Millivoltmeter of direct and alternating currents and ohmmeter with a linear scale. High Frequency Linear Scale Millivoltmeter Variable Voltage Millivoltmeter

The high accuracy of measuring the magnitude of RF voltages (up to the third or fourth digit) in amateur radio practice is, in fact, not needed. The qualitative component is more important (the presence of a signal of a sufficiently high level - the more, the better). Usually, when measuring the RF signal at the output of the local oscillator (generator), this value does not exceed 1.5 - 2 volts, and the circuit itself is tuned to resonance according to the maximum value of the RF voltage. With settings in the IF paths, the signal rises in stages from units to hundreds of millivolts.

For such measurements, tube voltmeters are still often offered (type VK 7-9, V 7-15, etc.) with measurement ranges of 1-3V. High input impedance and low input capacitance in such devices is the determining factor, and the error is up to 5-10% and is determined by the accuracy of the pointer measuring head used. Measurements of the same parameters can be carried out using home-made pointer devices, the circuits of which are made on field-effect transistors. For example, in B. Stepanov's RF millivoltmeter (2), the input capacitance is only 3 pF, the resistance at various subranges (from 3 mV to 1000 mV), even in the worst case, does not exceed 100 kOhm with an error of +/- 10% (determined by the head used and instrumentation error for calibration). At the same time, the measured RF voltage with the upper limit of the frequency range of 30 MHz without an obvious frequency error, which is quite acceptable in amateur radio practice.

Because modern digital devices are still expensive for most radio amateurs, last year in the Radio magazine B. Stepanov (3) suggested using an RF probe for a cheap M-832 type digital multimeter with a detailed description of its circuit and application methods. Meanwhile, without spending any money at all, it is possible to successfully use pointer RF millivoltmeters, while freeing up the main digital multimeter for parallel measurements of current or resistance in the circuit being developed ...

In terms of circuitry, the proposed device is very simple, and a minimum of used components can be found “in the box” of almost every radio amateur. Actually, there is nothing new in the scheme. The use of DU for such purposes is described in detail in the amateur radio literature of the 80-90s (1, 4). The widely used K544UD2A (or UD2B, UD1A, B) microcircuit with field-effect transistors at the input (and hence with high input resistance) was used. You can use any operational amplifiers of other series with field devices at the input and in a typical connection, for example, K140UD8A. The technical characteristics of the millivoltmeter-voltmeter correspond to those given above, since B. Stepanov's circuit (2) became the basis of the device.

In the voltmeter mode, the gain of the op amp is 1 (100% OOS) and the voltage is measured by a microammeter up to 100 μA with additional resistances (R12 - R17). They, in fact, determine the subranges of the device in the voltmeter mode. When the OOS decreases (switch S2 turns on resistors R6 - R8) Kus. increases, the sensitivity of the operational amplifier increases accordingly, which allows it to be used in the millivoltmeter mode.

feature The proposed development is the ability to operate the device in two modes - a DC voltmeter with limits from 0.1 to 1000 V, and a millivoltmeter with upper limits of the subranges of 12.5, 25, 50 mV. In this case, the same divider (X1, X100) is used in two modes, so, for example, on the subrange of 25 mV (0.025 V) using the X100 multiplier, a voltage of 2.5 V can be measured. To switch the sub-ranges of the device, one multi-position two-board switch is used.

With the use of an external RF probe based on a GD507A germanium diode, it is possible to measure the RF voltage in the same subranges with a frequency of up to 30 MHz.

Diodes VD1, VD2 protect the pointer measuring device from overloads during operation. Another feature protection of the microammeter during transients that occur when the device is turned on / off, when the arrow of the device goes off scale and can even bend, is the use of a relay shutdown of the microammeter and closing the output of the op-amp to a load resistor (relays P1, C7 and R11). In this case (when the device is turned on), it takes a fraction of a second to charge C7, so the relay operates with a delay and the microammeter is connected to the output of the op-amp a fraction of a second later. When the device is turned off, C7 is discharged through the indicator lamp very quickly, the relay is de-energized and breaks the microammeter connection circuit before the power supply circuits of the op-amp are completely de-energized. Protection of the actual op-amp is carried out by switching on the input R9 and C1. Capacitors C2, C3 are blocking and prevent excitation of the OS. The device is balanced (“setting 0”) by a variable resistor R10 on the subrange of 0.1 V (it is possible on more sensitive subranges, but when the remote probe is turned on, the influence of the hands increases). Capacitors are desirable type K73-xx, but in their absence, ceramic 47 - 68n can also be taken. In the remote probe-probe, a KSO capacitor is used for an operating voltage of at least 1000V.

Setting millivoltmeter-voltmeter is carried out in this sequence. First set up the voltage divider. Operating mode - voltmeter. Trimmer resistor R16 (subrange 10V) is set to maximum resistance. On the resistance R9, controlling with an exemplary digital voltmeter, set the voltage from a stabilized power source of 10 V (position S1 - X1, S3 - 10v). Then, in position S1 - X100, trimming resistors R1 and R4 are set to 0.1v using a standard voltmeter. In this case, in position S3 - 0.1v, the microammeter needle should be set to the last mark on the instrument scale. The ratio 100/1 (the voltage across the resistor R9 - X1 - 10v to X100 - 0.1v, when the position of the arrow of the tuned device at the last division of the scale on the subrange S3 - 0.1v) is checked and corrected several times. In this case, a prerequisite: when switching S1, the exemplary voltage of 10V cannot be changed.

Further. In the DC voltage measurement mode, in the position of the divider switch S1 - X1 and the subrange switch S3 - 10v, the microammeter pointer is set to the last division with a variable resistor R16. The result (at 10 V at the input) should be the same instrument readings on the sub-range 0.1v - X100 and the sub-range 10v - X1.

The method for setting the voltmeter on the sub-ranges 0.3v, 1v, 3v and 10v is the same. In this case, the positions of the sliders of the resistors R1, R4 in the divider cannot be changed.

Operating mode - millivoltmeter. At the entrance 5 in. In position S3 - 50 mV, the divider S1 - X100 with resistor R8 sets the arrow to the last division of the scale. We check the readings of the voltmeter: on the subrange 10v X1 or 0.1v X100, the arrow should be in the middle of the scale - 5v.

The tuning procedure for the 12.5mV and 25mV subranges is the same as for the 50mV subrange. The input is 1.25v and 2.5v, respectively, at X 100. Checking the readings is carried out in the voltmeter mode X100 - 0.1v, X1 - 3v, X1 - 10v. It should be noted that when the arrow of the microammeter is in the left sector of the instrument scale, the measurement error increases.

Peculiarity such a technique for calibrating the device: it does not require an exemplary power supply of 12 - 100 mV and a voltmeter with a lower measurement limit of less than 0.1 V.

When calibrating the device in the mode of measuring RF voltages with an external probe for subranges of 12.5, 25, 50 mV (if necessary), you can build corrective graphs or tables.

The device is assembled by surface mounting in a metal case. Its dimensions depend on the dimensions of the measuring head used and the power supply transformer. For example, I have a bipolar power supply unit assembled on a transformer from an imported tape recorder (primary winding for 110v). It is best to assemble the stabilizer on MS 7812 and 7912 (or LM317), but it can also be simpler - parametric, on two zener diodes. The design of the remote RF probe and the features of working with it are described in detail in (2, 3).

Used Books:

1. B.Stepanov. Measurement of small RF voltages. Zh. "Radio", No. 7, 12 - 1980, p.55, p.28.
2. B.Stepanov. High frequency millivoltmeter. Zh. "Radio", No. 8 - 1984, p.57.
3. B.Stepanov. RF head to digital voltmeter. Zh. "Radio", No. 8, 2006, p.58.
4. M. Dorofeev. Voltmeter on the OU. Zh. "Radio", No. 12, 1983, p.30.

Vasily Kononenko (RA0CCN).

These instruments are mainly used for measuring small voltages. Their maximum measurement limit is 1÷10 mV, internal resistance is about 1÷10 mΩ.

The input voltage is supplied to a three-section L-shaped FS filter, the purpose of which is to reduce interference of industrial frequency - 50 Hz in the input signal.

Then the voltage is modulated, amplified by the amplifier Y 1, consisting of Y "(1st and 2nd stages) and Y" (3rd - 5th stages), then demodulated, fed to a matching amplifier Y 2 , which is made according to the scheme of a cathode follower and serves to match the resistance μA with the resistance Y 2 . Voltage is measured in μA (100 μA), the scale of which is graduated in units of voltage.

A vibration transducer was used as a modulator. DM - diode ring demodulator.

