Electronic voltmeters of alternating voltage. Analog electronic voltmeters

REPUBLIC OF KAZAKHSTAN

AVIEK university

FACULTY OF INFORMATION SCIENCE

DISCIPLINE: "Standardization and measurement technologies"

TEST: "ANALOGUE ELECTRONIC VOLTMETERS."

Completed:

St-t gr. ZPOS-96-1

Grinev M.V.

Associate Professor, Ph.D.

Nurmanov M.Sh.

Almaty 2000

VOLTAGE MEASUREMENT WITH ELECTRONIC ANALOGUE VOLTMETER

Electronic analog voltmeters are the first example of electronic measuring instruments covered in the course. Among them, there are both direct conversion voltmeters and comparison voltmeters. Consider the principle of operation, structural diagrams and the main functional units of analog voltmeters for direct conversion and comparison.

ANALOGUE DIRECT CONVERSION VOLTMETER

The block diagram of an electronic analog direct conversion voltmeter corresponds to the typical diagram of fig. 2.1 and, as can be seen from Fig. 3.13, in the most general case, it includes an input device (ID), to the input of which the measured voltage is applied Ux, IP and a magnetoelectric device used as an IU.

input device in the simplest case, it represents a divider of the measured voltage - an attenuator, with the help of which the measurement limits of the voltmeter are expanded. In addition to exact division Ux, The VU should not reduce the input impedance of the voltmeter, which affects, as has been repeatedly emphasized, the methodological measurement error Ux- Thus, the use of a VU in the form of an attenuator is, in addition to additional

Rice. 3.13. Generalized block diagram of a direct conversion analog voltmeter.

resistances and measuring voltage transformers, another way to expand the measurement limits of voltmeters. It is this method that is used in electronic voltmeters and other radio measuring instruments.

As an IP in voltmeters direct current(B2) a DC amplifier (UCT) is used, and in AC and pulsed current voltmeters (VZ and V4), a detector is used in combination with a DC amplifier or an amplifier alternating current. Converters in voltmeters of other types have a more complex structure. In particular, the converters of selective voltmeters (B6) should provide, in addition to detecting and amplifying the signal, its selection in frequency, and the converters of phase-sensitive voltmeters (B5) should provide the ability to measure not only the amplitude, but also the phase parameters of the signal under study.

The block diagram of the analog DC voltmeter corresponds to the generalized circuit of fig. 3.13. The main functional unit of such voltmeters is the UPT. Modern DC voltmeters are designed primarily as digital instruments.

Voltmeters of alternating and pulsed current, depending on the purpose, can be designed according to one of two block diagrams (Fig. 3.14), which differ in the type of IP. In voltmeters of the first modification (Fig. 3.14, a) measured voltage Ux^ converted to constant pressure Ux=, which is then measured with a DC voltmeter. On the contrary, in voltmeters of the second modification (Fig. 3.14, b) the measured voltage is first amplified by an AC amplifier and then detected and measured. If necessary, a UPT can be additionally connected between the detector and the DUT.

Comparing the block diagrams of Fig. 3.14, even before considering the circuit solutions of their functional units, it is possible to draw certain conclusions regarding the properties of voltmeters of both modifications. In particular, the voltmeters of the first modification in relation to the frequency range of the measured voltages do not have such restrictions as the voltmeters of the second modification, where this parameter depends on the bandwidth of the AC amplifier. But the voltmeters of the second modification have a high sensitivity. From the course "Amplifying Devices" it is known that with the help of an AC amplifier it is possible to obtain a significantly higher gain than with the help of UPT, i.e., to design microvoltmeters with a lower limit Ux^. limited by the amplifier's own noise. Through the change

Rice. 3.14. Structural diagrams of analog voltmeters of alternating and pulsed current:

a - with a detector at the entrance; b - with an AC amplifier at the input.

the division factor of the VU and the gain of the amplifiers, the range of measured voltages can be large for voltmeters of both modifications.

Detector type in block diagrams fig. 3.14 determines whether voltmeters of both modifications belong to voltmeters of amplitude, rms or medium-rectified voltage. At the same time, pulsed current voltmeters (B4) are designed only as voltmeters of the first modification in order to avoid distortion of the pulse shape in the AC amplifier. When measuring the voltage of single and rarely repetitive pulses, either diode-capacitive pulse expanders are used in combination with detectors, or the amplitude-time conversion of pulses, which is typical for digital voltmeters.

