Digital multimeters models M830, M838, MV-63 and similar are widely distributed; radio amateurs use them to check and configure various electronic equipment. But such devices, of course, have drawbacks, and one of the most significant from the point of view of a radio amateur is the impossibility of measuring the voltage of the radio frequency range.

The prefix to the digital multimeter, which is made in the form of a high-frequency probe, will help to eliminate this drawback. It has a fairly large input impedance (about 50 kOhm), a small input capacitance (no more than 1 pF) and operates in the frequency range of 0.1 ... 200 MHz, and with a decrease in sensitivity - up to 500 MHz. Together with a multimeter, it allows you to measure the effective voltage in the range from 5 ... 10 mV to 10 V (range 60 ... 65 dB), which in most cases is quite enough for amateur practice.

The main feature of the device is that the measurement results are displayed not in volts or millivolts, but in relative units - dBV, that is, in decibels relative to a voltage level of 1 V. It should be noted right away that relative units of measurement are widely used in measuring technology, for example , for power measurement - dBW (relative to 1 W), dBmW or dBm (relative to 1 mW), and for voltage measurement - dBμV (relative to 1 μV) or, in this case, dBV (relative to 1 V).

The use of such a unit of measurement with the proposed prefix has obvious advantages. Firstly, there is no need to switch the measurement sub-ranges of the multimeter, since one is enough: the device is set to the limit of 2 V DC voltage. Secondly, it becomes very simple to determine the gain of a quadripole in decibels, since the result of interest is obtained as the difference between the two values ​​at the input and output of this quadripole. Thirdly, it will be much more convenient to measure the bandwidth at various rolloff levels: -3 dB, -6 dB, -40 dB or otherwise. The disadvantages include the non-proliferation of such a unit of measurement as dBV, but it is quite convenient and you quickly get used to it. In table. 1 shows the correspondence between relative units of measurement of levels (dBV) and voltages in volts or millivolts for a load with a resistance of 50 ohms.


The probe-attachment scheme is shown in fig. 1. An input amplifier with a large input impedance and a low input capacitance is assembled on a specialized DA1 chip (SOT23-5 package). This microcircuit is a buffer amplifier with a gain that can be set in the range 1 ... 2, an upper cut-off frequency of about 200 MHz, a large input impedance (3 MΩ at low frequency), a low output impedance (6 Ω) and a low input capacitance (1 pf). In addition, it has built-in overvoltage protection at the input. Resistive divider R2R3 provides the chip mode for direct current. To increase the input resistance of the device at a high frequency and the ability to work with an input voltage of up to 10 V, a resistor R1 is installed at the input.

A logarithmic detector is made on the DA2 chip. It converts the input high frequency AC voltage into a DC voltage proportional to the input signal voltage. The transformation law is logarithmic. This microcircuit is operable at high frequencies up to 900 MHz in the range of input signal levels from -72 dBm to 16 dBm. At pin 4 of DA2, a constant voltage is formed, proportional to the voltage of the input signal with a slope of 25 mV / dB. This guarantees a deviation from the law within ±1 dB over the entire range of input voltages. A voltage regulator is assembled on the DA3 chip (SOT23-5 package), from which the first two microcircuits are powered. Diode VD1 protects the device from incorrect polarity of the supply voltage.

Thanks to the use of small-sized parts for surface mounting, the dimensions of the stylus attachment have been made small. Most of the parts are placed on a board made of double-sided foil fiberglass with a thickness of 115 mm and dimensions of 10 × 70 mm, a sketch of which is shown in Fig. 2. Chokes and capacitors SYU, S11 are placed on the second side. Most of the plating on the second side is used as a common wire and is connected through edges and holes to the common wire on the mounting side. The board is connected to the multimeter with a two-wire shielded wire, it is also desirable to apply the supply voltage through a shielded cable.

