Sine wave interpolation for pwm generation

[con3]

Hey everyone,

I'm still busy scoping out how to do this. I've been looking int methods to generate sine waves and so far it looks like some of the methods use lookup tables or trig functions from cmsis.

Although I don't think this would be very ideal, as I essentially want to generate a sine sweep. I'd like to generate a small amplitude (maybe 0.1 or 0.05V) sine wave with frequency varying from 500 Hz to 90 kHz.

I also want to let this signal have a DC offset of varying from 0.1 o 0.9V

I don't think generating this would be to hard if it can just be fed to the dac, although I have no idea how to accurately and efficiently generate the sine wave.

Any help will really be appreciated!

[Ozone]

Not sure what you want, PWM and DAC are different things.

With DAC output, you could generate the value on-the-fly via periodic timer interrupt, and write it to the DAC. For sweep, i.e. continuous frequency change, you probably need to.

Adding a DC offset and scaling down the output voltage is no problem.

However, check the settling times and output impedance of the DAC, and the intrinsic offset with enabled output buffer.

For PWM, you could apply the same on-the-fly generated amplitude value to the duty cycle of a PWM (timer unit).

> I've been looking int methods to generate sine waves and so far it looks like some of the methods use lookup tables or trig functions from cmsis.

I would suggest to use a value table instead of trigonometric functions. The FPU has no trigonometry support.

Using this method (and DAC output) I managed 100kHz (2us DAC update cycle) on a F103, with sufficient headroom for other tasks.

[con3]

Hey @Ozone​ ,

Thank you for the reply.

Yeah this explanation I gave is horrible.

I'm going to try and do it a bit better. My end goal is to generate a pwm signal.

The pwm generator usually has some dc reference value that it compares to a triangular waveform to produce the variable duty cycle. I want to generate this dc reference, which will be in the range of 0.2 to 0.9V.

But Essentially I don't just want to feed this 0.2 - 0.9 V reference to the pwm , instead I want to interpolate a small sinusoidal signal on it. So it'll look like a small sinusoid that has a variable offset of 0.2 - 0.9V.

This sine wave will have an amplitude of around 0.05 -> 0.1 V with a frequency which will vary from 500 Hz to about 90 kHZ.

I then want to send this interpolated signal through to a pwm generator to generate the pwm signal.

I'm not sure if that was a better explanation but I really hope so?

> Not sure what you want, PWM and DAC are different things.

Yes, I want to use the DAC to generate the varying frequency interpolated sinusoid and send this to the PWM generator.

> With DAC output, you could generate the value on-the-fly via periodic timer interrupt, and write it to the DAC. For sweep, i.e. continuous frequency change, you probably need to.

I'll need to look into this, you also mention you used a lookup table, although I don't think varying the frequency is possible with a lookup table as far as I've seen?

Thank you for the help!

[Ozone]

I think I understood.

For the STM32, carefully study the datasheet. Analog peripherals in general-purpose MCUs are always a compromise, and so is the DAC.

Especially the settling times and the output impedance are to consider. I didn't check the F7 DAC implementation, but assume it is similar to the F1 .. F4 one's.

ST's DAC use to have two output options, unbuffered (1MOhm) or buffered (15kOhm). However, buffered also used to mean up to 200mV offset. This is quite much, compared to your signals. Consider a larger output, and scale it down externally. Or perhaps a cheap serial DAC.

> I'll need to look into this, you also mention you used a lookup table, although I don't think varying the frequency is possible with a lookup table as far as I've seen?

A lookup table just defines a number points (values) of one sinus period. Actually, you need not more then a half or a quarter period, because of symmetries(sin(x) = -sin(x+PI)) . Only the index calculation gets slightly more elaborate .

The output frequency determines the "speed" you cycle trough the lookup table. Instead of using the trigonometric function, you just access the table, with the argument scaled to (Nr_Of_Elements / 2*PI).