The 2000 THD series of Keithley Instruments are "Total Harmonic Distortion" analyzers, which are used for audio analysis and distortion measurements. The frequency range starts from 20Hz to 20kHz and a built-in low distortion sine wave Generator provide the adjustable signal for testing amplifier stages or speakers for distortion. Thus, for example in the production of mobile telephones, the microphone, the amplifier and the speaker part may be tested with this single device. The full digital multimeter part was taken from the model 2000 and allows measurements of 100 nV to 1000V.