To connect to the points of the controlled node, a metal probe (XI) is soldered at the input of the device, for example, a sewing needle, and a piece of flexible soft wire or a small clamp (X2) is soldered to the common wire. The board can be placed in a plastic case from the marker (see photo in Fig. 3), in this case, to reduce interference on the board above the DA1, DA2 microcircuits, it is necessary to install a foil screen.

Some other details can also be used in the device: the DA1 chip can be replaced with an AD8079 or an op-amp AD9631, AD849, but the board topology will have to be changed; in addition, it will be necessary to use a bipolar power supply. The integrated stabilizer DA3 can be replaced by 78L05 or similar. As a protective diode, you can use any small-sized rectifier, polar capacitors - tantalum for surface mounting, non-polar - K10-17v or similar imported ones. Fixed resistors - P1-12 and similar imported, tuned - 330W-3, POZ3 or SPZ-19, but in the latter case, the dimensions of the board will have to be increased.


Adjustment is carried out in the following sequence. The device is connected to an RF generator with an output calibrated in dBV and loaded with a standard load, and the output is connected to the input of a multimeter (measurement limit - 2 V). A signal is supplied with a frequency of 20 ... 30 MHz and a level ranging from -30 dBV to 0 dBV. changing output voltage RF generator within the specified limits, control the output voltage and set the slope of the output signal to 10 mV / dB with a tuned resistor R6. Then a signal with a voltage level of 0 dBV is supplied and the resistor R10 is set to zero readings on the multimeter. The setting must be repeated several times. After that, you need to check the readings in the frequency range and input voltages. In table. Figure 2 shows the readings of the author's layout of the device when a signal with a voltage of 1 V is applied to the input in a wide frequency range. As can be seen from this table, the device can be successfully used up to a frequency of 500 MHz by introducing appropriate adjustments to the multimeter readings. By selecting the capacitance of capacitor C1, you can change the lower operating frequency of the device. It is undesirable to make it too low, since the influence of low-frequency pickups will increase. To correct the frequency response at high frequencies, a capacitor with a capacity of several units to several tens of picofarads can be installed between pin 4 of the DA1 chip and the common wire.

The probe attachment can be powered from a power source with a voltage of 8…20 V, the current consumption is 12…15 mA. In this case, the multimeter and the probe should not be connected via power circuits. Input parameters the probes were evaluated using a device for measuring the inductance and quality factor of inductors E4-11. At a frequency of 100 MHz, the quality factor of the inductor was measured with and without a probe connected. The input resistance was 40 ... 45 kOhm, the input capacitance was 0.6-0.7 pF.

LITERATURE
1. Afonsky A., Kudrevatykh E., Pleshkova T. Compact multimeter M-830V. - Radio, 2001, No. 9, p. 25-27.
2. Nechaev I. Field strength indicator on the AD8307 chip. - Radio. 2003, no. 3, p. 64, 65.

I. NECHAYEV, Kursk
“Radio” No. 11 2004

Frequency counters are essential instruments for every radio amateur. They allow you to measure the repetition period and pulse duration, as well as other important indicators. To increase the sensitivity of the frequency counter, a special probe is required, which can be purchased at Aliexpress .

Remote probe for a frequency meter on Aliexpress: catalog, photo

As we said, the frequency counter is important for every radio amateur. Today, devices assembled on microcontrollers are very popular. They are relatively easy to manufacture.

Depending on which microcontroller is being used, maximum frequency measurements can range from hundreds of kilohertz to tens of megahertz. For stable operation the input of the microcontroller needs to be fed a signal with logic levels, so the frequency meter has an input signal amplifier on an op-amp or transistors, or a comparator.

To enhance the sensitivity of the frequency meter, amplifiers and comparators are often made in the form of a special remote probe. You can buy this device from Aliexpress .

Input active probe for a frequency meter on Aliexpress: catalog, photo

Many homemade digital frequency counters have low input impedance, high input capacitance, and poor sensitivity. All these factors adversely affect the frequency measurement accuracy. To avoid such problems, a broadband input probe with Aliexpress .

It is an input probe with high sensitivity and shaper rectangular pulses. It features high input impedance and low input capacitance. The device saves working condition from 2 Hz to 38 MHz. This allows it to be used in many situations where other devices fail.

Probe for a frequency meter on Aliexpress: sales, discounts, free shipping

Free shipping also makes it possible to save a lot on purchases. To see the goods, for example, the same probe for a frequency meter, with free shipping, you need to select the appropriate filter under the search bar:

Free shipping with Aliexpress

Probe for frequency meter on Aliexpress: best sellers and stores

On the Aliexpress there are a lot of shops where you can buy a probe for a frequency meter. The most reliable of them are.

The author offers remote probes that expand the measurement limits of the frequency meter. They divide by 100 the frequency of the measured signal, have differential inputs, and in one variant the same outputs. In the second variant, the output is normal, unbalanced. Probe supply voltage - 5 V, current consumption - 51 mA. They are built on the ADCMP553 analog comparator and frequency dividers MC12080 and KS193IE3.

On fig. 1 shows a diagram of a probe with a balanced output. The measured signal from the input contacts via the C1R1 and C2R2 circuits is fed to the symmetrical input of the ADCMP553 (DA1) voltage comparator, made on field-effect transistors, the isolated gates of which are protected by reverse-biased diodes. Pins 1 and 2 of DA1 control an internal "latch" that allows you to fix the state of the comparator outputs at the right time. With the connection of these outputs shown in the diagram, the "latch" is disabled.

Rice. 1. Diagram of a probe with a balanced output

As established experimentally, the sensitivity of the ADCMP553 comparator depends on the common-mode bias voltage at its inputs, which comes from an internal positive voltage source. If there are no resistors R3 and R5 in the input circuits connecting them to a common wire, the voltage at the inputs is more than 3 V, and the sensitivity of the comparator is reduced. Maximum sensitivity is achieved at a bias voltage of 1 ... 1.15 V, which is set by a selection of these resistors.

With a resistance of 150 kΩ indicated on the diagram, the input impedance of the probe is about 230 kΩ. The input signal swing at which the probe operates stably in the frequency band from 1 MHz to 600 MHz is at least 0.3 V, 0.7 V at a frequency of 0.9 GHz and 1 V at a frequency of 1.2 GHz.

The maximum operating frequency of the ADCMP553 comparator, according to technical description in , - only 800 MHz. Between its inputs, using connector X1, you can connect a resistor R4 with a resistance of 51 ohms. In this case, the input impedance of the probe decreases to 1 kOhm, and the band expands towards high frequencies. At frequencies from 0.6 GHz to 1 GHz, the sensitivity is not worse than 0.3 V, at a frequency of 1.4 GHz - 0.7 V, at a frequency of 1.55 GHz - 1 V. However, at frequencies below 0.6 GHz, the frequency counter , to which the probe is connected, overestimates the readings.

Resistors R6 and R7 in the comparator output circuits (pins 5 and 6) are connected to a common wire. Their resistance is not 100 ohms, as recommended, but 390 ohms to prevent exceeding the permissible output current. In this case, the load resistance is not exceeded, since the inputs of the first frequency divider are connected to the outputs of the comparator - the MC12080 (DD1) microcircuit, which has an input differential resistance of less than 100 ohms.

Experiments have shown that this divider operates at frequencies from 1 MHz to 1.6 GHz, although in its documentation the region of stable operation extends from 100 MHz to 1.1 GHz. The control inputs SW1-SW3 of the MC12080 divider are connected to the power plus, which sets its division ratio to 10. From the output of the first divider, a signal with an amplitude of 1.2 V with steep drops enters the input of the KS193IE3 (DD2) microcircuit - the second frequency divider by 10.

