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From: "Helmut Sennewald"
Subject: Re: How generate a 3phase sine wave ?
Date: Fri, 8 Nov 2002 07:34:00 +0100
References: <3DC915AA.firstname.lastname@example.org> <email@example.com>
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"Ben Bradley" schrieb im Newsbeitrag
> In sci.electronics.design, "Sparky" wrote:
> >"Helmut Sennewald" wrote in message
> >> Hello,
> >> I fully agree up to here. The 3 DACs can be done easily by 3 PWM
> >> (pulse width modulation) outputs of a single microcontroller chip.
> >> The only extra hardware needed are 3 resistors plus 3 capacitors
> >> to filter the digital PWM outputs.
> >> Look for devices with 3 or more PWM channels.
> >> http://www.atmel.com/atmel/products/prod199.htm
> >> Best Regards
> >> Helmut
> >Good point Helmut, I fully agree, assuming he can get the resolution
> With a maximum output frequency of 5 Hz, I can't imagine an 8-bit
> processor that couldn't give very good resolution PWM'ed sine waves on
> three output bits totally in software. Hardware PWM channels just
> makes for a little less code required.
I have done it one time and achieved better than -60dB THD for any
frequency from close to zero to some kHz.
You need a software DDS with a sine table in ROM for the PWM value.
The SW updates the PWM synchronously with their period (eg. 8MHz/256).
It is nothing magic there. The best results are achieved with "centered"
DDS: Direct Digital Synthesizer.
PWM: Pulse Width Modulator
"centered" : Pulse is symmetric to center of PWM period
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