The Hewlett-Packard Models 333A and 334A Distortion Analyzers are solid state instruments for measuring distortion and ac voltages. The Models 333A and 334A include two control loops that automatically tune both legs of a bridge circuit which rejects the fundamental when the rejection circuit is initially set within the range of the loops. A high impedance detector which operates from 500 Kc to greater than 65 Mc provides the capability” of monitoring the distortion of the amplitude modulation on an rf carrier. 1-3. Distortion levels of O. 1% to 1OO% full scale are measured in seven ranges for any fundamental frequency of 5 cps to 600 Kc. Harmonics are indicated up to 3Mc. The high sensitivity of these instruments requires only O. 3 v rms for the 100% set level reference. The distortion characteristics can be monitored at the OUTPUT connectors with an oscilloscope, a true rms voltmeter, or a wave analyzer. The instruments are capable of an isolation voltage of 400 volts above chassis ground. 1-4. The voltmeter can be used separately for general purpose voltage and gain measurements. It has a frequency range of 5 cps to 3 Mc (20 cps to 500 Kc for 300 pv range) and a voltage range of 300 pv to 300 v rms full scale. 1-5. The AM detector included in the Model 334A is a broadband dc restoring peak detector consisting of a semiconductor diode and filter circuit. AM distortion levels as low as O. 3% can be measured on a 3 v to 8 v rms carrier modulated 30% in the standard broadcast band, and lower than 1% distortion can be measured at the same level of the carrier up to 65 Mc.