Let us now consider a typical block diagram of selective voltmeters, which are used in measuring low harmonic voltages under interference, in the study of the spectra of periodic signals, and in a number of other cases. As can be seen from fig. 3.15, the voltmeter is essentially a superheterodyne receiver, the principle of operation of which is explained in the course "Radio Circuits and Signals".

Frequency selection of the input signal is carried out using a tunable local oscillator, a mixer (Cm) and a narrow-band intermediate frequency amplifier (IFA), which provides high sensitivity and the required selectivity. If the selectivity is insufficient, a two-fold and sometimes a three-fold frequency conversion can be applied. In addition, selective voltmeters must have an automatic frequency control system and a calibrator. Calibrator - exemplary

source (generator) AC voltage a certain level, which makes it possible to eliminate systematic errors due to changes in the local oscillator voltage during its restructuring, changes in the transmission coefficients of the voltmeter nodes, the influence of external factors, etc. The voltmeter is calibrated before the measurement when the switch P is set from position 1 to position 2.

Rice. 3.15. Block diagram of a selective voltmeter.

In conclusion, we note that in one device it is not difficult to combine the functions of measuring direct and alternating voltages, and with the help of additional functional units and appropriate switching (similar to rectifier devices) to form combined devices, called universal voltmeters (B7). Modern types of such voltmeters, as a rule, are designed as digital instruments, which allows them to further expand their functionality and improve accuracy. In this regard, the features of constructing structural diagrams of universal voltmeters will be considered in the works of colleagues.

ANALOGUE COMPARISON VOLTMETER

Rice. 3.16. Measuring potentiometer circuit.

Electronic analog comparison voltmeters for the most part implement the most common modification of the comparison method - the zero method. Therefore, they are often called compensatory voltmeters. Compared to direct conversion voltmeters, these are more complex, but, as emphasized earlier, more accurate instruments. In addition, from the diagram in Fig. 2.2 it can be seen that at the moment of compensation DX=0 and the device does not consume power from the source x. With regard to compensation voltmeters, this means the possibility of measuring not only voltage, but also the EMF of low-power sources. In the practice of electroradio measurements, such measurements are performed both with the help of electronic compensation voltmeters and electromechanical ones. To explain the use of the zero method in measuring EMF and voltage, let us first consider the classical circuit of an electromechanical DC compensator shown in Fig. 3.16.

One of the main functional units of any compensator is a high-precision variable resistor. R, on the scale of which the measured value of EMF is counted (Ex) or voltage (Ux). Therefore, it is customary to call compensators according to GOST 9245-79 measuring potentiometers. As an exemplary measure of EMF, normal element(NE) - electrochemical source, EMF (Ea) which is known with a very high degree of accuracy. However, the NE capacitance is small, and a long-term comparison during measurements Ex(Ux) With Yong impossible. Therefore, the potentiometer circuit is supplemented with an auxiliary source of high-capacity EMF (Eo). For comparison with Ex(Ux) the voltage drop across the reference resistor is used Rn., created by the current from the source Eabout- operating current (Ip), which is pre-set. So the measurement process Ex{ Ux) should be in two stages.

At the first stage, the required value of Ir is set. To do this, set the switch to position 1 and use the potentiometer Rp achieve a zero reading of the indicator AND (as a rule, a magnetoelectric galvanometer). As can be seen from fig. 3.16, this corresponds to IPRn=En, i.e., the operating current Ip, which must then remain constant, will reproduce the value during the measurement process En.

At the second stage, the value of Ex(Ux) is measured. To do this, the switch is moved to the position 2, and changing the resistance of the potentiometer R again achieve a zero reading of I. When Ip = const, this corresponds to Ex (Ux) = IPR, i.e. the desired value Ex(U^}^. R and can be measured on a scale R.

Thus, the metrological characteristics of DC measuring potentiometers are determined by the parameters of the NO, reference resistors, indicator, and source Eu. As NE, saturated and unsaturated reversible galvanic cells are used, the positive electrode of which is formed by mercury, and the negative electrode is formed by cadmium amalgam. Accuracy classes of NE are regulated by GOST 1954-82 within 0.0002 ... 0.02 and determine the accuracy class of the potentiometer as a whole. Potentiometer R is performed according to a special scheme that ensures the constancy of /p when changing R and the required number of characters (decades) when counting Ex(Ux). These requirements are met by circuits with replacement and shunt decades.

Measuring potentiometers can also be used to measure alternating voltages. However, the compensating voltage must be regulated in this case not only in absolute value, but also in phase. Therefore, such potentiometers have a more complex circuit than direct current potentiometers, and are significantly inferior to them in accuracy due to the lack of an exemplary measure on alternating current, similar in its characteristics to NE. In the practice of electrical radio measurements, they are completely replaced by electronic compensation voltmeters.

In compensation voltmeters, the measured voltage (DC, AC, pulse) is compared with a constant compensation voltage, which in turn is accurately measured by a DC voltmeter and is a measure Ux. A typical block diagram of such a voltmeter is shown in fig. 3.17.

As can be seen from fig. 3.17, the basis of the voltmeter is a compensation IP, consisting of a measuring diode V s load R, controlled source constant compensating voltage -Ek, amplifier and bistable indicator. With absence Ux indicator implemented using

functional nodes is in the first stable state, and at a certain threshold value it goes into the second state. Measurement process Ux is reduced to a gradual increase Ek until the indicator enters the second stable state. Meaning Ek, corresponding to the moment of transition, is measured by a DC voltmeter and is a measure Ux.

Rice. 3.17. Block diagram of a compensation voltmeter.

In combination with other circuit solutions (the use of an indicator with a low threshold voltage, a lamp measuring diode with a stable characteristic, etc.), it is possible to design high-precision compensation voltmeters.

The disadvantage of the considered scheme is the need to install Her manually. Therefore, in most voltmeters, the IP circuit is complicated by providing automatic compensation Ux and Ek. Auto-compensation voltmeters are direct-reading instruments and are more convenient to use.

MAIN PARTS OF ANALOGUE VOLTMETER

Consider circuit solutions of the main functional units that determine the metrological characteristics of analog voltmeters. Most of these nodes are used in other types of electronic measuring instruments.

input device

As mentioned above, the WU is designed to expand the measurement limits of the voltmeter. In the simplest case, it is an attenuator made according to resistive (Fig. 3.18, a), capacitive (Fig. 3.18, b) or combined (Fig. 3.18, c) schemes.

The fulfillment of the remaining requirements and, above all, the provision of a high input resistance and a minimum input capacitance of the voltmeter in some cases leads to a complication of the WU structure. The most versatile and frequently used in modern AC voltmeters is the VU, the block diagram of which is shown in fig. 3.19.

The fundamental feature of this circuit is the change in Uv using a low-resistance resistive attenuator with a constant input and output impedance. This improves measurement accuracy. Ux~, but requires the introduction of an impedance converter (PI) into the structure of the VU, which ensures the transformation of the high input resistance of the voltmeter into a low input impedance of the attenuator. As a PI, a voltage follower on a field-effect transistor with deep negative feedback is most often used. By using

Rice. 3.18. Voltmeter attenuator circuits:

a-on resistors; b - on capacitors; c - combined.

Rice. 3.19. Structural diagram of the universal input device.

input voltage divider (VDN) provides an additional opportunity to expand the measurement limits of the voltmeter. VDN is a fixed resistor-capacitive divider (see Fig. 3.18, in)

At high frequencies, the input resistance of the voltmeter decreases, and the input capacitance and inductance of the conductors form a series oscillatory circuit, which has almost zero resistance at the resonant frequency. To neutralize these effects, the PI is designed as a remote probe with VDN in the form of a nozzle.

Amplifiers

DC amplifiers, as can be seen from the block diagrams (see Fig. 3.13 and 3.14, o), provide sufficient power to drive the IM of a magnetoelectric device, and match the input impedance of the DUT with the output impedance of the VU or detector. Two main requirements are imposed on the UPT: high constancy of the gain and negligible fluctuations of the output value in the absence of Ux= (Drift zero). Therefore, all practical UPT circuits have deep negative feedback (NFB), which ensures their stable operation and insensitivity to overloads. Radical methods of combating zero drift are its periodic correction, as well as the transformation Uх= into an alternating voltage with subsequent amplification and rectification of this voltage.

