24-11-2012, 05:11 PM
HARMONIC DISTORTION ANALYZER
HARMONIC DISTORTION ANALYZER.docx (Size: 31.35 KB / Downloads: 34)
THD analyzers are important tools in the design and construction of high performance audio equipment, as well as in the design and construction of oscillators and other test equipment.
How THD analyzers work
Total Harmonic Distortion is measured by using a rejection filter system to remove the fundamental frequency (the first harmonic) of a waveform, usually a sine-wave, and then measuring whatever is left over -- second and higher harmonics, and noise.
Naturally, the measuring system has to have wider bandwidth than the fundamental frequency being measured. For very low distortion signals, say under 0.05% THD, the significant harmonic content will be in the 2nd thru 5th harmonics -- above the 5th, noise tends to dominate. So a measuring bandwidth of 5 to 10 times the fundamental will be needed, with 10 times usually preferred.
Most analog analyzers, like the HP 334A and the Heath IM-5258 for example, basically take the input signal, buffer it from the outside world, adjust its voltage to a convenient level, split it into two paths, sometimes inverting one path to get both non-inverted and inverted signals, and then pass one of the two resulting signals through a narrow-bandwidth filter of some kind -- sometimes a peak (band-pass), sometimes a notch (band-reject). The filter is essential to isolate the fundamental from the other signal components, allowing the fundamental to be eliminated while the remaining harmonics and noise are processed for measurement (there are other circuit topologies for measuring THD which I'll discuss below). Then the two signals are summed (if they one was inverted) or they are differenced (if they both have the same polarity).
Once the phases (either precisely in-phase or precisely out) and the levels of the two signal paths are exactly matched at the fundamental frequency, the summing or differencing eliminates the fundamental and leaves everything else. Feedback is used to sharpen the notch, raising the filter's Q. Given sufficient feedback, the fundamental will be eliminated but the harmonics will all be passed through with little or no attenuation -- a requirement for accuracy. The notch filter is not infinitely deep or sharp, so some noise signals are attenuated along with the fundamental, but this usually is not meaningful. And if the cancellation is not complete, some residual fundamental can be passed through the filter, setting a floor on the maximum resolutionb. Ultimately, the filter used serves as a notch (band-reject) filter once all the processing is done.