Goal of the work- study of metrological characteristics of electronic voltmeters
Familiarize yourself with the equipment used and instructions for its use. Receive a specific assignment from the teacher to complete the work.
Determine the main error of an electronic voltmeter over the measurement range specified by the teacher. Plot on one graph the dependence of the relative and reduced errors on the readings of the electronic voltmeter. Draw a conclusion about the compliance of the voltmeter being verified with its accuracy class.
Determine the amplitude-frequency characteristic of the electronic voltmeter. Plot the frequency response graph and determine the operating frequency band of the voltmeter at the level of frequency response attenuation determined by the regulatory and technical documentation for the voltmeter being verified.
Experimentally evaluate the frequency response of a digital voltmeter. Conduct a comparative analysis of the amplitude-frequency characteristics of electronic, digital and electromechanical 11 Note 1. Take the results of research on electromechanical voltmeters from laboratory work No. 1, if it was previously performed. voltmeters. Construct graphs of the frequency response of the devices under study.
Using an electronic voltmeter, measure voltages of various shapes (sinusoidal, rectangular and triangular) with the same amplitude at frequencies lying in the operating frequency band of this device. Explain and confirm the results obtained with calculations. Draw a conclusion about the influence of the shape of the measured voltage on the readings of an electronic voltmeter.
Description and order of work
Devices used
Electronic voltmeter with analog output - GVT-417V
Universal measuring device with digital display - GDM-8135
Harmonic Signal Generator - SFG-2120
Electronic oscilloscope - GOS-620
Descriptions of the devices are attached at the stand.
To perform the work, use the diagram presented in Fig. 2.1, where GS is a generator (synthesizer) of sinusoidal, rectangular and triangular signals, CV is a digital voltmeter, EV is an electronic voltmeter, ELO is a cathode ray oscilloscope.
1. The main error of the electronic voltmeter determined by comparison method, i.e. by comparing its readings with the readings of a standard, in this case, a digital voltmeter, at a sinusoidal voltage. The readings of the reference voltmeter are taken as the actual voltage values.
The GVT-417B electronic voltmeter is checked at a frequency of 1 kHz on scales with upper limits of 1V or 3V, which is due to the regulation range of the output voltage of the generator used.
Verification is carried out for n= (610) scale marks, evenly distributed along the instrument scale, with a smooth increase and decrease in its readings
Verified voltage points U n are installed on the electronic voltmeter being verified, and the actual voltage values U oh uv, U O the value is taken from a standard digital voltmeter, respectively, when approaching the mark being verified U n scales as the readings increase and decrease.
The results of measurements and calculations are presented in the form of a table.
Absolute, relative, reduced errors and variations in readings are determined using the formulas given in laboratory work 1 or in; also determine the maximum reduced error max = Max(| i|) and maximum variation H max = Max( H i) obtained as a result of the experiment.
Based on the results of tests and calculations, plot on one graph the dependence of the relative and reduced errors on the readings of the electronic voltmeter, = F (U P), = F (U P); The graph also contains lines defining the limits of the maximum permissible reduced error corresponding to the accuracy class of the device being tested.
Based on the analysis of data on the main error and variation of readings, a conclusion is made about the compliance of the specified characteristics with the requirements determined by the accuracy class of the device being tested.
2. Amplitude-frequency characteristic of an electronic voltmeter is defined as the dependence of voltmeter readings on the frequency of the input sinusoidal signal at a constant value of its voltage.
In practice, the concept of the operating frequency band of a measuring instrument is widely used. The operating frequency band of a voltmeter refers to the frequency range f, for which the unevenness of the frequency response of the voltmeter does not exceed a certain pre-established permissible value. Thus, for the GVT-417B electronic voltmeter, within the operating band, no more than a 10 percent change in the instrument readings from the readings at the frequency is allowed f 0 = 1KHz.
The extreme values of the frequency range that satisfies the specified requirement are called the lower f H and top f In the limiting frequencies of the operating band of the electronic voltmeter.
The frequency response is also determined according to the scheme shown in Fig. 2.1. The SFG-2120 generator is used as a signal source, which ensures a constant amplitude of the output signal when the frequency changes in its operating range.
The frequency is preliminarily set on the GS generator f 0 =1kHz with a sinusoidal waveform. Using the GS generator output voltage regulator, set the reading of the electronic voltmeter at the scale mark in the range (0.7-0.9) from the upper measurement limit and record the set voltage value U P ( f 0 =1kHz) = … .
In the future, when determining the frequency response, only the frequency of the GS signal generator is changed, and the voltage taken from the generator is not changed.
