We recommend repeating. Attachment to a multimeter for measuring RF voltages - Measurements - Other - Catalog of articles and diagrams

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.

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 input. 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.

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.

A feature of 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 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. 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.

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.

One of the most necessary devices in the arsenal of a shortwave radio amateur is certainly a high-frequency voltmeter.
Unlike low-frequency multimeters and inexpensive, compact LCD oscilloscopes, such devices are much rarer, and new, branded ones are also quite expensive.
Therefore, it was decided to assemble a home-made device, taking into account the usual requirements.

When choosing a display option, I settled on analog. Unlike digital, analog indication allows you to easily and visually evaluate changes in readings quantitatively, and not just by comparing results. This is especially important when setting up circuits where the amplitude of the measured signal is constantly changing.
At the same time, the accuracy of measurements with the appropriate circuitry is quite sufficient.

As a rule, there are two types of RF voltmeters. Firstly, broadband amplifiers are used, which ensure the operation of the detector element in the linear section of the current-voltage characteristic, or by including a rectifier in the OOS circuit of such an amplifier.

Secondly, a simple detector is used, sometimes with a high TCA. The scale of such an RF voltmeter is non-linear at the lower measurement limits and requires the use of special tables or individual calibration of the scale.
An attempt to linearize the scale to some extent, as well as to shift the sensitivity threshold down, by passing a small current through the diode, does not solve the problem. The resulting RF voltmeters before the start of the linear sections of the CVC remain, in fact, indicators. Nevertheless, such RF voltmeters, both in the form of complete instruments and in the form of attachments to digital multimeters, are very popular, as evidenced by numerous publications in magazines and on the Internet.

There is another way to linearize the measuring scale, when a linearizing element is included in the DCF circuit, providing the necessary gain change depending on the amplitude of the input signal.
Such circuits are often used in professional equipment units, for example, in broadband high-linear instrumentation amplifiers with AGC. It was on the basis of such a solution that the device described here was created.

The author of this article first assembled such a device around the years of its publication, recently reassembled, moved to another case, on new printed circuit boards and for new components.
With all the obvious simplicity of the circuit, this RF voltmeter provides very good parameters.
The range of measured voltages (final divisions of the scale) is from 10mV to 20V. Frequency range from 100Hz to 75MHz, input impedance not less than 1MΩ, with input capacitance not more than a few pF (determined mainly by the design of the RF head). And, of course, it has a linear scale, eliminating problems with graduation. Measurement accuracy, with high-quality settings, is not worse than 5%.

The scheme of the device is shown in Figure 1.

Rice. 1

Structurally, the device consists of three parts. Measuring detector (HF head), UPT board with linearization unit and stabilizer board.
The linearizing unit is made on the OP1 chip with a diode in the OOS circuit. Due to the presence of the diode D2 in the negative feedback circuit, the gain of this stage of the UPT increases at low input voltages. Due to this, the decrease in the output voltage of the detector is compensated and the scale of the device is linear.

Capacitors C4, C5 prevent self-excitation of the UPT and reduce possible pickups.
The device used in the voltmeter for a current of 1mA.
Resistors of non-standard ratings consist of 2. You can use any op amp with a high input impedance. Capacitor C3 is mounted directly on the input BNC connector.
Resistor R7 happens to promptly set the head arrow to 0. In this case, the RF head must be closed at the input.
Setting up the device begins with balancing the amplifier on the op-amp OP2. To do this, the measurement limit switch is set to 5V, the RF head is closed and the instrument needle is set to 0 with a tuning resistor R13. Next, we switch to 10mV, apply the same voltage, set the arrow to the last division of the scale with resistor R14. We apply 5mV to the input, the arrow should be approximately in the middle of the scale. We achieve linearity by selecting the resistor R2.
Next, we calibrate the device on all subranges with the corresponding tuning resistors.