The probe board is connected to the output connector X2 by a bundle of four wires 80 cm long. Resistor R11 is located in close proximity to the connector contacts. The X2 connector is designed to connect to the balanced input of the FC250 frequency counter that I have modified. A power supply voltage of 5 V is supplied to the probe through the harness from the voltage regulator available in the FC250, and an anti-phase signal with a swing of 0.6 V is supplied to the differential inputs of this frequency meter, modified in accordance with the outputs of the divider DD2 of the probe.

Since the counting time of input pulses in the FC250 frequency meter is only 0.1 s, without a divider probe, its indicator displays the frequency value in tens of hertz (if the decimal point is not taken into account). Taking into account the division of the frequency by the probe by 100, it will be expressed in kilohertz.

The drawing of the board of the considered probe is shown in fig. 2, and the location of the parts on it - in Fig. 3. Drawing printed circuit board for connector X2 and resistor R11 is available in fig. 13th c. The boards are made of fiberglass 1.5 mm thick, covered with foil on both sides (for the probe board) or on one side (for the connector board). The edges of the probe board are "brushed" with tinned copper wire 0.5 mm in diameter, which is soldered to the foil on both sides of the board. From the same wire are made and soldered to the foil shown in Fig. 3 jumpers. The input contacts of the probe are made of hard tinned wire with a diameter of 0.75 mm.

Rice. 2. Drawing of the probe board

Rice. 3. Location of parts on the probe board

Resistor R4 - MLT-0.25. Before soldering to the pins of the male connector X1, its leads should be cut to a minimum length. The remaining resistors and capacitors are sizes 0805 or 1206 for surface mounting. Connector X1 - any four-pin pair of plug-socket with a pitch of 2.54 mm contacts located in one row (for example, CHU-4 and CWF-4), in which only the extreme contact pairs are left, and the middle ones are removed. Fork X2 - WF-4R. The connector housings are glued to the respective boards.

Under the DA1 and DD1 microcircuit cases, cover the board with varnish or a thin layer of hot melt adhesive before soldering them. Capacitor C8 and resistor R9 are installed on the board in the process of setting up the probe.

By inserting the plug X2 with the resistor R11 into the corresponding connector of the frequency meter, the resistance of the resistor R9 is selected until the counting by the DD2 chip stops, after which the capacitor C8 is mounted on the board. Then the main board of the probe tested in action and the connector boards are degreased and coated with a moisture-proof varnish. The main board is placed in a heat-shrinkable tube with a diameter of 25/12.5 mm, and the X2 connector board is placed in the same tube with a diameter of 12.5/7 mm. Shielding of the probe is not provided, it would increase its input capacitance and reduce sensitivity. Appearance probe is shown in the photograph of fig. four.

Rice. 4. Appearance of the probe

To work with a frequency meter that has a conventional unbalanced input, a second version of the probe was made, differing only in that its output circuits are made according to the circuit shown in Fig. 5. This probe is connected to the frequency meter with a three-wire harness. There is no load (resistor R11) at the end of the "Out" wire connected to the frequency meter. Output signal levels - TTL. The drawing of the printed circuit board of this probe is shown in fig. 6. Elements are located on it in accordance with fig. 7.

Rice. 5. Diagram of output circuits

Rice. 6. Drawing of the printed circuit board of the second version of the probe

Rice. 7. Location of elements on the board

On fig. 8 shows the measurement of the local oscillator frequency of a medium wave radio tuned to a radio station operating at a frequency of 612 kHz. The measured local oscillator frequency (1077 kHz) is 465 kHz (the value of the intermediate frequency of the receiver) above the carrier frequency of the radio station.

Rice. 8. Measurement of the local oscillator frequency of a medium wave radio receiver

Rice. 9. Demonstration of the probe operation

The signal frequency of a source that creates a sufficiently powerful electromagnetic field around itself (for example, a wireless handset), can be measured without connecting a probe to it, but by turning its input terminals into an antenna - a half-wave vibrator. On fig. 9 this is done with crocodile clips. The operating frequency of the handset transmitter is 927076 kHz.

