A Converters | Analog Devices

ATxmega32E5 - 8-bit PIC Microcontrollers (DAC 12b)

ATMEGA4809 - 8-bit Microcontrollers (DACREF 8b to AC and ADC)

Digital to Analog Converters|Intelligent Analog|Peripherals|8-bit | Microchip Technology

via ATTINY3217 - 8-bit Microcontrollers (DAC 8b)

 

 

 

How  "pure"  do you need you sinewave?  DDS & digital methods will inject a bit of noise & quantization of levels.   In that regards, some analog methods might be quieter.  However DDS is generally the method of choice these days.   

 

for $4 you can get an actual DDS chip:

https://www.newegg.com/p/2S7-01JK-02AB9?item=9SIABKS8X78902&source=region&nm_mc=knc-googlemkp-pc&cm_mmc=knc-googlemkp-pc-_-pla-new+ocean+tech-_-gadgets-_-9SIABKS8X78902&gclid=EAIaIQobChMI4-jegb2u5QIVrfbjBx36VAL1EAkYBiABEgI0LPD_BwE&gclsrc=aw.ds

 

Wonder what the lowest cost , low freq, one is?

 

Note also, that for real low freqs, you can/could calculate the values (each point), on-the-fly, as needed.  There are some simple fixed-point methods out there.   That saves  space (no big table), which might allow it to be in a chaepo 8-pin AVR talking to a cheapo dac. chip. if you had 5Hz & 720 points per cycle (1/2 degree) that would only be about 4000 points per second to figure out.  

Of course, that isn't DDS per se, unless you build it around a phase accumulator.

 interesting stuff:

https://www.coranac.com/2009/07/sines/

https://ccrma.stanford.edu/~juhan/pubs/jnam-emile05.pdf

 

See Tip #1 for how to put the image in the post - so that we can see it in context:

I like the i2c DAC’s, like the mcp4706/4716/4726.

Either will work in your project.

 

jim