Appearance of the finished device:

RF detector head

Drawings of printed circuit boards of a voltmeter and stabilizers can be taken

V. Kostychev, UN8CB.

Petropavlovsk.

This simple device allows you to measure the effective (effective) value of the voltage and power of RF oscillations, both sinusoidal and modulated, as well as, with the improvement of the device, and peak power. The basis of this device is a simple diode high-frequency voltmeter, which is used in SWR meters, as well as in imported devices SX-100, SX-200. Such a similar diode voltmeter is also used in the BB-10 device, the diode of which is supplied with high-frequency voltage through a current transformer (Fig. 1).

(Details of blue color are installed additionally for the peak indicator, with the improvement of the device). When the device is operating in the mode of an absorbing power meter, a load resistor Rl is connected to the “ANT” connector by switch S1. When operating in the transmission power meter mode Rn, the antenna is turned off and the antenna is connected. Switch S2 sets the measurement limit to 100 W or 500 W.

For current transformer T1, a ring 1000NN-2000NN with a diameter of 12-16 mm is used, winding with PEL wire 0.5; 4 - 5 turns. A sufficiently thick insulated wire is passed through the ring of the transformer T1, connecting the "ANT" and "PER" connectors, located about 5 cm apart on the rear wall of the device. Microammeter RA - type M2001 with a total deflection current of 100 μA. The load resistor consists of 30 MLT-1.5 k resistors, 2 W power (total resistance 50 ohms). The total power Rn - 60 watts. Resistors are soldered between two boards made of foil fiberglass. (Fig.2).

Mounting of parts of the device hinged, using reference points, in a case of a suitable size

The scale of the device is graduated in volts and watts. To do this, an RF voltmeter (type B7-15) is connected in parallel with Rn. The transmitter is connected to the “PER” connector, the switch S2 is set to the 100 W position. The carrier transmission mode is switched on at a frequency of 14 MHz, by gradually increasing the output power, set the RF voltage to Rn equal to 70.7 V, which will correspond to a power of 100 W. Resistor R3 sets the arrow of the microammeter to the last mark of the scale - 100 μA. Decreasing the output power of the transmitter, we determine the readings of the microammeter for other power values, based on the expression: Рeff = (Ueff)2/Rn. The result is entered in the calibration table 1.

Table 1.

To calibrate the scale at the limit of 500 W, switch S2 to the 500 W position, set the transmitter power to 100 W and, with the resistor R4, fix the microammeter needle at 44.5 μA. Then, reducing the transmitter power, and then increasing it, calibrate the rest of the scale for this limit. This table can be used later when working with the device. You can stick it on the top cover.

When working with the device, you need to remember that Rн is designed for a power of 60 W, therefore, at high powers, the measurement time should not be long, with interruptions.

The operating instructions for the SX-100, SX-200 devices state that these devices are not capable of showing all 100% of peak power, but only 70% - 90%. Also, a significant drawback of the SX-100, SX-200 devices is the lack of a more or less long-term fixation of readings when measuring the usual conversational peak power, which makes it difficult to read it. In the BB-10 device, these shortcomings are eliminated if you use the peak indicator in the form of an additional attachment to the BB-10 on an operational amplifier, for example, which is offered by DJ7AW (Radio No. 7, 2011, p. 63). Such a peak indicator was tested and showed good results. Fig.3.

To connect it in the circuit in Fig. 1, it is necessary to make some changes. The switch S3 is switched on in the gap between the points "a-a" and connected, as shown in the diagram in Fig. 1 in blue. Position 1 of switch S3 measures the effective power, while position 2 measures the peak power. In the peak power measurement mode, a constant voltage from the rectifier of the voltmeter-wattmeter is fed through the operational amplifier DA1.1 to the peak detector VD1, R4, C2. The time constant of this detector (about 6.8 s) is quite sufficient to record the usual conversational peak power. The repeater on the operational amplifier DA1.2 eliminates the shunting of the load of the peak detector, which allows you to increase the time of fixing the readings of the measuring device. The peak indicator is assembled on a scarf 45x38 mm in size, on patches by hinged mounting, fig. 4.

