generate sin wave with PWM

hi all,

How do I generate a sin wave using PWM?
The way I have thought is just to create a triangular wave with equal steps
of the PWM duty cycle (depending on freq) and then filter out the harmonics
with a hw filter.
But what if I want to generate a sin wave instead of a triangular wave, how
do I calculate the steps for the duty cycle of the PWM? These steps are not
equal, so I will need to store them in a table.
Any ideas?

thanks
Ivan

In comp.arch.embedded ivan <wolfsafety@wolfsafety.it> wrote:

> How do I generate a sin wave using PWM?  The way I have thought is
> just to create a triangular wave with equal steps of the PWM duty
> cycle (depending on freq) and then filter out the harmonics with a hw
> filter.  But what if I want to generate a sin wave instead of a
> triangular wave, how do I calculate the steps for the duty cycle of
> the PWM? These steps are not equal, so I will need to store them in a
> table.

Well, the steps are not equal, so you will need to store them in a table :)

Using sine tables is common for generating sine waves, since
calculating sines is a very expensive operation.

> Any ideas?

You could use something like the following snippet:

#include <stdio.h>

int main(int argc, char **argv)
{
    int x = 0, y = 64; 

    while(1) {
        x += y/8; 
        y -= x/8; 

        printf("%d %d\n", x, y);
    }   
    return 0;
}

-- 
:wq
^X^Cy^K^X^C^C^C^C

Write a program on your favorite computer in your favorite language
that emits a bunch of .pword assembler directives with the sine values.


Note that the table only needs to cover 1/4 cycle. At run time you use
symmetry to get the values for other parts of the sine wave. All 4
combinations of indexing into the table forwards and backwards, and
negating the result or not yield the full sine wave.


ivan wrote:
> hi all,
>
> How do I generate a sin wave using PWM?
> The way I have thought is just to create a triangular wave with equal steps
> of the PWM duty cycle (depending on freq) and then filter out the harmonics
> with a hw filter.
> But what if I want to generate a sin wave instead of a triangular wave, how
> do I calculate the steps for the duty cycle of the PWM? These steps are not
> equal, so I will need to store them in a table.
> Any ideas?
> 
> thanks
> Ivan

"ivan" <wolfsafety@wolfsafety.it> wrote in message
news:egg63k$ekg$1@news.flashnet.it...
> hi all,
>
> How do I generate a sin wave using PWM?
> The way I have thought is just to create a triangular wave with equal
steps
> of the PWM duty cycle (depending on freq) and then filter out the
harmonics
> with a hw filter.
> But what if I want to generate a sin wave instead of a triangular wave,
how
> do I calculate the steps for the duty cycle of the PWM? These steps are
not
> equal, so I will need to store them in a table.
> Any ideas?

Or calculate them on the fly. Neither method seems challenging. After all if
the PWM is clocking at the CPU frequency you've got lots of cycles to the
next PWM update unless you got no resolution from the PWM. Now, is the sin
wave you want at a fixed frequency or fixed amplitude?

As to calculating you might want to apply you mind to the trig formula:

sin(A+B) = sin(A)*cos(B) + cos(A)*sin(B)

Peter

ivan wrote:
> hi all,
> 
> How do I generate a sin wave using PWM?
> The way I have thought is just to create a triangular wave with equal steps
> of the PWM duty cycle (depending on freq) and then filter out the harmonics
> with a hw filter.
> But what if I want to generate a sin wave instead of a triangular wave, how
> do I calculate the steps for the duty cycle of the PWM? These steps are not
> equal, so I will need to store them in a table.
> Any ideas?

You can output widths that are samples of a sine. they can be generated 
directly as you need them or taken from a table. The derivative of a 
sine is a cosine. The difference between the widths of successive pulses 
that simulate a sine follow a cosine. A pair of DDAs, like a pair of 
op-amps, connected back to back can generate sine and cosine waves 
simultaneously. (DDA stands for "digital differential analyzer". Nobody 
builds them any more, but they are easily created in software.) Software 
DDAs are often used for circle generation, and they can supply pulse 
widths for uni- or bipolar PWM.

The software version is related to Bresenham�s algorithm. Look it up.