Literature

1. Single-Supply, High Speed ​​PECL/ LVPECL Comparators ADCMP551 /ADCMP552/ ADCMP553. - URL: http://www.analog. com/media/en/technical-documentation/data-sheets/ADCMP551_552_553.pdf (27.02.17).

2. MC12080 1.1GHz Prescaler. - URL: http://www.nxp.com/assets/documents/data/en/data-sheets/MC12080.pdf (02/27/17).

3. Panshin A. Pre-amplifier-shaper for frequency counter FC250. - Radio, 2015, No. 2, p. 18-20.

4. Panshin A. Refinement of the frequency meter FC250. - Radio, 2016, No. 3, p. 23, 24.

Publication date: 23.06.2017

Readers' opinions

• Panshin A.V. / 30.07.2017 - 20:21
  There is an inaccuracy in the text of the article: the 3rd paragraph after Fig.1. It says "the frequency meter to which the probe is connected is overestimating." It should read: "the frequency meter to which the probe is connected with R4 connected to it overestimates."

Active Probe

See detailed article in VRL No. 95 p. 12

Active probes with low input capacitance. I. Shiyanov.

________________________________________________________________________

http://nowradio. *****/pribory%20dly%20nastroyki%20KV-UKV%20apparatury. htm

http://*****/forum/download/file. php? id=16793

Setting up radio receivers often requires checking heterodynes measurement of the parameters of the RF voltage generated by it. Unfortunately, it can be difficult to do this directly with an RF oscilloscope or millivoltmeter. The input capacitance of the device, the input impedance, has a very great influence on the operation of a micropower generator (local oscillator). For example, the input of the popular C1-65 oscilloscope with a capacitance of 30 pF and a resistance of 1 M can not only distort the measurement results, but even disrupt the generation of the local oscillator. And then there is a coaxial cable with a wave impedance of 50 ohms. Of course, you can connect the input through a 1 pF capacitor, but this can greatly distort the measurement result (the RF voltage level that reaches the input of the measuring device can be 100 times or more underestimated). It is best to use an active probe, which is a source follower on a high-frequency field-effect transistor with an input capacitance of less than 1 pF, and an input resistance of more than 10 MΩ with an output resistance of 50 Ω. Such a probe, made in the form of a separate shielded box, can be placed in the immediate vicinity of the measurement point, connected to it with the shortest conductors, completely eliminating the influence of the wave resistance of the device capacitance cable and the device input impedance cable on the measurement result. Moreover, himself measuring device can be located at a considerable distance from the measuring point (very long connection cables can be used).

A schematic diagram of an active probe on a BF998 field effect transistor is shown in the figure. In the diagram, the transistor is shown in the case so that its pinout is understood. The input capacitance of the probe is approximately 0.7 pF; it is formed by three capacitors C1-C3 connected in series. Input impedance 10 megohm. The measured RF voltage is applied to the first gate of the transistor. The bias voltage at this gate is equal to half the supply voltage and is created by a resistive divider R2-R3. The bias voltage is applied to the gate through a resistor R1 with a resistance of 10 MΩ. The input capacitance of the BF998 transistor is 2.1 pF, so the voltage obtained as a result of the measurement must be multiplied by 3. The load is the resistor R4, its resistance should be the same as the wave resistance of the cable. The probe operates in the frequency range from 100 kHz to 1 GHz with a voltage gain unevenness of no more than 7 5dB. At frequencies above 1 GHz, the error increases significantly. The power source is network adapter from a TV game console of the “Dandy” type (output constant unstable voltage 8-11V) The voltage is stabilized at the level of 5V by the integral stabilizer A1. Diode VD1 serves to protect against erroneous wrong connection source. The probe can also be powered from a laboratory source with a voltage of 8 ... 20V. Structurally, the probe is made in a shielded case of a faulty all-wave tuner of the LG TV. Mounting the first gate of the field-effect transistor on R1 and capacitors C1-C3 must be done "in the air" in order to exclude the influence of the capacitance of the printed circuit board and the shielded case on the input circuit. Input - two mounting wires no longer than 10 cm. The wire connected to C1 must not come into contact with the insulation with the board or the housing screen.