AC amplifiers, in accordance with their functional purpose (see Fig. 3.14, b), must have high sensitivity, great importance and high gain stability, low non-linear distortion and wide bandwidth (with the exception of the IF selective voltmeter). Only multi-stage amplifiers with OOS and links for correcting the frequency response can satisfy these conflicting requirements. In some cases, logarithmic amplifiers are used to obtain a ^ linear scale in decibels. If the task is to minimize the additive error of the voltmeter, the amplifiers can be two-channel with amplification of the main signal and the signal that corrects the additive error. To expand the functionality, many voltmeters have a special amplifier output and can be used as broadband amplifiers. Moreover, amplifiers can be produced as independent measuring instruments, forming a subgroup U.

DC and AC amplifiers are discussed in detail in the Amplifying Devices course.

Detector

The type of detector determines, as already mentioned, whether AC voltmeters belong to amplitude, rms, or average-rectified voltage voltmeters. In accordance with this, the detectors themselves are classified as follows: according to the parameter Ux~^ which corresponds to the current or voltage in the output circuit of the detector: peak detector, rms and average rectified voltage detectors; according to the input scheme: detectors with open and closed DC voltage inputs;

according to the detection characteristic: linear and quadratic detectors.

Rice. 3.20. Peak detector circuits:

A - with an open entrance; B - c closed entrance.

Peak Detector - it is a detector whose output voltage directly corresponds to t/max or<7min (Ov or Us). The peak detector is linear and can have an open (Fig. 3.20, a) or closed (Fig. 3.20, b) DC voltage input.

The principle of operation of peak detectors is specific and consists in charging the capacitor C through a diode V up to the maximum (peak) value Ux~ , which is then stored if the discharge time constant C (via R) much larger than the charge time constant. Switching polarity V defines a Ux= match, or Umax(Uin), or Umin(Un), and possible pulsations U x= are smoothed by a chain RF, SF. If the detector has an open input, U x= is determined by the sum U and Uin(Un), i.e. corresponds to Umax (Umin) With closed inlet U x= corresponds Uin(Un). If Ux~ does not contain a constant component, then the circuits shown in Fig. 3.20, a, b, are identical, and U x= corresponds um. In some cases, full-wave peak detectors with voltage doubling are used, allowing direct measurement of the voltage peak-to-peak value.

An essential advantage of peak detectors is their large input impedance (equal to R/2 for the circuit in fig. 3.20, a and R/3- for the circuit in fig. 3.20, b) and the best frequency properties compared to other types of detectors. Therefore, peak detectors are most often used in voltmeters of the first modification (see Fig. 3.14, o), being structurally designed together with the VU in the form of an external probe. In this case, the cable connecting the probe to the device transmits Ux=.

RMS detector - it is an AC to DC converter (voltage) proportional to U 2 ck. The detection characteristic in this case should be quadratic, and when on. If U- a detector with an open input is required. In modern types of voltmeters, mainly quadratic detectors with thermal converters are used, similar to converters of thermoelectric ammeters. Their main disadvantage, as noted earlier, is the quadratic character of the instrument scale. In voltmeters, this drawback is eliminated by using a differential circuit for switching on two (or more) thermal converters, as shown in Fig. 3.21.

Rice. 3.21. Structural diagram of the RMS voltage detector.

When the measured voltage is applied to the thermal converter TP1 Ux~ output voltage TP1 by analogy with (3.26) U 1 =k t U 2 sk.

In addition to TP1, the circuit has a second thermal converter TP2, which is connected opposite to TP1. A feedback voltage is applied to TP2, so it

output voltage U 2 == k t BU 2 3 .

Thus, at the input of the UPT, there is a resulting voltage

U 1 - U 2 = kt(U 2 sc - BU 2 3)

what does

U 3 \u003d k upt k t (U 2 sk - BU 2 3).

If the scheme parameters are chosen so that

k upt k t BU 2 3 >> U 3 ,

then finally U 3 º Uck, i.e. the DUT scale will be uniform.

Average rectified value detector - this is a converter of alternating voltage to direct current, proportional to Usv. Schematically, it is based on a full-wave semiconductor rectifier, considered in the analysis of rectifier ammeters (see § 3.4.1). However, it should be added that the linearity of the characteristics of such detectors will be the better, the more Ux~(for small Ux~ detector becomes quadratic). Therefore, detectors of the average rectified value, as a rule, are used in voltmeters of the second modification (Fig. 3.14, b).

