MW1008P, user manual
33
PHASE DETECTOR AND A/D CONVERTER
The phase and module of the unknown impedance are obtained by multiplying the voltage of
an alternative measure (current and voltage) by a square wave whose frequency is identical to
the frequency of the sinusoidal signal, and whose phase can be changed. The average value of
the signal produced is proportional to the part of signal in phase or quadrature. A total of four
measures for which the phase of the square signal is advanced very precisely by 90 ° allows
vector determination of the signal. This process is performed both for the voltage and current.
Then eight basic steps are necessary for the final calculation. The components out of phase
are added (0 ° to 180 ° and 90 ° to 270 °), removing any common offsets and reducing noise.
Each of these measures has no meaning in itself, because the absolute phase between the sine
wave test signal and the square is not known and because the current flowing through the
unknown impedance is not controlled. We can represent the operation in vector form, where
the system determines a vector for the current and a second vector for the voltage. Each of
these two vectors have no specific meaning, but the ratio of these two vectors ( division of the
modules and difference of the phases) gives the impedance. Please also refer to Chapter
DIAGNOSTICS for more details.
The signal coming from the phase detector is integrated over a period multiple of test
frequency, and the closest possible to 20 ms or 16.67 ms depending on whether one is dealing
with an mains of 50 Hz or 60 Hz. The resulting value is obtained by measuring the discharge
of the integrating capacitor with a meter.
