378CHAPTER 12. AC METERING CIRCUITS1357 911 13Total distortion = 43.7 %Power Quality MeterFigure 12.26: Power quality meter is a low frequency spectrum analyzer.phase AC power systems (predominant for large power applications), even-numbered harmonics tendto be canceled out, and so only harmonics existing in signiﬁcant measure are the odd-numbered.Meters like these are very useful in the hands of a skilled technician, because diﬀerent types ofnonlinear loads tend to generate diﬀerent spectrum “signatures” which can clue the troubleshooterto the source of the problem. These meters work by very quickly sampling the AC voltage atmany diﬀerent points along the waveform shape, digitizing those points of information, and using amicroprocessor (small computer) to perform numerical Fourier analysis (the Fast Fourier Transformor “FFT ” algorithm) on those data points to arrive at harmonic frequency magnitudes. The processis not much unlike what the SPICE program tells a computer to do when performing a Fourieranalysis on a simulated circuit voltage or current waveform.12.5AC bridge circuitsAs we saw with DC measurement circuits, the circuit conﬁguration known as a bridge can be a veryuseful way to measure unknown values of resistance. This is true with AC as well, and we can applythe very same principle to the accurate measurement of unknown impedances.To review, the bridge circuit works as a pair of two-component voltage dividers connected acrossthe same source voltage, with a null-detector meter movement connected between them to indicatea condition of “balance” at zero volts: (Figure 388,12.27)Any one of the four resistors in the above bridge can be the resistor of unknown value, and itsvalue can be determined by a ratio of the other three, which are “calibrated,” or whose resistancesare known to a precise degree. When the bridge is in a balanced condition (zero voltage as indicatedby the null detector), the ratio works out to be this: