The Hewlett-Packard HSCH-3486 zero-bias Schottky diode is used as the detector. To avoid using a modulation method of detection, a chopperstabilized op amp is used. The chopper op amp basically converts the input de voltage to ac, amplifies it, and converts it back to de. Amplifying the de output from the detector 150 times with a chopper op amp puts the signal at a level that simpler op amps, such as the LMll, can handle.
Offset voltages in the amplifier are nulled with two pots, one for the high range and one for the lower three ranges.