The blue color indicates a piece of wire in insulation (instead of a track), passed under the socket for the microcircuit soldered to the contact pads. Capacitor C2 is non-polar. The board is connected to points A and B of the circuit in Fig. 1. One thing is bad, to power this circuit you need a 12V power source.

The journal does not provide a method for setting and grading this peak indicator. I did this on the basis that in linear mode the effective power and the peak power of the sinusoidal oscillation (carrier) are equal, and the peak power of the modulated signal when pronouncing a moderate sound “aaa” in front of the microphone is approximately equal to the effective power of the carrier. The voltage level supplied from the detector to the DA1 opamp must be such that it does not enter saturation mode. To do this, the R1 engine was set to approximately 1/3 of its resistance from the "ground". Calibration when measuring the peak power of the modulated signal (S3 in position 2) is performed by resistor R6 (with a transmitter output power of about 100 W) in the long “a-a-a” mode, with which the microammeter readings are set the same as when measuring the effective power in carrier mode (S3 in position 1). Then, when measuring the peak power of modulated oscillations, a more or less real result should be obtained. For the BB-10 device, this figure is about 95%.

high frequency voltmeter

http://*****/vom. htm

We measure the RF voltage with a digital multimeter.

To measure RF voltages with a conventional DC voltmeter, you can make a detector attachment to it. Such an attachment allows you to measure RF voltages from several hundred millivolts to the breakdown voltage of diodes in the attachment. On Fig. 1 shows two types of simple diode detectors, series and parallel. One can argue about the advantage of one or the other for a long time, but parallel is more convenient, given the design of the microwave diodes used in the set-top box, which should be directly fixed to the device case with one output. The design of the device for measuring RF voltages can be done in different ways, but the details of the left part of the circuit, separated by the curve, must be connected to each other by short leads. A resistor (active) equivalent load (for example, 50 Ohm) can be connected in parallel with the input to measure the voltage at the loaded outputs of amplifiers and signal sources.

Fig.1. Basic schemes of detectors. When measuring power in parallel

the input should be connected to a load equivalent of 50 or 75 Ohm

RF - HF. GND - Case, "ground". Shunt detector - parallel detector.

Series detector - serial detector. Keep these leads short

the conclusions of the details here should be short. 10Megohm Voltmeter -

DC voltmeter with 10 MΩ input impedance. Output

Voltage - output voltage. Input Voltage - input voltage.

You can also use diodes: 1N21, 1N23, 1N830…833, HP diodes

5082-28xx, 5082-23xx, 5082-29xx or other point diodes,

low-signal Schottky barrier diodes.

For detector use, a small signal Schottky barrier diode such as the available 1N5711 is well suited, however germanium diodes such as 1N60, 1N34 or even 1N270 also provide excellent sensitivity. The commonly used 1N914 silicon diode can also be used in the detector, but it. will cause an error of approximately 300 mV (due to the “step”, below which the diode does not perform its functions - U.A.9 LAQ) when measuring the amplitude values of the RF voltage. In this case, the error with the 1N5711 diode will be about 100 mV, and with germanium diodes - 60 mV. When measuring rather high RF voltages, the result is reduced by the amount of error. When measuring voltages below the double voltage of the “step”, the diode attachment begins to give significant errors (nonlinearity of detection - U.A.9 LAQ). On Fig. Figure 2 shows how to eliminate the "step", as a result - the voltmeter shows the correct voltage value, starting from a few hundred millivolts.

Fig.2. Sequential type detector with “step” correction.

The 82 kΩ resistance can be changed to get the best

compensation individually for each type and copy of the diode. Another

the diode can be installed in place of the 82 kΩ resistor (or in

combination with it) to provide thermal compensation (voltage

“steps” depends on the temperature). 82k with 1N5711 (select for zero

offset) - when using a 1N5711 diode, the resistance of the resistor is

approximately 82 kOhm and is selected according to full voltage compensation

"steps".

The battery and resistors provide a negative (compensating - U.A.9 LAQ) a voltage of about 100 mV, which is well suited for 1N5711. Other diodes require a different compensating voltage, and the 82 kΩ resistor can be adjusted to obtain an accurate RF voltage reading down to a few volts. If you choose an adjustable compensation voltage, then instead of a fixed resistor of 82 kΩ, you can install a 100 kΩ potentiometer. If you need to measure the RMS value of the signal voltage, add a 4.15 MΩ resistor in series to the voltmeter circuit with an input resistance of 10 MΩ (a 3.9 MΩ resistor in series with a 270 kΩ resistor will work well). On Fig. Figure 3 shows a differential version of the meter suitable for measuring RF voltages directly on components in a circuit. The probe should provide the same readings of the meter, regardless of the “polarity” of connecting its outputs.

Rice. 3. Scheme of a differential attachment - a probe for measuring RF

voltage with a DC voltmeter.

1.5 VDC - DC voltage 1.5 V.

Diode detectors have a quadratic response for input signals below about 100 mV. Power detectors (rectifiers) can be designed taking into account the quadratic characteristic, but their calculation and design are not considered here. However, several factors must be taken into account when calculating them. To achieve greater sensitivity, the diodes must be matched as closely as possible to the signal source. Because diodes typically have very high dynamic impedance, this matching is the best that can be done, including an optional passive matching circuit to achieve better SWR. Diodes are typically biased through a resistor of several hundred ohms to reduce dynamic resistance and extend the quadratic response to higher levels of the rectified (detected) signal. The diode bias resistor may be an NTC thermistor, selected to thermally compensate for the change in diode resistance. A second DC biased diode can be used to provide a temperature dependent voltage to the differential amplifier. Another circuit includes a second diode in the voltage follower feedback circuit as shown below:

Rice. 4. Version of the probe on the op amp.

The 50 ohm input resistor can be replaced with a 200 kilo ohm resistor to

turn the probe into one with a linear characteristic for signals

with a level greater than 100 mV.

This version has no impedance matching. - In this version of the probe

there is no impedance matching. Large value bias resistors

high-resistance bias resistors.

A 50 ohm resistor is included in this circuit because no attempt has been made to match the detector diode. The optimal resistance value will depend on the diode current, and the optimal diode current will depend on the choice of instance and type of the diode and the matching circuit, if any. A lot of parameters have to be "taken off" before you get what you need, however, the above circuit gives reasonably encouraging results. Since the measured signal levels are small, the limiting factor in the application of the circuit is noise. Noise, on the one hand, and deviations from the quadratic characteristic, on the other, are limitations in the use of a quadratic detector.

The op amp circuit above works well as a line detector for signals above 100 mV. Resistors can be connected to the zero power bus - this will allow you to power the circuit with one voltage. Choose an op amp with a ground input operating in a negative supply voltage circuit, such as LM358 or LM10, connect 10 MΩ resistors and 0.1 uF capacitors.

http://*****/index. php? s=1144102f26fef4c60e9abc&showtopic=10860&st=20

Look w. Radio No. 7 for 92, page 39. Article "High-frequency millivoltmeter with a linear scale", author Pugach. I wanted to do this once, but I got a B3-43 and abandoned it. The circuit is really more complicated, the linearizer and UPT are made on two op-amps, the binding is also not weak, again switchable environmental protection circuits, but there are only two diodes.

Millivoltmeters with a linear scale, described in the literature, are traditionally performed according to the scheme with a diode rectifier included in the negative feedback circuit of an AC amplifier. Such devices are quite complex, require the use of scarce parts, in addition, rather stringent design requirements are imposed on them.