To monitor the signal level and its shape, a cathode ray oscilloscope is used. On the oscilloscope screen, by selecting the deviation coefficients (VOLTS/DIV) and sweep coefficients (TIME/DIV), an oscillogram convenient for observations and measurements is obtained - an image of several periods of a sinusoid with a sufficiently large amplitude; record the amplitude l A (or l 2A - double amplitude) image of the signal for subsequent monitoring of the signal level.
It is convenient to determine the frequency response separately for the high- and low-frequency regions.
In the high-frequency region, the frequency response begins to be taken in steps of 100 kHz: 1 kHz (initial frequency), 100 kHz, 200 kHz, ... until the frequency at which the readings of the electronic voltmeter drop to a value of the order of 0.8-0.9 from the initially set reading U P ( f 0 =1kHz). To clarify the upper frequency f in the operating frequency band f electronic voltmeter in the region of a 10 percent decline in the frequency response, it is necessary to additionally remove several points of the frequency response with a smaller step in changing the frequency of the input signal.
During testing, the constant level of the GS output signal is monitored with an electronic oscilloscope.
Write the results of tests and calculations in the table:
For EV f B = ... for CV f B = ...
Where U P ( f) - voltmeter readings at frequency f; K(f) = U P ( f) /U P ( f o = 1 kHz) - frequency response of the voltmeter, presented in relative units for the corresponding frequencies, f c is the upper limit frequency of the voltmeter’s operating band, found in the experiment.
When performing a task in a similar way at the same frequencies, the frequency response of a digital voltmeter is evaluated. The test results are entered into the same table. Since this work requires comparing the operating frequency bands of electronic and digital voltmeters in a qualitative sense, it is not necessary to clarify the frequency response of a digital voltmeter at additional frequency points. In this case, the values of the limiting frequencies of the digital voltmeter will be determined with less accuracy.
Lower cutoff frequency f n working strip f for electronic AC voltmeters it is usually in the region of units and the first tens of Hz. Therefore, the procedure for determining the frequency response in the low-frequency region can be as follows: first, reduce the frequency from the original f 0 =1000Hz through 200Hz, and then from 50Hz through 10Hz. If necessary, clarify the lower frequency f n of the working band, at which the frequency response drops to a level of 0.9 from its value at f 0 =1000Hz, removing additional points in 1Hz increments.
The frequency response of a digital voltmeter is assessed at the same frequencies.
The test and calculation results are presented in table form:
For EV f n = …Hz, for CV f n = ...Hz.
Based on the results of the research, frequency response graphs are constructed for high and low frequencies. It is convenient to construct graphs along the frequency axis on a logarithmic scale.
3. Determination of the influence of the input signal shape on the readings of AC voltmeters.
In electronic AC voltmeters, AC to DC voltage converters are used, as, for example, shown in Fig. 2.2, where: u in( t) - input voltage, U - alternating current amplifier, IM - magnetoelectric measuring mechanism, - deflection angle of the measuring mechanism.
Converters of amplitude, average rectified or effective values of alternating voltage into direct voltage are used. At the same time, all electronic AC voltmeters, regardless of the type of converter, are calibrated in effective values of sinusoidal voltage. This may lead to additional errors when measuring non-sinusoidal voltages.
The GVT-417B electronic voltmeter has an average-rectified value converter. For such voltmeters, the angle of deflection of the pointer is proportional to the average rectified value U cf input voltage
Where: k V- voltmeter conversion coefficient, u in( t) - input alternating voltage with period T.
Indications U p voltmeter are calibrated in current U sinusoidal voltage values
Where: k F = U/U CP - voltage waveform coefficient, for sinusoidal voltage kФ = 1.11. Therefore, for another voltage form ( k F? 1.11) voltmeter readings may differ significantly from its actual value, which leads to an additional error in the measurement result.
In such cases, the required voltages with a known signal shape can be found by calculation.
Based on the principle of operation of the voltmeter and the accepted calibration, it is possible according to the readings U P of the device to determine the average rectified value of any (within the frequency response of the voltmeter) measured voltage
U SR = U P/1.11.
Effective value U non-sinusoidal voltage can only be determined if the coefficient is known k F voltage waveform, k F = U/U CP (or the signal shape is known from which this coefficient can be determined)
U=k F U SR.
Numerical values of shape factors for some signals are presented in the table.
To experimentally evaluate the influence of the voltage shape on the readings of an electronic voltmeter, signals of sinusoidal, rectangular and triangular shapes are sequentially measured at the same amplitude.