The feedback circuit serves to stabilize the gain and change it when switching the measurement limits.

The switch of measurement limits, in addition to the OS link, includes a DN voltage divider located between the second and third stages Y 1 .

LFO - carrier frequency generator provides voltage supply to M and DM.

According to this scheme, a DC voltmeter of type B2-11 was built with measurement limits
V, internal resistance 10÷300 mΩ and error 6÷1%.

Universal voltmeters

At Universal voltmeters are built according to a scheme called the "rectifier-amplifier" scheme. An important part of the circuit is the rectifier "B". As a rule, in universal voltmeters, V amplitude values ​​are used, built according to a half-wave rectification circuit (since it is impossible to create a grounded bus in the case of full-wave rectification) with an open or closed input, but, as a rule, a circuit with a closed input is used, which is explained by the independence of the voltage on its output from the constant component at the input.

Universal voltmeters have a wide frequency range, but relatively low sensitivity and accuracy.

Universal voltmeters V7-17, V7-26, VK7-9 and others have become widespread. Their basic error reaches ±4%. Frequency range up to 10 3 MHz. Measurement limits from 100÷300 mV to 10 3 V.

AC Voltmeters

PPI - switch of measurement limits.

Electronic AC voltmeters are mainly intended for measuring low voltages. This is due to their "amplifier-rectifier" structure, i.e. pre-amplification of the voltage. These devices have a high input impedance due to the introduction of circuits with deep local feedbacks, including cathode and emitter followers: rectifiers of average, amplitude and effective value are used as VP. The scale, as a rule, is graduated in units of the effective value, taking into account the ratios
and
for sinusoidal voltages. If the scale is calibrated to U Wed or U t, then it has the corresponding designations.

In general, devices according to the "amplifier-rectifier" scheme have greater sensitivity and accuracy, but their frequency range is narrowed, it is limited by the U amplifier.

If B is used for the average or amplitude value, then the devices are critical to the shape of the input voltage curve when grading the scale in units. U d .

When using the average B, it is usually performed in a full-wave rectification scheme. When using an amplitude detector - according to the scheme with open or closed inputs.

A feature of electronic voltmeters of the current value is the squareness of the scale due to the presence of a squaring device in V. There are special methods to eliminate this drawback.

AC millivoltmeters of the V3-14, V3-88, V3-2, etc. types have become widespread.

Among electronic voltmeters, the diode compensation voltmeter (DKV) has the highest accuracy. Its error does not exceed hundredths of a percent. The principle of operation is explained by the following diagram.

NI - null indicator

When applying
and offset compensation voltage the latter can be adjusted so that NI shows 0. Then we can assume that
.

Pulse voltmeters

Pulse V are designed to measure the amplitudes of periodic pulses of signals with a large duty cycle and the amplitudes of single pulses.

The difficulty of measurement lies in the variety of pulse shapes and a wide range of changes in temporal characteristics.

All this is not always known to the operator.

The measurement of single pulses creates additional difficulties, since it is not possible to accumulate information about the measured value by repeated exposure to the signal.

Impulse V are built according to the above scheme. Here PAI is a converter of amplitude and impulse into voltage. This is the most important block. In a number of cases, it provides not only the specified transformation and the storage of the converted value during the reference time.

Most often, diode-capacitor peak detectors are used in PAI. The peculiarity of these detectors is that the pulse duration τ U may be small, but the duty cycle - large. As a result for τ U"C" will not be fully charged, and for "T" it will be significantly discharged.

Homemade measuring instruments

Main parameters:

Range of measured voltages, mV 3...5*І0^3;

Operating frequency range, Hz 30.. .30* 10^3;

Frequency response unevenness, dB ±1;

Input resistance, mOhm:

on "within 10, 20, 50 mV 0.1;

within 100 "mV .. .5 V 1.0;

Measurement error, % 10.

Device diagram

The device consists of an input emitter follower (transistors V1, V2), an amplifying stage - (transistor V3) and an AC voltmeter (transistors V4, V5, diodes V6-V9 and microammeter P1).

The measured alternating voltage from connector X1 is fed to the input emitter follower through a voltage divider (resistors R1, R2* and R22), with which this voltage can be reduced by 10 or 100 times. A 10-fold decrease occurs when switch S1 is set to X 10 mV (the divider is formed by resistor R1 and resistor R22 connected in parallel and the input resistance of the emitter follower). Resistor R22 is used to accurately set the input resistance of the device (100 kOhm). When switch S1 is set to X 0.1 V, 1/100 of the measured voltage is supplied to the input of the emitter follower.

