[time-nuts] OT: AC voltage standard

[Didier Juges]

The DDS chip is a good idea. That's what they are made for. They still need an external frequency reference though, and unless you use a part with direct frequency entry (which may be high pin-count), you will probably need a microcontroller to set the frequency.

Silabs sells a very small development system called Toolstick (www.silabs.com/toolstick).

You can buy the development system, including the USB JTAG adapter , a target board with a microcontroller (Toolstick 330DC) with Flash memory and a built-in 10 bit DAC, and the compiler with sample programs for less than $30 all included.

The DAC is intended for waveform generation and has a glitch-free output. The DAC can be updated by the internal timer, so you would not need software loops to generate low distortion output. The core runs at up to 25 MHz (the chip runs one instruction/clock), so a waveform generator would not use much of the processor capability.

For <$30, you have a small PWB (les than 1" square) which, when suitably programmed and provided with a 3VDC supply will produce a 2.5Vp-p sinewave with 10 bit resolution. The DAC reference is internal and may or may not have sufficient stability or accuracy for the intended purpose though. The internal clock is also not as stable as a crystal, but you could not make simpler (single chip). The chip support an external oscillator or crystal if you need better frequency stability. Note that the chip also has a built-in UART, so it could be easily make the generator programmable via a PC. 

The little board has a potentiometer already installed, which could be used to vary the frequency under software control.

I have done a number of projects (mostly at work, but some home projects too) with the Silabs chips, and the chips and tools are first rate.

I may write the code for a sine generator myself on one of these boards just for fun.

Didier KO4BB

---- Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote: 
> Didier Juges wrote:
> > The problem with a mercury relay is that the switching delay is significant and not well controlled, so the duty cycle of the resulting waveform is not well controlled, and so would be the RMS value.
> >
> > I believe CMOS analog switches would provide better control, and with series resistance that is easily below 10 ohm, that would give you negligible error when driving loads in the megohm, such as a voltmeter.
> >
> > Four switches in a full bridge configuration will give you a true AC square wave output, and if you know the DC voltage feeding the bridge (using your voltmeter calibrated with the Weston cell), you will have an accurate AC source that will not require further calibration, at least good enough for most home lab uses. 
> >
> > Now, for a sinewave, it's another matter, but Bruce's suggestion of a DAC powered from a precise DC source would work extremely well (limited by the DAC) and provide a low distortion sinewave, which is just as important as controlling the peak voltage. A simple microcontroller is all that's required to drive the DAC. Make sure you understand the delays involved with making software loops. Alternately, a counter driving a suitably programmed EPROM driving the DAC will take software out of the equation, but it sounds like the 70's all over again...
> >
> > Didier KO4BB
> >
> >   
> A DDS chip with its own DAC could be used to generate a programmable
> frequency sinewave with amplitude stable to better than 1%.
> 
> Bruce
> 
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