16th July 2013 NCSLI 2013, Nashville 1

The Design of a Calculable AC voltage reference using digital waveform generation

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Abstract

The design of an AC voltage reference source using a digital to analogue converter controlled by a microcontroller to produce a calculable RMS AC voltage reference with accuracy suitable for calibrating high performance Digital multimeters.

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Technical Objectives

• To provide a method for secondary and below laboratories with the ability to generate high accuracy AC Voltages • Investigate errors associated with digitally generated AC Voltages, as well as the practical application of digitally generated waveforms in commercial calibration 16th July 2013 NCSLI 2013, Nashville 4

Learning Objectives

• To investigate alternatives to traditional AC voltage verification methods • To enhance calibration laboratories understanding of verification of high performance modern DMM’s 16th July 2013 NCSLI 2013, Nashville 5

The Requirement

The calibration of Today ’ s Modern high performance Digital Meters which offer ACV Accuracy in the order of 80-100 ppm To achieve a stand off ratio of just 4 to 1 requires an accuracy of around 20 ppm. This is not possible with a multi-product calibrator 16th July 2013 NCSLI 2013, Nashville 6

The Solutions

1) Multi–Junction Thermal Transfer Standard 2) Calculable AC voltage reference 16th July 2013 NCSLI 2013, Nashville 7

The Solutions : Multi-Junction Thermal Transfer Standard

Advantages

Wide Frequency Range Proven Accuracy 16th July 2013

Disadvantages

Requires both a stable DC and AC voltage source Equipment is expensive Calibration is costly, especially when a wide range of points is required Even with protection, older Thermal Transfer devices are easy to damage by accidently over-ranging NCSLI 2013, Nashville 8

The Solutions : Calculable AC Voltage Reference

Advantages

Due to the nature of the device, does not require an AC or DC source Instrument can be internally verified with a DC volt meter

Disadvantages

Limited frequency range Output voltages are limited Due to solid state design, instrument is hard to damage with incorrect connections Low cost due to ‘simple’ hardware 16th July 2013 NCSLI 2013, Nashville 9

The Concept of the Calculable waveform

• Digitally create an AC waveform using a digital to analogue (DAC) converter. In our experiments we chose a waveform based on 256 steps.

• Single step the waveform to allow the level of each step to be measured as a DC level.

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The Concept of the Calculable waveform

The RMS value of the waveform is then calculated using the following formula

V rms

=

1

n n

å

i

= 1

(

V i

2

)

=

(

V

1 2

)

+

(

V

2 2

)

+

........

+

(

V n

2

)

n

Where : Vrms = RMS Ac voltage output V = measured DC voltage of ‘step’

n

= number of steps 16th July 2013 NCSLI 2013, Nashville 11

History

This is not a new idea.

This technique was pioneered as long ago as 1988 where results against Thermal converters gave uncertainties at the 7 volt level of just 5ppm.

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What has changed

There is now a much greater need for high accuracy ACV. In 1988 we were only seeing the first developments in High accuracy AC DMM ’ s. Now almost every laboratory has a high performance DMM 16th July 2013 NCSLI 2013, Nashville 13

What has changed

Digital Electronics along with the rapid development of digital to analogue converters for high quality audio has now made it much easier to implement this concept.

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The Circuit

The design uses a PIC micro controller, programmed with the waveform to directly drive a DAC. The PIC is directly controlled from an RS232 interface.

Clock 10V Reference Input Control Interface 16th July 2013 Micro Controller PIC 16F84 DAC NCSLI 2013, Nashville AC Voltage Output 15

The DC Reference

The D to A converter needs a short term stable 10Volt DC reference. The Linear Technology LTZ100 reference can easily provide better than 1ppm stability over the short term 2/3 hours required.

As the LTZ1000 is temperature stabilised temperature variations over the measurement period will not add any significant contributions to an uncertainty budget 16th July 2013 NCSLI 2013, Nashville 16

Improvements Vs Performance

The circuit used is extremely simple. Early design ’ s used two converters, with the output being switched between them to remove glitches.

With modern converters this is not necessary. The PIC micro controller directly drives the converter, loading the code into the converter directly from it ’ s memory, also clocking the converter after each data load. This very simple approach requires some machine code for the PIC to make it run as fast as possible. All timing comes from the crystal oscillator driving the PIC. 16th July 2013 NCSLI 2013, Nashville 17

The Software

The AC reference is very simple to control, with just a few commands needed.

S Y 1 15Hz 4 1000Hz Stop waveform and set to zero X 2 3 60Hz 200Hz Advance 1 step Advance 10 steps Z Advance 64 steps 16th July 2013 NCSLI 2013, Nashville 18

Taking the DCV Measurements

The DCV measurements can be taken with a high performance 8 digit DMM.

This will typically give full scale & linearity uncertainties of less than 5ppm.

As there are over 256 measurements to be made it is recommend that the process be automated by using a PC & software.

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Measuring Software

A procedure to measure each step was written using Procal. As each step is measured it is squared and added to a running total.

