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Integrating-type digital electronic voltmeter
Working and Construction Working principle of an integrating-type
digital electronic voltmeter is dependent on the measurement
of the true average of the input voltage over a fixed measuring
period, in contrast to the ramp-type digital electronic voltmeter
which samples the voltage at the end of a measuring cycle.
This voltage employs an integration technique which uses a
voltage-to-frequency (V/F) conversion. This V/F converter
functions as a feedback control system that governs the rate
of pulse generation in proportion to the magnitude of the
input voltage.
Working and Construction Working principle of an integrating-type
digital electronic voltmeter is dependent on the measurement
of the true average of the input voltage over a fixed measuring
period, in contrast to the ramp-type digital electronic voltmeter
which samples the voltage at the end of a measuring cycle.
This voltage employs an integration technique which uses a
voltage-to-frequency (V/F) conversion. This V/F converter
functions as a feedback control system that governs the rate
of pulse generation in proportion to the magnitude of the
input voltage.
A block diagram of an integrating-type digital electronic
voltmeter is show in Fig. 3.11 (a). The input d.c. voltage
to be measured is applied to the input attenuator which isolates
the meter circuit from the test circuit. The attenuated input
voltage is applied to the voltage-to-frequency (V/F) converter.
The V/F circuit consists of an integrating amplifier, a level
detector (comparator circuit) and a pulse generator. The integrating
amplifier produces an output voltage proportional to the input
voltage.
If the input voltage is constant, the output is a linear
ramp.
The voltage-to-frequency (V/F) conversion waveform is shown
in Fig. 3.11 (b). When the attenuated input voltage is applied
to the integrator, the output voltage of the integrator starts
rising and it is a linear ramp waveform. When the ramp reaches
a certain negative voltage level, the level detector triggers
the pulse generator, which applies a negative voltage step
to the summing junction of the integrating amplifier. The
sum of the input voltage and the pulse voltage is negative,
causing the ramp to reverse its direction. The retrace is
very rapid since the pulse is large in amplitude compared
to the input voltage.
Advantage of Integrating-type digital electronic voltmeter
Since the input is integrated, integrating-type digital electronic
voltmeter is capable of providing accurate measurements in
the presence of large amounts of superimposed noise.
Disadvantage of Integrating-type digital electronic voltmeter
Integrating-type digital electronic voltmeter requires excellent
characteristics in linearity of the ramp.
Continuous-balance or Servo-balance digital electronic voltmeter
Working and Construction The block diagram of a servo-driven
continuous-balancing digital electronic voltmeter. As shown
in the diagram, the d.c. input voltage to be measured is applied
to an input attenuator that provides suitable range switching.
The input attenuator works as a front panel control that also
causes a decimal point indicator to move on the display area
in accordance with the input range selected. After passing
through the input attenuator, the input voltage passes through
an over-voltage protection and a.c. rejection filter, and
is applied to one side of a mechanical chopper comparator.
The other side of the comparator is connected to the variable
arm of the motor-driven precision potentiometer, connected
across a reference supply. The output of the chopper comparator,
which is driven by the line voltage and vibrates at the line
frequency rate, is a square-wave signal. The amplitude of
the square-wave signal is a function of the difference in
magnitude and polarity of the d.c. voltages connected to the
opposite side of the chopper. The square-wave signal is amplified
by a high-impedance, low-noise pre-amplifier and fed to a
power amplifier. The power amplifier has special damping to
minimize overshoot and hunting at the null position.
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The servo-motor, on receiving the amplified square-wave difference
signal, drives the arm of the precision potentiometer in the
direction required to cancel the difference voltage across
chopper comparator. The servo-motor also drives a drum-type
mechanical indicator that has the digits 0 to 9 imprinted
about the periphery of its drum segments. The position of
the servo-motor shaft corresponds to the amount of feedback
voltage required to null the chopper input, and this position
is indicated by the drum-type indicator. The position of the
shaft, therefore, is an indication of the magnitude of the
input d.c. voltage. The continuous-balance digital electronic
voltmeter uses the principle of balancing the input voltage
against the internally generated reference instead of sampling,
because of the different mechanical movements involved in
the mechanism such as positioning of the potentiometer arm
and the rotation of the indicator mechanism.
Advantages of Continuous-balance or Servo-balance digital
electronic voltmeter Following are the advantages of continuous-balance
or servo-balance digital electronic voltmeter:
(i) It is a low cost instrument that provides excellent performance.
(ii) The accuracy of this digital electronic voltmeter is
quite satisfactory, usually of the order of .01% of its input
d.c. voltage.
(iii) It has high input impedance, of about 10 M.
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