The lower arm of the divider in this case consists of the input resistance of the follower and resistors R22 and R2*.

At the output of the emitter follower, another voltage divider is included (switch S2 and resistors R6-R8), which makes it possible to attenuate the signal that is fed further to the amplifier.

The next stage of the millivoltmeter - the voltage amplifier AF on the transistor V3 (gain of about 30) - provides the ability to measure low voltages / From the output of this stage, the amplified voltage 34 is fed to the input of the AC voltage meter with a linear scale, which is a two-stage amplifier (V4, V5) covered by negative feedback through the rectifier bridge (V7-V10). A microammeter P1 is included in the diagonal of this bridge.

The non-linearity of the scale of the described voltmeter in the range of marks 30 ... 100 does not exceed 3%, and in the working area (50 ... 100) -2%. When calibrating, the sensitivity of the millivoltmeter is adjusted by the resistor R13.

The device can use any low-frequency low-power transistors with a static current transfer coefficient h21e = 30...60 (at an emitter current of 1 mA). Transistors with a large coefficient h21e should be installed in place of V1 and V4. Diodes V7-V10 - any germanium from the D2 or D9 series.

The KS168A zener diode can be replaced by two KS133A zener diodes by turning them on in series. The device uses capacitors MBM (C1), K50-6 (all others), fixed resistors MLT-0.125, trimmer SPO-0.5.

Switches S1 and S2 (sliding, from the Sokol transistor radio) are modified so that each of them becomes two-pole in three positions: in each row, the extreme fixed contacts (two moving contacts) are removed, and the remaining moving contacts are rearranged in accordance with the diagram switching.

The adjustment of the device is reduced to the selection of the modes indicated on the diagram by resistors marked with an asterisk, and the graduation of the scale according to the exemplary Device.

I needed an accurate AC millivoltmeter, I really didn’t want to be distracted by searching for a suitable circuit and picking up parts, and then I took and bought a ready-made set “AC Millivoltmeter”. When I delved into the instructions, it turned out that I had only half of what I needed on my hands. I left this venture and bought an ancient, but in almost excellent condition, LO-70 oscilloscope at the market and did everything perfectly. And since over the next time I got pretty tired of shifting this bag with the designer from place to place, I decided to assemble it anyway. There is also curiosity about how good it will be.

The kit includes a K544UD1B microcircuit, which is an operational differential amplifier with high input resistance and low input currents, with internal frequency correction. Plus a printed circuit board with two capacitors, with two pairs of resistors and diodes. There is also an assembly instruction. Everything is modest, but no offense, a set costs less than one chip from it in retail.

A millivoltmeter assembled according to this scheme allows you to measure voltage with limits:

• 1 - up to 100 mV
• 2 - up to 1 V
• 3 - up to 5 V

In the range of 20 Hz - 100 kHz, input impedance about 1 MΩ, supply voltage
from + 6 to 15 V.

The printed circuit board of the AC millivoltmeter is shown from the side of the printed tracks, for “drawing” in Sprint-Layout (“mirroring” is not necessary), if necessary.

The assembly began with changes in the component composition: I put a socket under the microcircuit (it will be safer), the ceramic capacitor was changed to a film capacitor, the denomination was naturally the same. One of the D9B diodes fell into disrepair during installation - it soldered all the D9I, since the last letter of the diode is not spelled out in the instructions at all. The ratings of all components installed on the board were measured, they correspond to those indicated in the circuit (for electrolyte).

The kit included three resistors with a nominal value of R2 - 910 Ohm, R3 - 9.1 kOhm and R4 - 47 kOhm, however, there is a clause in the assembly manual that their values ​​\u200b\u200bmust be selected during the setup process, so I immediately set the trimming resistors to 3, 3 kOhm, 22 kOhm and 100 kOhm. They needed to be mounted on any suitable switch, I took the available brand PD17-1. It seemed very convenient, miniature, there is something to attach to the board, it has three fixed switching positions.

As a result, I placed all the nodes from the electronic components on the circuit board, connected them to each other and connected them to a low-power AC source - the TP-8-3 transformer, which would supply a voltage of 8.5 volts to the circuit.

And now the final operation - calibration. A virtual one was used as an audio frequency generator. A computer sound card (even the most mediocre one) copes quite well with frequencies up to 5 kHz. At the input of the millivoltmeter, a signal with a frequency of 1000 Hz is supplied from the audio frequency generator, the effective value of which corresponds to the limiting voltage of the selected subrange.

The sound is taken from the headphone jack (green). If, after connecting to the circuit and turning on the virtual sound generator, the sound “does not go” and even after connecting the headphones it will not be heard, then in the “start” menu, hover over “settings” and select “control panel”, where select “sound effects manager ” and in it click on “S / PDIF Output”, where several options will be indicated. Ours is the one with the words "analog output". And the sound will go.

The subrange “up to 100 mV” was chosen and with the help of a tuning resistor, the arrow was deflected by the final division of the microammeter scale (you do not need to pay attention to the frequency symbol on the scale). The same has been successfully done with other subranges. Manufacturer's instructions in the archive. Despite its simplicity, the radio designer turned out to be quite efficient, and what I especially liked was adequate in setting. In a word, the set is good. Putting everything in a suitable case (if necessary), installing connectors and so on will be a matter of technique.

Discuss the article AC MILLIVOLTMETER

This article focuses on two voltmeters implemented on the PIC16F676 microcontroller. One voltmeter has a voltage range of 0.001 to 1.023 volts, the other, with an appropriate 1:10 resistive divider, can measure voltages from 0.01 to 10.02 volts. The current consumption of the entire device with a stabilizer output voltage of +5 volts is approximately 13.7 mA. The voltmeter circuit is shown in Figure 1.

Two voltmeter circuit

Digital voltmeter, circuit operation

To implement two voltmeters, two outputs of the microcontroller are used, configured as input for the digital conversion module. The RA2 input is used to measure low voltages, in the region of a volt, and a 1:10 voltage divider is connected to the RA0 input, consisting of resistors R1 and R2, which allows you to measure voltages up to 10 volts. This microcontroller uses ten-bit ADC module and in order to implement a voltage measurement with an accuracy of 0.001 volts for a range of 1 V, it was necessary to apply an external reference voltage from the ION of the DA1 K157XP2 microcircuit. Since the power AND HE the microcircuit is very small, and in order to exclude the influence of external circuits on this ION, a buffer op-amp on the DA2.1 microcircuit was introduced into the circuit LM358N. This is a non-inverting voltage follower with 100% negative feedback - OOS. The output of this op-amp is loaded with a load consisting of resistors R4 and R5. From the trimmer resistor R4, a reference voltage of 1.024 V is applied to pin 12 of the microcontroller DD1, configured as a reference voltage input for operation ADC module. At this voltage, each bit of the digitized signal will be equal to 0.001 V. To reduce the effect of noise, another voltage follower, implemented on the second op amp of the DA2 chip, was used when measuring small voltage values. The OOS of this amplifier sharply reduces the noise component of the measured voltage value. The voltage of impulse noise of the measured voltage also decreases.

A two-line LCD was used to display information about the measured values, although one line would be enough for this design. But having the ability to display some more information in reserve is also not bad. The brightness of the indicator backlight is regulated by resistor R6, the contrast of the displayed characters depends on the value of the resistors of the voltage divider R7 and R8. The device is powered by a voltage regulator assembled on the DA1 chip. The +5 V output voltage is set by resistor R3. To reduce the total current consumption, the supply voltage of the controller itself can be reduced to a value at which the indicator controller would remain operational. When checking this circuit, the indicator worked steadily at a microcontroller supply voltage of 3.3 volts.

Voltmeter setting

Setting up this voltmeter requires at least a digital multimeter capable of measuring 1.023 volts to set the reference voltage of the reference. And so, using a control voltmeter, we set a voltage of 1.024 volts at pin 12 of the DD1 microcircuit. Then, at the input of the op-amp DA2.2, pin 5, we apply a voltage of a known value, for example, 1,000 volts. If the readings of the control and adjustable voltmeters do not match, then the trimming resistor R4, by changing the value of the reference voltage, achieves equivalent readings. Then, a control voltage of a known value is applied to the input U2, for example, 10.00 volts, and by selecting the resistance value of the resistor R1, it is possible and R2, or both can achieve equivalent readings of both voltmeters. This completes the adjustment.