The procedure being 256 steps long, takes about 40 minutes to run automatically. Although the measurement procedure could be written in any language Procal has the advantage to work with any DMM without changing code.

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Basic Measurement Diagram

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Practical Set up

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Voltage Range

With a 10V input the

peak

output of the converter will be 10Volts; giving an RMS voltage of 7.07V

To get a 1V output the 10Volt output can be resistively divided down allowing the output to still be single stepped and measured on DC.

Note an IVD

cannot

be used to divide down the DC output. However it can be used on the AC output 16th July 2013 NCSLI 2013, Nashville 23

Errors

Switching Glitches and transition

This error is most difficult to evaluate. It is dependent on the DAC used. The DAC we have chosen is typically used for audio will be very ‘ clean ’ . The approach used by Transmille has been to look at an individual step on a scope and estimate an error based on the size and duration of the transitions. 16th July 2013 NCSLI 2013, Nashville 24

Errors

Switching Glitches and transition

Typical figures for this, at a frequency of 50Hz, where there is a step every 80uS, worst case measurements give glitch times of around 8nS, with a glitch size of around 10% of the step. This gives transition errors around the 10ppm level. In practice as some glitches will add and some will subtract the real error will be much less.

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Errors

from Rise / fall times & DC offsets

Rise and Fall time

Providing the rise and fall times are equal they do not contribute to errors. The error caused by a difference in rise/fall times can be mathematically calculated

DC offsets

Any DC offsets in the DAC will be measured and added to the calculated RMS figure. However if the device being calibrated is AC coupled DC offsets in the AC output will give an error and it may be best to trim any offsets out to avoid this problem.

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Errors

from Loading effects

Output Loading

When measuring the output with a thermal converter the load effect on the 2 wire output will need to be considered.

When using measuring the output with a typical DMM loading effects will be negligible 16th July 2013 NCSLI 2013, Nashville 27

Errors

from drift in DC Reference Source & DAC

As the AC source is a transfer device it is only the short term drift between the DC measurement of the system and the subsequent use on AC that contributes to the error. Long term changes in the reference or the DAC linearity or gain do not contribute as the system is calibrated before use. 16th July 2013 NCSLI 2013, Nashville 28

Uncertainties

Typical laboratory figures for uncertainty Imported DC Voltage Measurement Resolution of DC voltage Measurement Switching transition errors Short term stability of DC reference and DAC* 5ppm 0.1ppm

10ppm 2ppm Combining for 95% (K=2) gives 13ppm * Includes effects of temperature for +/- 2 ’ C from Tcal 16th July 2013 NCSLI 2013, Nashville 29

Verifying the accuracy

Measurements made by Transmille

Transmille best AC measurement capability uses our Fluke 540b thermal transfer, the accuracy of this Instrument is enhanced by measuring the output of the thermal converter directly. Loading effects on both AC and DC measurements were compensated for. Tests performed against a Wavetek 4920, Transmille 8081, Agilent 3458A and Fluke 8508A provided similar results.

Typical readings obtained

Single step calculation = Measurement by thermal Transfer 540B = Error = 14 ppm 16th July 2013 NCSLI 2013, Nashville 7.055681

7.055779

30

Frequency Range

To date Transmille’s interest has been In the frequency range 40Hz to 1kHz.

The design could generate frequencies below 1Hz with no additional errors. Frequencies up to 10kHz could also be generated with increased error from switching.

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Improvements

……

Extending the Voltage range.

A simple resistive divider to provide 1V and 100mV outputs.

A DC voltage amplifier to provide a 100V output.

Both could be again ‘ calibrated ’ at DC using a known DMM to provide the calculated AC RMS value 16th July 2013 NCSLI 2013, Nashville 32

Other Functions……

Non sine wave outputs With this method it would be very easy to generate other wave shapes to evaluate performance of converters including crest factor.

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Power and Phase…..

Future development of this technique could be to add additional converters to allow for several outputs. Up to 6 outputs could easily be provided for 3 phase simulation of power, with an accurate phase, or delay between the outputs.

If all the outputs were at the same 7V level this would provide a very affordable solution for a phase and power reference.

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References

[1] R. Kinard, and L. A. Harris “ A digitally synthesized sine wave source with +/- 1 ppm amplitude stability. “ in

Euromeas ’77 Conf Digest

. Institute Elect. Eng. Conf. Pub no 152, Sept. 1977 [2] Oldham N., Hetrick P., Zeng X., “A Calculable, Transportable Audio Frequency AC Reference Standard”. IEEE Transactions on I & M, April 1989, Vol. 38.

[3] Oldham N., Bruce W., FU C., Cohee A., Smith A., “An Intercomparison of AC Voltage Using A Digitally Synthesized Source”. IEEE Transactions on I & M Vol. 39 No. 1, February 1990 16th July 2013 NCSLI 2013, Nashville 35

Conclusions

Digital synthesized waveforms can provide a low cost and easy to use solution for DC to AC voltage transfer.

Copies of our paper and presentation are available on our booth on USB memory sticks 16th July 2013 NCSLI 2013, Nashville 36