At the same time, there are very simple millivoltmeters with a non-linear scale, where the rectifier is assembled in an external probe, and a simple DC amplifier (UCA) is used in the main part. According to this principle, a device was built, the description of which was proposed in the journal Radio, 1984, No. 8, p. 57. These devices are broadband, have high input impedance and low input capacitance, and are structurally simple. But the readings of the device are conditional, and the true value of the voltage is found either from calibration tables or from graphs. When using the node proposed by the author, the scale of such a millivoltmeter becomes linear.

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Fig.2

The millivoltmeter, made by the author, allows you to measure voltage in the range of 2.5 mV at 11 subranges. Operating frequency band 100 Hz...75 MHz. The measurement error does not exceed 5%.
The schematic diagram of the device is shown in Fig.2. The linearizing stage, made on the operational amplifier DA1, operates on the subranges "0 ... 12.5 mV", "0 ... 25 mV", "0 ... 50 mV" "0 ... 125 mV", " 0...250 mV", "0...500 mV", "0...1.25 V". On the remaining subranges, the amplitude characteristic of the VD1 diode is close to linear, so the input of the final stage (on the DA2 chip) is connected to the probe output through a resistive voltage divider (R7 - R11). Capacitors C4-C6 prevent self-excitation of the operational amplifier DA2 and reduce possible pickups at its input.
The device uses a milliammeter with a total deflection current of 1 mA. Adjusted resistors R14, R16-R23 - SP5-2. Resistor R7 is made up of two 300 kΩ resistance connected in series, R10 and R11 - of two 20 kΩ each. Diodes VD1, VD2 - germanium high-frequency.
Operational amplifiers KR544UD1A can be replaced by any other with a high input impedance.
There are no special requirements for the design of the device. Capacitors Cl, C2, diode VDI and resistor RI are mounted in a remote head, which is connected to the device with a shielded wire. The axis of the variable resistor R12 is displayed on the front panel.
The adjustment begins with setting the arrow of the measuring device to zero. To do this, switch SA1 is switched to the “25 V” position, the input of the device is connected to the case, and the necessary adjustment is made by resistor R14. After that, they switch to the “250 mV” range, by adjusting the resistor R12 set the arrow of the measuring device to zero and by selecting the resistor R2 they achieve the best linearity of the scale. Then check the linearity of the scale on the remaining ranges. If linearity cannot be achieved, one of the diodes should be replaced with another instance. Then trimming resistors R16-R23 calibrate the device on all ranges.

Note. We draw the attention of readers that, according to the reference data, the maximum constant and pulsed reverse voltages for the remote probe used by the author of the article (GD507A diode) are 20 V. Therefore, not every instance of this type of diodes will be able to ensure the operation of the device on the last two subranges.

A. Pugach Tashkent

Radio, No. 7, 1992

1. "Radio" No. 7 1982 p.31
2. "Radio" No. 8 2006, p. 58, 59.
3. "Radio" No. 1 2008, pp. 61, 62.
4. "Radio" No. 7 1992, p.39

High frequency voltmeter with linear scale.

www. /articles. php? article_id=4

The scheme of the device is shown in Figure 1.

Drawings of printed circuit boards of the voltmeter and stabilizers can be found here.

Any questions about the device can be asked on the ARCalc forum.

1. "Radio" No. 7 1982 p.31
2. "Radio" No. 8 2006, p. 58, 59.
3. "Radio" No. 1 2008, pp. 61, 62.
4. "Radio" No. 7 1992, p.39

RF Voltmeter 100KHz - 70MHz, 1000V

http://nowradio. *****/VCH-voltmeter%2010Gc-70Kgc%202.5Mv-1000v. htm

In their practice, radio amateurs quite often have to deal with measuring alternating voltage both when setting up and repairing equipment, and when removing the parameters of a device. However, industrial voltmeters, although they have high parameters, are still inaccessible for most radio amateurs and, moreover, they have large dimensions. Millivoltmeters used by radio amateurs must meet a number of requirements:

