RF voltmeter with linear scale. Voltmeter circuit to measure signal Voltmeter make operational amplifier

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. It 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. Output voltage+5 V 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.

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 is sufficient high level- the bigger, 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.

When setting up local oscillators, IF paths, lamp voltmeters (such as VK 7-9, V7-15, etc.) with measurement ranges of 1 - 3v are still often used. 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 microcircuits with field effect transistors at the entrance. 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). In this case, the measured RF voltage with upper bound frequency range of 30 MHz without a clear frequency error, which is quite acceptable in amateur radio practice.

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. Specifications millivoltmeter-voltmeter correspond to the above, since the basis of the device was the B. Stepanov circuit (2).

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.

A feature of the proposed development is the ability to operate the device in two modes - a voltmeter direct current with limits from 0.1 to 1000 V, and a millivoltmeter with upper limits of 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 of protecting the microammeter during transients that occur when the device is turned on and 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 the millivoltmeter-voltmeter is carried out in the following 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 the exemplary digital voltmeter, set the voltage from a stabilized power source 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. Wherein required condition: when switching S1, the reference voltage of 10V cannot be changed.

Further. In measurement mode constant voltage in the position of the divider switch S1 - X1 and the subrange switch S3 - 10v, the microammeter needle 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.

The peculiarity of this technique for calibrating the device is that 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. In the above diagram, a bipolar power supply unit, assembled on a transformer from an imported tape recorder, works (primary winding for 110v). The stabilizer is best assembled on MS 7812 and 7912 (or two LM317), but it can be easier - 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.

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 detailed description its schemes and methods of application. 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).

Quite a few motorists are faced with such a problem as an unexpected battery discharge. It is especially frustrating when this happens on a journey far from home. One of the reasons may be the failure of the auto generator. Help prevent impending battery drain car voltmeter. Below are a few simple circuits similar device.

Automotive voltmeter on the LM3914 chip

This car voltmeter circuit is designed to control the voltage of the car's on-board network in the range from 10.5V to 15V. 10 LEDs are used as an indicator.

The basis of the circuit is integrated. This microcircuit is able to evaluate the input voltage and display the result on 10 LEDs in dot or column mode. The LM3914 chip is capable of operating in a wide power range (3V ... 25V). The brightness of the LEDs can be set using an external variable resistor. The outputs of the microcircuit are compatible with TTL and CMOS logic.

Ten LEDs VD1-VD10 display the current value of the battery voltage or the voltage of the car's on-board network in point mode (pin 9 is not connected or connected to minus) or column (pin 9 is connected to plus power).

Resistor R4 connected between pins 6.7 and minus the power supply sets the brightness of the LEDs. Resistors R2 and variable resistor R1 form a voltage divider. Using the variable resistor R1, the upper voltage level is adjusted, and with the help of R3 the lower one.

As mentioned earlier, this automotive voltmeter provides an indication of 10.5 to 15 volts. Circuit calibration is performed as follows. Apply a voltage of 15 volts from the power supply to the input of the voltmeter circuit. Then, by changing the resistance of the resistor R1, it is necessary to ensure that the VD10 LED lights up (in point mode) or all VD ... VD10 LEDs (in column mode).

Then apply 10.5 volts to the input and use the variable resistor R3 to ensure that only the VD1 LED is lit. Now, increasing the voltage in 0.5 volt increments, the LEDs will light up one by one, and at 15 volts, all the LEDs will light up. Switch SA1 is designed to switch between dot/column indication modes. When the switch SA1 is closed, it is a column, when it is open, it is a dot.

Automobile voltmeter on transistors

The following automotive voltmeter circuit is built on two. When the battery voltage is less than 11 volts, the zener diodes VD1 and VD2 do not pass current, which is why only the red LED is on, indicating a low voltage of the car's on-board network.

If the voltage is between 12 and 14 volts, the zener diode VD1 opens the transistor VT1. The green LED lights up to indicate normal voltage. If the battery voltage exceeds 15 volts, the VD2 zener diode opens the VT2 transistor, as a result of which the yellow LED lights up, indicating a significant excess of voltage in the vehicle network.

Voltmeter on the operational amplifier LM393

This simple automotive voltmeter is based on an operational amplifier. As an indicator, as in the previous scheme, three LEDs are used.

When the voltage is low (less than 11V), the red LED lights up. If the voltage is normal (12.4 ... 14V), then the green light is on. In the event that the voltage exceeds 14V, the yellow LED lights up. Zener diode VD1 generates a reference voltage. This scheme is similar to the scheme.

Automotive voltmeter on the chip K1003PP1

This voltmeter circuit for a car is built on the K1003PP1 chip and allows you to track the voltage of the on-board network by the glow of 3 LEDs:

• At a voltage of less than 11 volts, the HL1 LED is on.
• At a voltage of 11.1 ... 14.4 volts, the HL2 LED is on
• At a voltage of more than 14.6 volts, the HL3 LED is on

Setting. After applying voltage to the input from any power supply (11.1 ... 14.4V), the variable resistor R4 must make the HL2 LED glow.

RF voltmeter with linear scale
Robert AKOPOV (UN7RX), Zhezkazgan, Karaganda region, Kazakhstan

One of the necessary devices in the arsenal of a shortwave radio amateur, of course, is a high-frequency voltmeter. Unlike a low-frequency multimeter or, for example, a compact LCD oscilloscope, such a device is rarely found on sale, and the cost of a new branded one is quite high. Therefore, when there was a need for such a device, it was built, moreover, with a dial milliammeter as an indicator, which, unlike a digital one, allows you to easily and visually evaluate changes in readings quantitatively, and not by comparing the results. This is especially important when setting up devices where the amplitude of the measured signal is constantly changing. At the same time, the measurement accuracy of the device when using a certain circuitry is quite acceptable.

