Calculate System THD Without Measuring Noise

Designing total-harmonic-distortion (THD) measurement into a system usually involves a notch filter and a broadband ac measurement. The problem with this technique is that it measures both THD and noise. DSP-based test equipment can calcula
April 13, 2006
2 min read

Designing total-harmonic-distortion (THD) measurement into a system usually involves a notch filter and a broadband ac measurement. The problem with this technique is that it measures both THD and noise. DSP-based test equipment can calculate just the THD, but this isn't practical for most portable equipment.

However, system designers can add THD measurement capability by using a bank of band-pass filters set at the harmonics of the fundamental, along with a notch filter. This approach doesn't require a microcontroller.

Figure 1 shows a circuit that measures THD directly, without noise. The MSHN1 is a switched-capacitor high-pass/notch filter that features three settings for the notch filter (narrow, wide, and deep). The MSFS5 is a switched-capacitor low-pass/ band-pass filter.

The output of the MSHN1 before the bank of MSFS5s provides the THD + noise signal needed for measurement. By setting the filter to a one-sixth-octave filter and generating clocks at 100 times the harmonic frequencies, the summed outputs supply a THD output that can be measured directly. Because three of the filter clocks are at divide-by-three, divide-by-five, and divide-by-six, the clock generator is more complex than a simple 74HC4040 binary divider. The 74HC163N, 74HC390, and 74HC393 generate the clocks for the third-and fifth-harmonic filters.

The TI TLC081 op amp sums the six band-pass filter outputs to generate the single summed signal. Figure 2 illustrates the op-amp output.

Depending on the complexity of the THD meter, the TLC081's output connects either to the analog input of the display controller, or to an LM3914 linear dot/bar display driver for a simple 10-segment THD display of the input waveform.

About the Author

John Ambrose

Vice President of Engineering and System Engineering

John R. Amrbose is the vice president of applications and system engineering at Mixed Signal Integration Corp. He can be reached at [email protected]

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