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 > One can always adapt the techniques used in NIST's quantum AC standard. This device is basically a single sigma delta DAC that turns a very accurate and stable voltage source on and off at 10GHz or so. The modulator output is then low pass filtered. To generate a stable AC voltage, at least for low frequencies, use CMOS switches controlled by a sigma delta bitstream to switch the input to a low pass filter between ground and the stable voltage. AC couple the output to eliminate the dc offset. Using a single bit DAC simplifies the calibration process over that when using a multibit DAC. NIST's AC standard is currently useful for generating frequencies up to 100kHz with 10MHz the projected useful limit for a 10Gb/s bit stream. With say a 1Mb/s bit stream output frequencies up to 1KHz or so should be feasible with high amplitude stability.
Bruce _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