Previously, the voltmeter readings are set on the sinusoidal signal in the range of 0.5 - 0.6 from the upper measurement limit of the selected scale at the nominal frequency f n =1 kHz, and then, at the same amplitude of the input signals, the voltage is measured with a voltmeter for other signal forms. Signal shapes (sinusoidal, triangular, rectangular) are set by pressing the “ key Wave” on the generator.
According to indications U The voltmeter determines the average U SR and current U voltage values for all waveforms.
To assess the influence of the voltage form on the readings of an electronic voltmeter with a medium-rectified voltage converter, determine the additional relative error (in percent)
100(U P - U)/U.
The results of measurements and calculations are recorded in a table.
It should be noted that an additional error will be included in the measurement result if the effective values of non-sinusoidal voltages are determined directly from the voltmeter readings without taking into account the signal shape and carrying out the corresponding calculations.
Based on the research results, draw a conclusion about the influence of the shape of the voltage curve on the results of its measurement with an electronic voltmeter.
Literature
Metrology, standardization and certification: a textbook for students. higher textbook institutions/[B.Ya.Avdeev, V.V.Alekseev, E.M.Antonyuk, etc.]; edited by V.V. Alekseev. - M.: Publishing center "Academy", 2007. pp. 136-140.
In practice, voltage measurements have to be performed quite often. Voltage is measured in radio engineering, electrical devices and circuits, etc. The type of alternating current can be pulsed or sinusoidal. Voltage sources are either current generators.
Pulse current voltage has amplitude and average voltage parameters. Sources of such voltage can be pulse generators. Voltage is measured in volts and is designated “V” or “V”. If the voltage is alternating, then the symbol “ ~ ", for constant voltage the symbol "-" is indicated. The alternating voltage in the home household network is marked ~220 V.
These are instruments designed to measure and control the characteristics of electrical signals. Oscilloscopes work on the principle of deflecting an electron beam, which produces an image of the values of variable quantities on the display.
AC voltage measurement
According to regulatory documents, the voltage in a household network must be equal to 220 volts with a deviation accuracy of 10%, that is, the voltage can vary in the range of 198-242 volts. If the lighting in your home has become dimmer, lamps have begun to fail frequently, or household devices have become unstable, then to identify and eliminate these problems, you first need to measure the voltage in the network.
Before measuring, you should prepare your existing measuring device for use:
- Check the integrity of the insulation of control wires with probes and tips.
- Set the switch to AC voltage, with an upper limit of 250 volts or higher.
- Insert the test leads into the sockets of the measuring device, for example. To avoid mistakes, it is better to look at the designations of the sockets on the case.
- Turn on the device.
The figure shows that the measurement limit of 300 volts is selected on the tester, and 700 volts on the multimeter. Some devices require that several different switches be set to the desired position in order to measure voltage: the type of current, the type of measurement, and also insert the wire tips into certain sockets. The end of the black tip in the multimeter is inserted into the COM socket (common socket), the red tip is inserted into the socket marked “V”. This socket is common for measuring any kind of voltage. The socket marked “ma” is used for measuring small currents. The socket marked “10 A” is used to measure a significant amount of current, which can reach 10 amperes.
If you measure the voltage with the wire inserted into the “10 A” socket, the device will fail or the fuse will blow. Therefore, you should be careful when performing measuring work. Most often, errors occur in cases where the resistance was first measured, and then, forgetting to switch to another mode, they begin to measure the voltage. In this case, a resistor responsible for measuring resistance burns out inside the device.
After preparing the device, you can begin measurements. If nothing appears on the indicator when you turn on the multimeter, this means that the battery located inside the device has expired and requires replacement. Most often, multimeters contain “Krona”, which produces a voltage of 9 volts. Its service life is about a year, depending on the manufacturer. If the multimeter has not been used for a long time, the crown may still be faulty. If the battery is good, the multimeter should show one.
The wire probes must be inserted into the socket or touched with bare wires.
The multimeter display will immediately display the network voltage in digital form. On a dial gauge, the needle will deviate by a certain angle. The pointer tester has several graduated scales. If you look at them carefully, everything becomes clear. Each scale is designed for a specific measurement: current, voltage or resistance.
The measurement limit on the device was set to 300 volts, so you need to count on the second scale, which has a limit of 3, and the readings of the device must be multiplied by 100. The scale has a division value equal to 0.1 volts, so we get the result shown in the figure, about 235 volts. This result is within acceptable limits. If the meter readings constantly change during measurement, there may be poor contact in the electrical wiring connections, which can lead to arcing and network faults.
