Agilent Swerleins Algorithm free download

File name Swerleins_Algorithm.pdf

This literature was published years prior to the establishment of Agilent Technologies as a company independent from Hewlett- Packard and describes products or services now available through Agilent. It may also refer to products/services no longer supported by Agilent. We regret any inconvenience caused by obsolete information. For the latest information on Agilent's test and measurement products go to: www.agilent.com/find/products Or in the U.S., call Agilent Technologies at 1-800-452-4844 (8am-8pm EST) A 1Oppm ACCURATE DIGITAL AC MEASUREMENT ALGORITHM August 09, 1991 Ronald L. Swerlein Hewlett Packard Co. ABSTRACT Digital sampling has been used for a number of years to make specialized and general purpose RMS AC voltage measurements. An algorithm is described that can be implemented using commercially available equipment to achieve a 1 year absolute accuracy approaching 10ppm. The implementation is usable for up to 1% distorted sinewaves with frequencies below 0.01 Hz and up to 1kHz with voltages from 10mV to 700V. Also, the error analysis serves as a tutorial on the limitations of integrating voltmeters used in a digitizing application. INTRODUCTION A digital sampling algorithm was developed and optimized for precision low frequency AC RMS measurement using the Hewlett Packard model HP3458A voltmeter. It's accuracy at high speed, precision time base, level triggering, and frequency measurement function made it ideal for this application. While optimized for this specific voltmeter, the principles discussed herein should be transportable to other digitizing equipment. A program written in HP BASIC implementing this algorithm is listed in the paper's appendix. Operating instructions and an address for obtaining more information and program copies are also included. Voltmeters have been used as samplers to digitally measure low frequency AC for many years (albeit in a somewhat handicapped fashion). By definition, an RMS (Root MEAN Squared) measurement involves averaging, which for low frequencies requires large amounts of time. For periodic waveforms, this can be substantially reduced if the sampling interval is exactly one or more periods. But due to the voltmeter's timebase quantization, this can normally be done only for specific frequencies. For example, if the voltmeter can space samples at multiples of 0.1s, it's pretty easy to see that 100 samples can exactly sample 10 periods of a 1 Hz waveform. If the waveform had a frequency of 1.3Hz, however, things wouldn't be quite so simple. In general, sampling over integral number of periods can only be achieved to a precision of 1/2 the voltmeter's time quantization. This imprecision can be a source of significant measurement error. Another source of measurement error is aliasing. A pure sinewave can be perfectly reproduced (and measured) if the sample rate is greater t