They must be sensitive enough so that they can measure very small alternating voltages (up to a millivolt); To improve the accuracy of measurements, the scale should be linear;

The device must be portable, have low power consumption so that it can be used in the field, i.e. it must be powered by an independent power source (batteries or DC elements); Millivoltmeters must have a sufficiently wide range of measured frequency from 1 MHz to tens of MHz; Measure alternating voltages of large values up to hundreds of volts. The schemes of published millivoltmeters are assembled mainly on discrete elements (transistors) or on operational amplifiers (op-amps). The circuitry of millivoltmeters, which are described in the literature, is usually based on three principles:

Based on the use of an AC amplifier in a high-frequency head, from which amplified high-frequency voltage is supplied to several stages of an AC amplifier. To linearize its scale, frequency-dependent ac feedbacks are included in each stage. Next, the high-frequency voltage is rectified by the detector and fed to the measuring head. On fig. 1 shows a block diagram.

But to achieve the linearity of the scale of the device in a wide range of measured frequencies (1-100 MHz) is very difficult or almost impossible. At the same time, they have one very important advantage: the ability to measure very small values of alternating voltages (up to tens of microvolts). Construction of millivoltmeters with a non-linear scale. These millivoltmeters are simple in design. The rectifier in them is assembled in a remote head (probe), and in the device itself there are conventional DC amplifiers (UCAs), which can be based on transistors or operational amplifiers (op-amps). These devices are broadband, have high input impedance and low input capacitance. However, their indications are conditional, the true value must be determined from tables or graphs. In some schemes on the op-amp, to linearize the scale, the measuring device is included in one of the diagonals of the bridge, and the OS circuit of the high-speed op-amp is included in the other. This circuit requires the use of high-frequency diodes and high-speed op amps (although the frequency range itself remains small), has low sensitivity, and is sometimes prone to self-excitation, since the OS circuit is nonlinear. This non-linearity leads to the fact that the frequency response of the voltmeter changes at different levels. At the same time, if we apply the idea proposed at the beginning of the article, the scale of the instrument is linearized. The measured high-frequency voltage is rectified by the diode VD1 in the remote probe and through the resistor R1 is fed to the input of the UPT, made on the operational amplifier DA1. Due to the presence of the VD2 diode in the negative feedback, the amplification of the UPT at low voltages is compensated, and the scale of the device becomes linear. The proposed scheme of the millivoltmeter is free from most of these shortcomings. The millivoltmeter is designed to measure the effective value of a sinusoidal alternating voltage in the frequency range from 100 Hz to 70 MHz. With an external 1:100 divider, voltages up to 1000 V can be measured. However, it is not recommended to measure high-frequency voltages of 1000 V. It is desirable to limit the upper limit of measurements of a millivoltmeter by measuring voltages up to 300 V. The entire range of the device is divided into 4 subranges: 10 mV, 100 mV, 1 V, 10 V and with a divider of 1 V, 10 V, 100 V and more. The scale of the device for the convenience of the report is taken as a multiple of 10. The parameters of the proposed millivoltmeter are as follows:

1. Frequency band 100 Hz...70 MHz (no measurements were made above this frequency).

2. Range of measured voltages - 2.5 mV ... 1000 V

3. The maximum unevenness of the frequency response is not more than - 0.2 dB.

4. Current consumption in the +15 V circuit -7.5mA.

5. Input impedance 1 MΩ.

6. Measurement error does not exceed 10%.

The schematic diagram of the device is shown in the figure.