Such voltmeters consist of an AC-to-DC converter, an amplifier, and a magnetoelectric measuring mechanism. There are two generalized block diagrams of AC voltmeters (Fig. 4.17), which differ in their characteristics. In voltmeters according to the scheme of Fig. 4.17, a measured voltage them first converted to DC voltage, which is then applied to UPT and THEM, which are essentially a DC voltmeter. Converter Etc is a low-inertia non-linear link, so voltmeters with such a structure can operate in a wide frequency range (from tens of hertz to 10 3 MHz).

Fig.4.17. Structural diagrams of AC voltmeters

In voltmeters made according to the scheme of Fig. 4.17, b, due to the preliminary amplification, it is possible to increase the sensitivity. However, the creation of high-gain AC amplifiers operating in a wide frequency range is a rather difficult technical problem. Therefore, such voltmeters have a relatively low frequency range (1 - 10 MHz); the upper limit of measurement at maximum sensitivity is tens or hundreds of microvolts.

Depending on the type of AC-to-DC converter, deviations of the pointer of the measuring mechanism of voltmeters can be proportional to amplitude(peak), average(medium rectified) or current measured voltage values.

Peak value voltmeters have amplitude value converters (peak detectors) with an open (Fig. 4.18, a) or closed (Fig. 4.19, a) inputs, where and in and and out- input and output voltage of the converter.

Rice. 4.18. Scheme (a) and timing diagrams of signals (b and c) of the amplitude value converter (peak detector)

with open entrance

In amplitude transducers with an open input, the capacitor is charged almost to the maximum them m ax of the positive (with a given diode on) value of the input voltage b). The voltage ripple u out on the capacitor is explained by its recharging with the diode open and the discharge through the resistor R with the diode closed.

Average output voltage and wed" and xtah and, consequently, the angle of deviation of the pointer of the measuring mechanism

(4.29)

where k y- voltmeter conversion factor.

A feature of open input amplitude converters is that they pass the DC component of the input signal (positive for the diode turn-on shown)

At and in \u003d U o + U m sin ωt average output voltage and СР ≈ U o + Um. Consequently,

(4.30)

Obviously, for U BX<0 подвижная часть THEM will not deviate, since in this case the diode is closed D.

Rice. 4.19. Scheme (a) and timing diagrams of signals (b)

closed input amplitude transducer

In converters with a closed input (Fig. 4.19, a, b) in steady state on a resistor R regardless of the presence of a constant component of the input signal, there is a ripple voltage u R varying from 0 to -2 Um where Um- amplitude of the variable component of the input voltage. The average value of this voltage is almost equal to U m . To reduce the ripple of the output voltage in such converters, a low-pass filter R f C f is installed. Thus, the voltmeter readings in this case are determined only by the amplitude value of the variable component of the input voltage them, i.e. a = k v U m .

Since the scale of voltmeters is calibrated in the effective values ​​of the sinusoidal voltage, then when measuring voltages of a different form, it is necessary to make an appropriate recalculation if the amplitude factor of the measured voltage is known. The amplitude value of the measured voltage of a non-sinusoidal form

where k ac =\u003d 1.41 - amplitude coefficient of the sinusoid; U np- voltage value, read on the scale of the device.

Effective value of the measured voltage

where k a- amplitude factor of the measured voltage.

Average value voltmeters have AC-to-DC converters similar to those used in rectifiers. Such voltmeters usually have the structure shown in Fig. 4.17, b. In this case, a pre-amplified voltage is applied to the rectifier converter them, which increases the sensitivity of voltmeters and reduces the effect of non-linearity of the diodes. The deviation angle of the moving part of the measuring mechanism for such voltmeters is proportional to the average rectified value of the measured voltage, i.e.

(4.33)

The scale of such voltmeters is also calibrated in the effective values ​​of the sinusoidal voltage. When measuring a non-sinusoidal voltage, the average value of this voltage

and the current one:

where U OL- voltmeter reading; k fs\u003d 1.11 - sinusoid form factor; k F is the form factor of the measured voltage.

RMS voltmeters have an AC voltage converter with a quadratic static conversion characteristic and OUT = k u IN 2 . As such a converter, thermal converters, squaring devices with a piecewise linear approximation of a parabola, vacuum tubes, and others are used. Moreover, if the effective value voltmeter is made according to the block diagrams shown in Fig. 4.17, then regardless of the shape of the curve of the measured voltage, the deviation of the measuring mechanism pointer is proportional to the square of the effective value of the measured voltage:

(4.36)