Rotek 800A - 3 Theory of Operation; Introduction; Voltage

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3. Theory of Operation
3.1 Introduction
The Series 800 consists of two sources for generating Voltage and Current independent of
each other, with a means for controlling the phase relationship between them. The three separate
functions of the 800 Series are:
• Voltage
• Current
• Power Factor
The Series 800 is a microprocessor based system which, in conjunction with logic decoding,
provides an interface between the front panel controls and the three functions listed above. The
basic configurations for each section are described in this paragraph. The following paragraphs
describe the detailed operation of each section and logic interface.
3.1.1 Voltage
Figure 3-1 shows the arrangement of components for the generation of the AC voltage
output. The basic components in this block diagram are:
• A precision source of DC voltage, the Voltage Reference.
• A precision D to A converter, the Voltage Attenuator.
• An operational amplifier integrator.
• A voltage controlled oscillator.
• The Post Attenuator.
• A precision operational amplifier rectifier.
The voltage controlled oscillator injects a signal via the post attenuator to the power amplifier
which in turn drives the primary of the output step-up transformer; the selected voltage is then applied
to the output terminals. The precision rectifier senses this voltage at the sense terminals via the
appropriate range resistor. The rectifier output injects a current to the summing junction of the
operational integrator. This current is compared with the current injected by the DC reference, D to A
converter combination. The output of the integrator assumes a value that establishes an amplitude
from the voltage controlled oscillator such that the average of the currents injected into the summing
junction are equal and opposite; thus leveling the oscillator for the correct output voltage.
The power amplifier can supply sufficient current to cause an error due to lead drop,
particularly on low impedance meters. The points that sense voltage (i.e. the connection to Rf) are
connected to Sense High and Sense Low. These terminals are available for four wire measurement
to eliminate lead drop errors. For most meters the sense and output terminals are connected
together for four wire configuration. The 800 Series is placed in the External Sense mode via S20 on
the rear panel of the 800A/811A.