The Krohn-Hite Model 6900B is a Fully Automatic Distortion Analyzer designed or ultra-low distortion and ac voltage measurements in the 5Hz to 1MHz frequency range. The Distortion Analyzer will measure total harmonic distortion (THD) down to 0.005%, typically, with 0.001% resolution. A unique Auto Set Level feature eliminates the need to manually preset the input reference level before each measurement for any input voltage between 0.1-130 volts rms. An Auto-Null Network provides Auto Nulling of the fundamental frequency over the entire frequency range. The Analyzer also measures ac voltages from 10mV to 130 volts rms over the range of 5Hz to 1MHz. The Auto-Ranging Digital Display Meter (DDM) reads directly in percent distortion or ac volts, and eliminates the ambiguities commonly found in analog voltmeter. A 1kHz ultra-low distortion (0.003%) sinewave output is provided for use as a stimulus to test circuit or system distortion characteristics, or for self-checking of the Analyzer. A switch selectable High-Pass filter reduces the effects of hum and low frequency noise. The 400Hz, 30kHz and 80kHz selectable filters are effective only in the DISTORTION MODE. An internal low-pass tracking filter eliminates high frequency noise when making low distortion measurements. A DISTORTION OUTPUT is also provided for visual inspection or spectral analysis of the input signal after the fundamental has been filtered out. An optional BCD output is available.