There is a typo in the diagram in the magazine: R9 should be a resistance of 4.7 MΩ

RF voltmeters can be divided into three groups. The first ones are built on the basis of a broadband amplifier with the inclusion of a diode rectifier in the negative feedback circuit. The amplifier ensures the operation of the rectifier element in the linear section of the current-voltage characteristic. In devices of the second group, the simplest detector with a high-resistance DC amplifier (UPT). The scale of such an RF voltmeter at the lower measurement limits is non-linear, which requires the use of special calibration tables or individual calibration of the device. An attempt to somewhat linearize the scale and shift the sensitivity threshold down by passing a small current through the diode does not solve the problem. Before the beginning of the linear section of the I–V characteristic, these voltmeters are, in fact, indicators. Nevertheless, such devices, both in the form of finished designs and attachments to digital multimeters, are very popular, as evidenced by numerous publications in magazines and on the Internet.
The third group of instruments uses scale linearization, when the linearizing element is included in the DCF circuit to provide the necessary gain change depending on the input signal amplitude. Such solutions are often used in professional equipment units, for example, in broadband high-linear instrumentation amplifiers with AGC, or AGC units of broadband RF generators. It is on this principle that the described device is built, the circuit of which, with minor changes, is borrowed from.
With all the obvious simplicity, the RF voltmeter has very good parameters and, of course, a linear scale that eliminates calibration problems.
The measured voltage range is from 10 mV to 20 V. The operating frequency band is 100 Hz…75 MHz. The input resistance is at least 1 MΩ with an input capacitance of no more than a few picofarads, which is determined by the design of the detector head. The measurement error is no worse than 5%.
The linearizing unit is made on the DA1 chip. Diode VD2 in the negative feedback circuit helps to increase the gain of this stage of the UPT at low input voltages. The decrease in the output voltage of the detector is compensated, as a result, the readings of the device acquire a linear dependence. Capacitors C4, C5 prevent self-excitation of the UPT and reduce possible pickups. Variable resistor R10 is used to set the arrow measuring instrument PA1 to the zero mark of the scale before taking measurements. In this case, the input of the detector head must be closed. The power supply of the device has no special features. It is made on two stabilizers and provides a bipolar voltage of 2 × 12 V for powering operational amplifiers (the network transformer is not conventionally shown in the diagram, but is included in the assembly kit).

All parts of the device, with the exception of parts of the measuring probe, are mounted on two printed circuit boards ah from one-sided foil fiberglass. Below is a photograph of the UPT board, the power board and the measuring probe.

Milliammeter RA1 - M42100, with a current of full deflection of the needle 1 mA. Switch SA1 - PGZ-8PZN. Variable resistor R10 - SP2-2, all tuning resistors - imported multi-turn, for example 3296W. Resistors of non-standard ratings R2, R5 and R11 can be made up of two connected in series. Operational amplifiers can be replaced by others with high input impedance and preferably with internal correction (so as not to complicate the circuit). All fixed capacitors are ceramic. Capacitor C3 is mounted directly on the input connector XW1.
The D311A diode in the RF rectifier was chosen from the point of view of the optimal maximum allowable RF voltage and rectification efficiency at the upper measured frequency boundary.
A few words about the design of the instrument's measuring probe. The body of the probe is made of fiberglass in the form of a tube, on top of which a copper foil screen is put on.

Inside the case there is a board made of foil fiberglass, on which the probe parts are mounted. A ring of tinned foil strip approximately in the middle of the body is provided to make contact with the common wire of a detachable divider, which can be screwed on in place of the probe tip.
The adjustment of the device begins with the balancing of the op-amp DA2. To do this, switch SA1 is set to the "5 V" position, the input of the measuring probe is closed, and the pointer of the device PA1 is set to the zero mark of the scale with a trimming resistor R13. Then the device is switched to the “10 mV” position, the same voltage is applied to its input, and the arrow of the RA1 device is set to the last division of the scale with the resistor R16. Next, a voltage of 5 mV is applied to the input of the voltmeter, the arrow of the device should be approximately in the middle of the scale. The linearity of the readings is achieved by selecting the resistor R3. Even better linearity can be achieved by selecting the resistor R12, however, it should be borne in mind that this will affect the gain of the UPT. Next, the device is calibrated on all subranges with the corresponding tuning resistors. As a reference voltage when calibrating the voltmeter, the author used an Agilent 8648A generator (with a 50 Ohm load equivalent connected to its output), which has a digital output signal level meter.
The entire article from the magazine Radio No. 2, 2011 can be downloaded from here
LITERATURE:
1. Prokofiev I., Millivoltmeter-Q-meter. - Radio, 1982, No. 7, p. 31.
2. Stepanov B., RF head for a digital multimeter. - Radio, 2006, No. 8, p. 58, 59.
3. Stepanov B., Schottky diode RF voltmeter. - Radio, 2008, No. 1, p. 61, 62.
4. Pugach A., High-frequency millivoltmeter with a linear scale. - Radio, 1992, No. 7, p. 39.

The cost of printed circuit boards (probe, main board and power supply board) with a mask and marking: 80 UAH