DC voltage measurement
Sources of constant voltage are batteries, low-voltage or batteries whose voltage does not exceed 24 volts. Therefore, touching the battery poles is not dangerous, and there is no need for special safety measures.
To assess the performance of a battery or other source, it is necessary to measure the voltage at its poles. For AA batteries, the power poles are located at the ends of the case. The positive pole is marked “+”.
Direct current is measured in the same way as alternating current. The only difference is in setting the device to the appropriate mode and observing the polarity of the terminals.
The battery voltage is usually marked on the case. But the measurement result does not yet indicate the health of the battery, since the electromotive force of the battery is measured. The duration of operation of the device in which the battery will be installed depends on its capacity.
To accurately assess the performance of the battery, it is necessary to measure the voltage with a connected load. For a AA battery, a regular 1.5 volt flashlight light bulb is suitable as a load. If the voltage decreases slightly when the light is on, that is, by no more than 15%, therefore, the battery is suitable for operation. If the voltage drops significantly more, then such a battery can only serve in a wall clock, which consumes very little energy.
To measure alternating voltage, analog electromechanical devices (electromagnetic, electrodynamic, rarely inductive), analog electronic devices (including rectifier systems) and digital measuring instruments are used. Compensators, oscilloscopes, recorders and virtual instruments can also be used for measurements.
When measuring alternating voltage, one should distinguish between instantaneous, amplitude, average and effective values of the desired voltage.
Sinusoidal alternating voltage can be represented in the form of the following relationships:
Where u(t)- instantaneous voltage value, V; U m - amplitude voltage value, V; (U - average voltage value, V T - period
(T = 1//) desired sinusoidal voltage, s; U- effective voltage value, V.
The instantaneous value of the alternating current can be displayed on an electronic oscilloscope or using an analog recorder (chart recorder).
Average, amplitude and effective values of alternating voltages are measured by pointer or digital devices for direct assessment or alternating voltage compensators. Instruments for measuring average and amplitude values are used relatively rarely. Most devices are calibrated in effective voltage values. For these reasons, the quantitative values of stresses given in the textbook are given, as a rule, in effective values (see expression (23.25)).
When measuring variable quantities, the shape of the desired voltages is of great importance, which can be sinusoidal, rectangular, triangular, etc. The passports for devices always indicate what voltages the device is designed to measure (for example, to measure sinusoidal or rectangular voltages). In this case, it is always indicated which AC voltage parameter is being measured (amplitude value, average value or effective value of the measured voltage). As already noted, for the most part calibration of devices is used in the effective values of the desired alternating voltages. Because of this, all further considered variable voltages are given in effective values.
To expand the measurement limits of alternating voltage voltmeters, additional resistances, instrument transformers and additional capacitances (with electrostatic system devices) are used.
The use of additional resistances to expand the measurement limits has already been discussed in subsection 23.2 in relation to DC voltmeters and therefore is not considered in this subsection. Voltage and current measuring transformers are also not considered. Information on transformers is given in the literature.
With a more detailed consideration of the use of additional capacitances, one additional capacitance can be used to expand the measurement limits of electrostatistics of voltmeters (Fig. 23.3, A) or two additional containers can be used (Fig. 23.3, b).
For a circuit with one additional capacitance (Fig. 23.3, A) measured voltage U distributed between the voltmeter capacitance C y and additional capacity C is inversely proportional to the values S y and S
Considering that U c = U- Uy, can be written down
Rice. 23.3. Scheme for expanding electrostatic measurement limits
voltmeters:
A- circuit with one additional capacity; b- circuit with two additional containers; U- measured alternating voltage (rms value); C, C, C 2 - additional containers; Cv- capacity of the electrostatic voltmeter used V; U c- voltage drop across additional capacitance C; U v - electrostatic voltmeter reading
Solving equation (23.27) for U, we get:
From expression (23.28) it follows that the greater the measured voltage U Compared to the maximum permissible voltage for a given electrostatic mechanism, the smaller the capacitance should be WITH compared to capacity With u.
It should be noted that formula (23.28) is valid only with ideal insulation of the capacitors forming the capacitances WITH And C v . If the dielectric that isolates the capacitor plates from each other has losses, then additional errors arise. In addition, the voltmeter capacity C y depends on the measured voltage U, since from U The readings of the voltmeter and, accordingly, the relative positions of the moving and fixed plates that form the electrostatic measuring mechanism depend. The latter circumstance leads to the appearance of another additional error.
The best results are obtained if, instead of one additional capacitance, two additional capacitors C (and C 2) are used, forming a voltage divider (see Fig. 23.3, b).
