[time-nuts] OT: AC voltage standard

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Nov 6 20:49:11 UTC 2007


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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