9.3
Clock Synchronization
To provide accurate time synchronization for power measurements, the
933A can use either IRIG-B timing signal from a GPS clock, such as the
Arbiter 1084A/B/C or 1093A/B/C or an optional twelve-channel global
positioning system (GPS) receiver. Proprietary technology developed by
Arbiter Systems and refined in several generations of GPS timing products
also help to achieve accuracy. By comparing the internal 10-MHz crystal
oscillator to the 1 PPS from an IRIG input or the output of a GPS receiver,
the 933A can maintain its frequency at any time within a few parts in 10
10
,
and time within one microsecond. All of the internal timing signals are
derived from this accurate timebase. If no external timebase is provided,
the 933A keeps time with an internal real-time clock.
9.4
Current Inputs
The current inputs of the 933A are implemented in a removable module.
The standard current module is a three-phase, current input designed to be
accurate to a few ppm over time and temperature (see “Error Analysis”).
This current input section uses a two-stage process, similar to a two-stage
current transformer except that the first stage, with active feedback is
designed to cancel transformer errors and the second stage provides the
current-to-voltage transfer. The current inputs have an accuracy of a few
parts per million.
9.5
Voltage Inputs
The voltage inputs uses low-TC (temperature coefficient) voltage-divider
resistor networks as signal attenuators. Voltage inputs may be configured
as a three-phase, three-element input, with four connections (A, B, C, and
N), or configured as a two-element input, with independent connections to
each element (A+, A-, C+, and C-, for example).
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