For 5V power it is better to use bf1005 orbf1012 Sis in Platana.

Radioconstructor №12 2007

Active Oscilloscope Probe

Magazine "Radio", number 6, 1999

http://www. *****/literature/radio/199906/p28_29.html

Broadband amplifiers with high input impedance, low input capacitance and low output impedance are used in various devices. One of the applications is input probes for oscilloscopes and other measuring equipment. As shown in this article, modern op-amps from Analog Device make it possible to solve this problem with simple means.

An oscilloscope is one of the most versatile instruments that allows you to measure a wide variety of electrical signal parameters, and often greatly simplify the setup procedure. electronic devices. In some cases, it is simply irreplaceable. However, many are familiar with the situation when connecting an oscilloscope to a custom device leads to a violation of its modes. This is primarily due to the capacitance and resistance of the input of the oscilloscope and its connecting cable introduced into the circuit under study.

Most oscilloscopes used by radio amateurs have a high input impedance (1 MΩ) and an input capacitance of 5 ... 20 pF. In combination with a connecting shielded input cable about a meter long, the total capacitance increases to 100 pF or more. For devices operating at frequencies above 100 kHz, this capacitance can have a significant impact on measurement results.

To eliminate this drawback, radio amateurs use an unshielded wire (if the signal level is high enough) or a special active probe, which includes an amplifier with a high input impedance, usually made on field effect transistors. The use of such a probe significantly reduces the amount of capacitance introduced into the device. However, the disadvantages of some of them are the low gain or the presence of a level shift at the output, which makes it difficult to measure the DC voltage. In addition, they have a narrow operating frequency range (up to 5 MHz), which also limits their use and requires short connecting cables. The probe described in has some better parameters. It should be noted that all these probes can work effectively with oscilloscopes that have a high input impedance.

Currently, broadband oscilloscopes with an operating frequency range of up to 100 MHz and higher, with a low input impedance of 50 ohms, are becoming more common, so their connection to a custom device often becomes almost impossible. Not all of them are equipped with active probes, and the use of resistive dividers leads to a noticeable decrease in sensitivity.

The active probe, the description of which is offered to the attention of readers, is free from these shortcomings. It works with various oscilloscopes, the input impedance of which can be low-resistance - 50 Ohm or high-resistance - up to 1 MΩ, has an operating frequency range of 0 ... 80 MHz and a fairly high input impedance at low frequencies- 100 kOhm. Its transmission coefficient is 1 or 10, that is, it not only does not attenuate, but also amplifies the signal. The advantages of the probe include its small dimensions.

Such parameters were achieved through the use of a modern high-speed op-amp from Analog Devices. In particular, this probe uses the AD812AN op-amp (Chip - Dip - 180r Platan - 190r), which has the following main characteristics:

Upper operating frequency - not less than 100 MHz; input resistance - 15 MΩ with an input capacitance of 1.7 pF; input voltage - up to + 13.5 V, and the slew rate of the output voltage is 1600 V / μs; output current (with an output resistance of 15 ohms) - up to 50 mA; current consumption in the absence of an input signal - 6 mA.

In addition, the op amp has low harmonics (-90 dB at 1 MHz and a load of 1 kΩ) and low noise (3.5 nV / ^ Hz), protection against K3 (current limited to 100 mA), power dissipation in a small package large enough - 1 W. It should be added to this that the price of a microcircuit containing two op-amps with such parameters is relatively low ($3...4).

The scheme of the active probe is shown in fig. 1. Basically it matches standard scheme switching on the OS. The transmission coefficient KU is changed by switching SA1 of the circuit elements feedback and has two values: 1 and 10. Switch SA2 selects the operating mode: with a "closed" input, when the capacitor C1 is turned on at the input and the DC component of the voltage does not pass to the input, or with an "open" input when it passes.

