636 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 51, NO. 4, AUGUST 2002 Tracking Generator of Calibrated Harmonic Umberto Pogliano Abstract—A tracking generator ofcalibratedharmonics has been tested at the Istituto Elettrotecnico Nazionale Galileo Ferraris. This system acquires the sinusoidal input supplied by a precision ac calibrator, and the internal DSP produces, by means of a digital-to-analog converter (DAC), an output signal combination of calibrated harmonics components, which is added to the sinusoidal signal of the calibrator. In this way, an ordinary 12-bit DAC can produce a precision signal with an assigned THD less than 10%. Some algorithms for generating the signal and for tracking with high accuracy the internal reference to the external input signal have been developed, which allow the frequency and phase difference to be recovered within 10 4 rad in less than 0.1 s. Index Terms—AC-generator, distorted waveform, harmonic dis- tortion, phase locked loop, tracking generator. I. I NTRODUCTION T HE generationof sinusoidal signalswith calibrated harmonic components can be very useful in electrical metrology. At power frequency, electrical signals with known harmonics content can be applied in high-precision measure- ment for testing different electrical instruments. In particular, this generator can be used to evaluate the influence of the harmonics in the calibration of precision voltmeters, ammeters, and wattmeters. Characterization of the harmonics influence is, in fact, increasingly required for the calibration and the testing of modern instruments at power frequency and with distorted waveforms. At the Istituto Elettrotecnico Nazionale (IEN) Galileo Ferraris the generation of harmonics at power frequency was developed mainly to extend the capabilities of the present system for the precision measurement of electrical power [1], [2]. This system is based on a sampling method, and, although it was primarily builtfor precision measurement of electrical power resulting from sinusoidal waveforms, it can also evaluate the power in waveforms with a limited number of harmonics (up to the tenth). This system has been verified in international comparisons for high-precision measurement [3], but at the moment, it has not been completely characterized for its behavior to harmonics, due to the limitation of its present generator, which supplies only sinusoidal signals. The construction of a precise and stable generator at the metrological levelthatalso supplies, in addition to the sinu- soidal signal, the harmonic components, can be very expensive. This is because the whole signal needs an accuracy and stability at the level of a few parts in 10 , which is achievable only by using refined electronic systems. On the other hand, precision AC calibrators generating sinusoidal signals are currently avail- Manuscript received May 29, 2001; revised April 30, 2002. The author is with the Istituto Elettrotecnico Nazionale Galileo Ferraris, Torino, Italy(e-mail: pogliano@me.ien.it; URL: http://www.ien.it). Digital Object Identifier 10.1109/TIM.2002.802259 Fig. 1. Basic circuit of the generator of harmonics. able in metrological laboratories. So,a design of a generator based on a different principle has been undertaken. In this generator, different from other approaches (for example [4]), only the selected harmonic components of a sinusoidal signal already given are produced. In this way, if the total distortion of the test signal is not very high (less than 10%), the harmon components can be generated with sufficient resolution by means of an ordinary 12-bit digital-to-analog converter (DAC) II. THEGENERATOR OF H ARMONICS A. Hardware The basic circuit of the system for the generation of har- monics built at IEN is shown in Fig. 1. The hardware of the system is essential and comprises the following. • An analog-to-digital converter (ADC), whose input is con- nected to the sinusoidal voltage supplied by a calibrator. The calibrator acts also as a reference for the harmonic components produced by the generator; • a system with an internal DSP, which processes the signa in real time and creates the suitable codes for the output This system is connected to a computer that prepares by means of a suitable program all the codes necessary for t initialization of the generator and for the production of th required harmonic components; • a DAC,which generates the selected harmonic compo- nents from the sequential code produced by the DSP; • an analog adder to add the harmonic components to the fundamental. The adder has been realized by a double- input instrumentation amplifier. A different solution base on a double-stage transformer injector, which could be more suitable for precision measurement, is now under investigation. B. Software The key part of the system is the software and specifically t program for the generation of the codes for the output. The lo ical and functional structure of this internal program is graph 0018-9456/02$17.00 © 2002 IEEE