From: email@example.com (Ben Bradley)
Subject: Re: How generate a 3phase sine wave ?
Date: 7 Nov 2002 20:56:40 -0800
NNTP-Posting-Date: 8 Nov 2002 04:56:40 GMT
In sci.electronics.design, "Sparky" wrote:
>"Helmut Sennewald" wrote in message
>> 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.
>> Best Regards
>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.
But the Original Poster left a lot of unanswered questions, one
that quickly comes to mind is the number of '3-phase generator units'
needed. If it's just one, and the size and power consumption of the
sinewave generator is not a concern, I'd write a C program to run
under DOS on an old pentium or 486 to toggle three bits on the
parallel port. Have it read the keyboard so one can enter a new
frequency numerically, or ramp it up and down with the arrow keys.
Back to analog hardware, there's the idea of a phase-shift
oscillator using three cascaded stages, each consisting of a CA3080
OTA going into an op-amp integrator. With proper component selection
it should work fine over most of the frequency range, but the outputs
would start to fall apart as the frequency approached zero, due to
lack of feedback.
OTOH, with a (cheap 8-bit) microcontroller using a software phase
accumulator made of two or three 8-bit words, you can go to
arbitrarily low frequencies, including 0 Hz, with arbitrary precision.
Let's see, three bytes gives a resolution of 5/(2^24)=0.000000298 Hz.
OP, is 0 to 5 Hz in increments of 0.000000298 Hz acceptable?