The millivoltmeter consists of an external probe (detector), an attenuator, a DC amplifier (DCA), an overload protection unit DA3, VT1 and a calibration voltage generator VT2. The linearizing stage is made on the operational amplifier DA1. It works on three subranges of 10 mV, 100 mV, 1 V. In the last range of 10 V, the amplitude characteristic of the VD1 diode is close to linear, so the probe output in this case is connected to the input of the operational amplifier DA2 directly through a resistive voltage divider R8, R9 (attenuator). To protect the operational amplifiers from self-excitation and possible interference with their input, blocking capacitors C3, C4 are included. On the DA3 chip, a unit for protecting the measuring device from overloads is assembled. This node is a comparator, which, if the voltage at the output of DA2 is within the normal range, produces a negative voltage that opens the transistor VT1. When the input signal exceeds the voltage by 1.5 times, at which the meter needle deviates to the last division of the scale, the comparator produces a positive voltage, which closes the VT1 key, and the red glow of the VD3 LED indicates an overload condition. When the overload is reduced to 1.1 full deviation voltage, the normal mode is restored. The hysteresis of the comparator response occurs due to the fact that often the load DA3 is turned off when overloaded. The presence of an overload protection unit is absolutely necessary, since at the moment of switching on and off the device and an error in choosing a subrange, unacceptable and dangerous voltage surges occur for the microammeter, which can damage it. The device has an internal calibration voltage generator, the output of which is connected by a shielded wire to the connector on the rear panel of the device. The device is calibrated in the range of 10 mV. In stationary conditions, the millivoltmeter is powered by a 220 V network, the power consumption is not more than 10 W. The calibration voltage generator is a conventional LC generator. The circuit in the collector circuit of the transistor VT2 is tuned to a frequency of 500 kHz. The load of the generator is an ohmic divider, consisting of resistors R28-R30. The calibration voltage of 10 mV is set by a tuned resistor R29. The power supply circuit of the device is extremely simple, since the current consumption is small and is the simplest parametric stabilizer. With such meager power consumption (7.5 mA on the +15 V circuit), there is no need to install a stabilizer on rolls or their imported counterparts. If the millivoltmeter is supposed to be used in the field, then a converter can be recommended, with the help of which the voltage of +4.5 V is converted to a voltage of ±15 V. their careful selection depends on the accuracy of the instrument as a whole. The remote probe is connected to the device with a shielded wire or a RK-20 coaxial cable.

DESIGN

The device is assembled in a 4 mm thick duralumin case with 2.5 mm screws at the end and has a size of 160x120x50 mm. Front and rear panel, removable duralumin 2 mm thick, radio elements of the device are attached to them. On the front panel there is a power transformer, a microammeter, power supply elements, a switch, a zero-setting resistor, a coaxial connector, indicating LEDs (power and overload). The microammeter is fixed on the outside of the front panel with 4 M4 screws. The power supply unit and the calibrator are mounted on a section of a unified printed circuit board, which is fixed on the upper screws securing the RA1 head. The circuit elements of the high-frequency millivoltmeter are mounted on a separate board, which is mounted on threaded studs for connecting the millivoltmeter. A power switch, a fuse, a holder and a connector for the output of the calibrator are mounted on the rear wall of the instrument. The remote head of the device is a half-wave voltage rectifier VD1. As VD1 (GD 507A), high-frequency germanium diodes GD402, GD508, D18 can be used. A high-frequency probe (Fig. 6) is a copper or brass tube 1 with a diameter of 15 mm and a length of 70 mm, on one side of which a boss 2 is inserted, machined from nylon or fluoroplastic with a pointed rod 3 pressed into it with a probe. A capacitor is soldered to it from the inside C1, on the other hand, a brass bushing 4 is inserted into the tube, through the hole in which a piece of a RK-20 coaxial cable (shielded wire) 750 mm long with a pin part of the connector mating with the CP-50 input socket of the millivoltmeter is passed. The boss and sleeve are fixed in the body of the probe with M2 screws, a common wire with a crocodile clip at the end is soldered to tab 5 on the body. The parts of the probe are hinged and held on the mounting lug 6. To measure voltages over 10 V, use the second replaceable probe (Fig. 3). In the second remote probe, a D104A high-frequency diode (or two diodes in series) with a high reverse voltage (100 V) is used as a rectifier. To measure even higher voltages, a frequency-dependent voltage divider 1:100 can be proposed (Fig. 4). The axis of the variable resistor R5 (zero setting) is displayed on the front panel of the device.

