Posted on Feb 6, 2014

This THD circuit is somewhat different from the usual types: it can operate at the standard fre-quency of 1000 Hz, but it also is tunable from 970 Hz to 1030 Hz, and has an adjustable Q factor of 0. 3 to over 50. Op-amp UI, a TLC272 CMOS unit, contains the two voltage-followers required to buffer the input to the bootstrapped twin-T notch filter. T

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uning is accomplished by R1, R2, and R3, which are standard linear-taper slide pots "ganged" together by mounting them side-by-side and glu-ing their sliders together. The only other intportant construction hint is to use twisted pair at the cir-cuit`s input and output. To calibrate the circuit, input a 1000-V RMS signal at 2000 Hz, set 51 to TEST, and adjust R7 for a reading of 0. 99-V RMS on a true-RMS voltmeter at the output. To use the circuit, set 51 to TUNE, input a 1000-Hz sine-wave signal to the amplifier under test, and set the amplifier`s output to the THD adapter and tune R1/R2/R3 for the lowest output signal. Then, set 51 to TEST and read the RMS voltage. To calculate the percent THD use:

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