Chargers" href="/text/category/zaryadnie_ustrojstva/" rel="bookmark"> power supply with an output voltage of %12...15 V. It should be noted that the current consumed in the absence of a signal is 10...15 mA, when operating on a low-resistance load, when a signal is applied, the current can increase up to 100 mA.

Literature

1. Grishin A. Active probe for oscilloscope. - Radio, 1988, # 12, p. 45.

2. Ivanov B. The oscilloscope is your assistant (active probe). - Radio, 1989, # 11, p. 80.

3. Turchinsky D. Active probe for oscilloscope. - Radio, 1998, # 6, p. 38.

Oscilloscope RF probe with SV = 0.5 pF

http://www. *****/ot07_19.htm

For oscilloscope measurements in high-frequency devices, the input capacitance of the divider can introduce significant distortion into the tuned node (for example, when connecting a probe to the RF generator circuit, etc.). Dividers with a ratio of 1:1 have an input capacitance of the order of 100 pF or more (cable capacitance plus oscilloscope input capacitance), which significantly limits their frequency range. At the same time, standard 1:10 passive dividers with an input capacitance of 12 to 17 pF reduce the oscilloscope sensitivity to 50 mV per division (with a maximum input sensitivity of 5 mV / division, typical of most industrial oscilloscopes), and also still have too large input capacitance for non-distorting measurements in RF circuits where loop capacitances can be of the same value.

This problem is solved by using for measurements special active probes produced for this purpose (for example, by Tektronix). However, these devices are quite hard to find and their price (from $150 and up) is comparable to the price of a good used oscilloscope. At the same time, it is not very difficult to independently manufacture a simple active oscilloscope probe with a small input capacitance, which was done by the author.

An active oscilloscope probe is designed to measure variable voltages in low-voltage RF circuits and has the following characteristics:

Range of measured signal amplitude values ​​– from 10 mV to 10 V frequency response– Linear from 10 kHz to 100 MHz at low signal Output signal – inverted, with a division ratio of 1:2 Supply voltage – 12 volts (4 * CR2025) or external source Input capacitance – 0.5 pF (0.25 pF with external divider 1:10) Input resistance - 100 kOhm Current consumption - 10 mA Dimensions 60 x 33 x 16 mm

The appearance of the manufactured device is shown in the photo.

Instrument design

The schematic diagram of the probe is shown in the figure. The device is assembled on three low-noise microwave transistors 2SC3356 with a cutoff frequency of 7 GHz. The voltage gain is about 23 dB. The output emitter follower serves to additionally isolate the amplifier from the load and can be omitted if the probe is used with the same oscilloscope. A string of LED, 9 volt zener diode, and resistor serves as a power-on indicator and a threshold indicator for battery voltage. A supply voltage of 12 volts is necessary and sufficient in order to obtain the maximum amplitude value of the measured signal up to 5 volts at the output of the device, and thereby ensure the maximum dynamic range up to 50 dB when performing measurements with the deviation coefficient set, starting from 5 mV per division (sensitivity most oscilloscopes).

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Establishment

This stage of work must be carried out very carefully to obtain the desired result.