Jerry
-- 
        "The rights of the best of men are secured only as the
        rights of the vilest and most abhorrent are protected."
            - Chief Justice Charles Evans Hughes, 1927
���������������������������������������������������������������������

Ico wrote:

> In comp.arch.embedded ivan <wolfsafety@wolfsafety.it> wrote:
> 
> 
>>How do I generate a sin wave using PWM?  The way I have thought is
>>just to create a triangular wave with equal steps of the PWM duty
>>cycle (depending on freq) and then filter out the harmonics with a hw
>>filter.  But what if I want to generate a sin wave instead of a
>>triangular wave, how do I calculate the steps for the duty cycle of
>>the PWM? These steps are not equal, so I will need to store them in a
>>table.
> 
> 
> Well, the steps are not equal, so you will need to store them in a table :)
> 
> Using sine tables is common for generating sine waves, since
> calculating sines is a very expensive operation.
> 

Not necessarily. You could, for example, run a square wave into a wave 
digital LP filter. If you use Chebychev or elliptic it won't consume 
much in terms of MIPS. One can play with the coefficients a bit to 
reduce the number of shift-adds as long as the stop band attenuation 
remains sufficient. And use CSD.

-- 
Regards, Joerg

http://www.analogconsultants.com

ivan wrote:

> But what if I want to generate a sin wave instead of a triangular wave, how
> do I calculate the steps for the duty cycle of the PWM? These steps are not
> equal, so I will need to store them in a table.
> Any ideas?

Here are some ideas:

http://www.dattalo.com/technical/theory/sinewave.html

ivan wrote:

> hi all,
> 
> How do I generate a sin wave using PWM?
> The way I have thought is just to create a triangular wave with equal
> steps of the PWM duty cycle (depending on freq) and then filter out the
> harmonics with a hw filter.
> But what if I want to generate a sin wave instead of a triangular wave,
> how do I calculate the steps for the duty cycle of the PWM? These steps
> are not equal, so I will need to store them in a table.
> Any ideas?

Others will tell you of all sorts of formulae and about look-up tables but
one technique that many forget is called a "Digital Differential Analyser".
With some very simple maths and some logic these can be implemented and
produce sine and cosine streams of data which could be used more-or-less
directly by the PWM generation logic. 

Put the term "Digital Differential Analyser" and Sizer into Google's search
string and you will get a number of references that may prove useful
(especially if you register for the IEEE and CiteSeer sites.

-- 
********************************************************************
Paul E. Bennett ....................<email://peb@amleth.demon.co.uk>
Forth based HIDECS Consultancy .....<http://www.amleth.demon.co.uk/>
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-811095
Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
********************************************************************

In comp.arch.embedded Joerg <notthisjoergsch@removethispacbell.net> wrote:
> Ico wrote:
> 
>> In comp.arch.embedded ivan <wolfsafety@wolfsafety.it> wrote:
>> 
>> 
>>>How do I generate a sin wave using PWM?  The way I have thought is
>>>just to create a triangular wave with equal steps of the PWM duty
>>>cycle (depending on freq) and then filter out the harmonics with a hw
>>>filter.  But what if I want to generate a sin wave instead of a
>>>triangular wave, how do I calculate the steps for the duty cycle of
>>>the PWM? These steps are not equal, so I will need to store them in a
>>>table.
>> 
>> 
>> Well, the steps are not equal, so you will need to store them in a table :)
>> 
>> Using sine tables is common for generating sine waves, since
>> calculating sines is a very expensive operation.
>> 
> 
> Not necessarily. You could, for example, run a square wave into a wave 
> digital LP filter. If you use Chebychev or elliptic it won't consume 
> much in terms of MIPS. One can play with the coefficients a bit to 
> reduce the number of shift-adds as long as the stop band attenuation 
> remains sufficient. And use CSD.

That's what I said, very expensive 

:)

-- 
:wq
^X^Cy^K^X^C^C^C^C

ivan wrote:

> hi all,
> 
> How do I generate a sin wave using PWM?
> The way I have thought is just to create a triangular wave with equal steps
> of the PWM duty cycle (depending on freq) and then filter out the harmonics
> with a hw filter.
> But what if I want to generate a sin wave instead of a triangular wave, how
> do I calculate the steps for the duty cycle of the PWM? These steps are not
> equal, so I will need to store them in a table.
> Any ideas?

Don Lancaster got carried away with this awhile ago:

http://www.tinaja.com/magsn01.asp