1-1
IM 253421-01E
Functional Overview and Digital Display
1
1.1
System Configuration and Block Diagram
System Configuration
Equipment
under
test
Voltage
input
Current
input
PT
CT
Ext.
sensor
Analog output
GP-IB or
RS-232-C
Recorder
Personal
Computer
Ext. printer
or plotter
Contact / relay output
Input
either
one
Input
either
one
WT200
(253421)
Block Diagram
VOLTAGE INPUT
CURRENT INPUT
LPF
LPF
A/D
A/D
Zero Cross
Detector
Zero Cross
Detector
ISO
ISO
Lead/Lag
Detector
EEPROM
SAMPLING
CLOCK
CPU
FREQUENCY
PLL
HARMONICS
DMAC
RAM
COMPARATOR
EEPROM
D/A OUTPUT
RS-232-C
GP-IB
CONTROLLER
KEY&DISPLAY
ROM
RAM
or
DSP
INPUT
A/D
interface
Bus
Arbiter
Bus
Arbiter
(Option)
(Option)
(Option)
CPU
COUNTER
×
2
CLOCK
This instrument consists of various sections: input (voltage input and current input
circuits), DSP, CPU, display and interface section.
In the voltage input circuit, the input voltage is formalized by a voltage divider and
operational amplifier, then sent to the A/D converter.
In the current input circuit, the input current passes through a shunt resistor that forms
a closed circuit. The voltage across shunt resistor is amplified and normalized and
then input to the A/D converter. This method enables switching of the current range
without opening the current measurement circuit, so the current range can be switched
while electricitiy is supplied to the circuit. This also enables remote control via
communications outputs.
The output from the A/D converter in the current input and voltage input circuits is sent
to the DSP (Digital Signal Processor) via a photo-isolator, which is used to provide
insulation between the current input circuit (or voltage circuit) and the DSP. One DSP
is provided for each input element (current/voltage). For example, a total of 3 DSP’s
are used for the three-phase, four-wire model. The DSP performs averaging of
voltage, current and active power for each sampled data sent from the A/D converter.
After processing of a certain number of sets of data has been completed, computation
of apparent power, reactive power, power factor and phase angle starts.
Computation results are then sent from the DSP to the CPU, where computation such
as range conversion, and scaling is carried out. Control of display and outputs is also
performed by the CPU.
Chapter 1 Functional Overview and Digital Display