For a circuit with two additional capacitors, the following relation is valid:
Where U a - voltage drop across the capacitor C y
Considering that 
can be written down
Solving equation (23.30) for U, we get:
From expression (23.31) we can conclude that if the capacitance of the capacitor C 2 to which the voltmeter is connected significantly exceeds the capacitance of the voltmeter itself, then the voltage distribution is practically independent of the voltmeter reading. In addition, at C 2 " C y change in insulation resistance of capacitors C, and C 2 and frequency
Table 23.3
Limits and errors of measurement of alternating voltages
the measured voltage also has little effect on the instrument readings. That is, when using two additional containers, additional errors in measurement results are significantly reduced.
The limits for measuring alternating voltages with devices of different types and the smallest errors of these devices are given in Table. 23.3.
As examples, Appendix 5 (Table A.5.1) shows the technical characteristics of universal voltmeters that allow measuring, among other things, alternating voltages.
In conclusion, the following should be noted.
Errors in measuring currents (direct and alternating) with devices of the same type and under equal conditions are always greater than errors in measuring voltages (both direct and alternating). The errors in measuring alternating currents and voltages with devices of the same type and under equal conditions are always greater than the errors in measuring direct currents and voltages.
More detailed information on the issues raised can be obtained from.
The operating principle of an electronic alternating voltage voltmeter is to convert alternating voltage into direct voltage, directly proportional to the corresponding value of alternating voltage, and measure the direct voltage with an electromechanical measuring device or a digital voltmeter.
The AC voltage value measured by an electronic voltmeter is determined by the type of AC-to-DC measuring converter used. Let's consider the design of electronic voltmeters of alternating voltages, requirements for individual elements, design features and their metrological characteristics.
Amplitude voltmeters
The deviation of the amplitude voltmeter indicator is directly proportional to the amplitude (peak) value of the alternating voltage, regardless of the shape of the voltage curve. None of the electromechanical measuring instrument systems have this property. Electronic peak-to-peak voltmeters use peak detectors with open and closed inputs.
The required sensitivity (the lower limit of measured voltages is a few millivolts) is achieved by using a UPT with a high gain after the detector.
Fig. Figure 2 shows a simplified block diagram of an amplitude voltmeter with a closed input, built according to a balancing conversion circuit.
Measured voltage U x supplied through an input device to the input of a peak detector with a closed input (VD1, C1, R1). To an identical detector (VD2, C2, R2) a compensating voltage is supplied with a frequency of about 100 kHz, generated in the feedback circuit. DC voltages equal to the amplitude values of the measured signal and the compensating voltage are compared across resistors R1,R2. It should be noted that at low voltages the detectors will operate in quadratic mode, which will lead to an error in the amplitude value of the voltmeter.
The difference voltage is supplied to the UPT A1 with high gain. If the voltage at the output of the UPT has a positive polarity, which indicates that the signal voltage exceeds the compensating voltage or the absence of the latter, the previously locked generator-modulator is started, and the compensating voltage is supplied through the feedback divider to the detector VD2, R2, C2. The oscillator-modulator is a generator assembled using a capacitive three-point circuit, an amplifier and an emitter follower.
The excess of the compensating voltage over the measured one leads to blocking of the generator-modulator. The output voltage with an amplitude proportional to the amplitude of the measured voltage and a frequency of 100 kHz is supplied to the average rectified voltage detector U1 and is measured by a magnetoelectric voltmeter PV1.
An important requirement is the identity of the transfer characteristics of the signal detectors and the compensating voltage. Only with identical characteristics will the equality of the output voltages of the detectors indicate the equality of the input voltages.
In steady state on resistors R1 and R2 a certain voltage difference is formed and is equal to
(1)
Where TO and β are the transmission coefficients of the direct conversion and feedback circuit.
In this circuit, the direct conversion circuit includes a UPT, a generator-modulator, and the reverse circuit includes a divider in the feedback circuit and a compensating signal detector. Thus, to ensure high balancing accuracy, the gain of the amplifier and the generator-modulator must be quite high.
The components of the error are: the error of standard means during calibration, the random error of measuring direct voltage with a magnetoelectric device, the error caused by the instability of the feedback circuit transmission coefficient and the average-rectified transmission coefficient of the detector, non-identical characteristics of the detectors, and imbalance of the circuit.
Commercially produced amplitude millivoltmeters V3-6, V3-43 operate according to a similar scheme. The main error at frequencies up to 30 MHz is 4...6%, at frequencies up to 1 GHz – 25%. The scales of amplitude voltmeters are graduated in rms values of sinusoidal voltage. The disadvantage is the large error when measuring voltages with a high level of harmonic components.
