Device with automatic compensation of an ac attenuator

摘要

An ac measuring instrument automatically compensates an input attenuator to achieve precision frequency response. The output of the input attenuator is applied to both the conventional measurement portion of the instrument and to an amplifier of variable gain. The output of the variable gain amplifier drives either a resistive or capacitive impedance coupled to the output of the input attenuator. The impedance is thus driven by a voltage that is a scaled replica of the output of the input attenuator. That scale factor may vary from near zero to greater than one, and the impedance acts as a virtual trimmer whose apparent value varies as the scale factor of its actual value according to the gain established for the variable gain amplifier. The proper gain therefor is determined by a servo loop that applies a test pulse to the input attenuator and subsequently samples the output thereof at least twice. An amplitude difference in the samples indicates an exponential component that arise from an uncompensated attenuator. The servo loop is responsive to the signed difference between the samples to adjust the value of the virtual trimmer until the signed difference is sufficiently near zero. Proper choice of the time interval between samples will adjust the servo loop to be most sensitive to exponential components of certain time constants rather than others. This allows the servo loop to ignore the effects of dielectric absorbtion while compensating the attenuator.