DETAILS

Operational amplifiers (op-amps) of the KR574UD1A or KR544UD1A type with an appropriate correction circuit. The DA3 chip is a general-purpose operational amplifier K140UD6, UD7, 153 UD2. Microammeter PA1 type M24 with a total deflection current of 100 μA, when replacing it, it is necessary to use devices of class 1.5. For this design, it is most expedient to use measuring heads with a frontal size of 80x80 mm, since heads with smaller dimensions do not provide the required measurement accuracy, and it is not economically feasible to use heads with large dimensions, since this increases the current consumption from an autonomous source. The measurement limit switch (attenuator) is made on a P2K type key switch in three directions. Power transformer from old calculators or home-made using iron W8x12.5 and a core section of 2.6 cm / sq. The primary winding contains 3000 turns of PEV-2 wire with a diameter of 0.08 mm, and the secondary winding contains 350 turns with a diameter of 0.17 mm. Smoothing electrolytic capacitor C1 in the power supply 470 ... 1000 uF. In parallel with it, a 22 ... 47 nF capacitor is connected. It is desirable to use variable resistors R16-19 multi-turn (type SP-5 or carefully selected constant resistances), connecting them in series from several pieces. The generator circuit coil is wound on a 3-section frame and placed in a SB-12 ferrite core, its winding is made from 60 turns of PELSHO 10x0.07 wire. The millivoltmeter uses resistors OMLT, MLT, capacitors such as KD, KLS, KM, K50. CP-50 type input connector.

SETUP

Setting up a millivoltmeter must begin with checking the supply voltage of the parametric stabilizer. Supply voltages +15 V and -15 V must differ by no more than 5%. Moreover, in the specified stabilizer, the voltage received from the secondary winding of the transformer is not critical. It can vary from 25 to 35 V. It is only necessary with the help of resistor R1 to select the required current of 10-15 mA, through the zener diodes VD1, / D2 and select the voltage of the electrolytic capacitor C1 (Fig. 5). Adjustment of the millivoltmeter must be carried out using industrial signal generators G402, G4-8 or similar and high-frequency voltmeters VZ-25, VZ-48A, and so on. First you need to set the arrow of the device to zero. When the input of the millivoltmeter is shorted at the limit of 10 V, the instrument needle is set to "0" using the trimmer resistor R14. On the remaining ranges, zero is set with a variable resistor R5. Further, applying pre-known voltages to the input of the device, by adjusting the resistors R15-R19, the millivoltmeter is calibrated on the remaining ranges. In conclusion, the operation of the protection circuit is checked by applying overestimated voltage values to the input of the device and monitoring the overload by the glow of the LED. If desired, the protection threshold can be adjusted by selecting the resistor R21. The final value of the scale on all limits is adjusted using resistors R23, R24. Calibration of the device using diodes D104 in the remote head is carried out by adjusting the elements located in the head R1, R2, C1, C2. Voltage adjustment using a divider is performed using capacitor C1. If it is not possible to achieve the linearity of the device using R6, R7, R2, then it is necessary to select the diode VD2. Setting up the calibrator consists in setting its frequency to 500 kHz by rotating the L1 core, controlling the frequency with a frequency meter on the VT2 collector. The value of the calibration voltage of 10 mV is set using a variable resistor R29.

Radio amateur No. 5 2001 p. 19

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, a simple detector with a high-resistance DC amplifier (HPA) is used. 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. The variable resistor R10 serves to set the pointer of the measuring device 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 the parts of the measuring probe, are mounted on two printed circuit boards made of 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

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