The Hewlett-Packard Models 333A & 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 kHz to greater than 65 MHz provides the capability of monitoring the distortion of the amplitude modulation on an RF carrier.

The 333A is the same as the 331A, but Adds:

Auto-nulling which measures down to 0.1% full scale distortion and
High pass filter which attenuates frequencies below 400 Hz to minimize hum.

The 333A is the same as the 334A, but without the AM Detector, and Carrier Frequency.