After assembling the amplifier, it is first necessary to accurately set its operating point by selecting a 120 kilo-ohm resistor to obtain the maximum amplitude of the undistorted signal at the output. In this circuit and with fresh batteries, this mode is achieved by setting a constant voltage from +5.2 to +5.3 volts at the emitter of the second transistor. The operating point of the second emitter follower does not require adjustment with the specified resistor values. Next, you should accurately select the value of the lower resistor (in this case, 20 kilo ohms) input divider to obtain the required scale (1: 2) of signal transmission between the input and output of the device at a relatively low frequency (about 100 kHz). Note that the input impedance of the amplifier at the specified ratings of parts is about 5 kiloohms (at the same frequency), so that in the absence of the specified resistor, the gain of the device will be higher than required by about 3 dB (the attenuation of the input signal is (105 / 5) = 26 dB, while the overall gain of the circuit is 23 dB, and the required gain of the entire device must be equal to 0.5, i.e. minus 6 dB). The selection of compensating capacitances (0.5 pF in parallel with a 100 kilo-ohm resistor, and a tuning capacitor in the lower branch of the input divider) is carried out by comparing the transfer coefficient at two frequencies, for example, 1 MHz and 30 MHz, and selecting capacitances until the desired constant gain of the device is obtained. Next, a final check of the device is carried out at the upper operating frequency, if the radio amateur has such an opportunity. Finally, the actual input capacitance of the probe at high frequency is checked (for example, by connecting it to a circuit with known parameters of a running generator and monitoring the change in the frequency of the output signal using a digital frequency meter or receiver). With the correct design of the device, it should not differ significantly from the value indicated on the diagram (the total input capacitance in the probe manufactured by the author, measured at a frequency of 20 MHz, was 0.505 pF).

Remarks

This probe was created by the author for measurements in circuits of sinusoidal RF signals in the circuits of generators and amplifier stages of transistor circuits, and it generally solves the problem. It is for this reason that the above ratio was chosen in the probe between all the main parameters of the device - its frequency range, high sensitivity, a sufficiently large input resistance and the lowest possible input capacitance of the meter, as well as a small current consumption. Radio engineering is always a compromise with the limit values ​​of parameters set by the developer.

Active probe for C1-94.

http://*****/izmeren/369-tri-pristavki-k-s1-94.html

Aluminum "href="/text/category/aluminij/" rel="bookmark"> an aluminum cup from a validol. The probe is connected to the oscilloscope with any high-frequency shielded cable, preferably of a small diameter.

When setting up the probe, first select (if necessary) the resistor R1 to ensure the operation mode of the transistor VT2 indicated in the diagram. The transmission coefficient is set by selecting the resistor R4, and the upper limit of the bandwidth - by selecting the capacitor C4. The lower limit of the bandwidth depends on the capacitance of the capacitor C1.

It is advisable to check the amplitude-frequency characteristic of the probe. If a rise is detected on it at frequencies corresponding to the upper limit of the passband, it will be necessary to connect a 30 Ohm resistor in series with capacitor C4

Taken from here: http://www. *****/lcmeter3.htm

Frequency meter, capacitance and inductance meter - FCL-meter

On the transistor VT1, the signal amplifier of the frequency meter F1 is assembled. The circuit has no features, except for the resistor R8 (100 Ohm), which is necessary to power an external amplifier with a small input capacitance, which greatly expands the scope of the device. Its diagram is shown in rice. 2.

When using the device without external amplifier it must be remembered that its input is 5 volts, and therefore a decoupling capacitor is needed in the signal circuit.

The prescaler of the F2 frequency meter is assembled according to a typical circuit for most of these prescalers, only limiting diodes VD3, VD4 are introduced. It should be noted that in the absence of a signal, the prescaler is self-excited at frequencies of about 800-850 MHz, which is typical for high-frequency dividers. Self-excitation disappears when a signal is applied to the input from a source with an input impedance close to 50 ohms. The signal from the amplifier and prescaler goes to DD2.

Remote probe to the oscilloscope.

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On fig. 3 presented circuit diagram voltage follower, made in the form of an electronic probe to the oscilloscope. The repeater circuit contains four transistors. A matched pair of field-effect transistors VT1, VT2 with an n-channel operates in a differential stage, transistor VT3 is a current source for the specified stage, and transistor VT4 is included in the voltage amplifier circuit with a common emitter.

The device works as follows. The input signal is applied to the gate of transistor VT1. Voltage amplified field effect transistor VT1, goes to the base of the transistor VT4. The output voltage of the repeater is taken from collector load - resistor R10. At the same time, the output voltage is applied to the gate of the second transistor differential pairs VT1, VT2. Deep negative feedback and large differential resistance of the current source provide close to unity gain of the follower. By selecting the collector current of the transistor VT4 (about 4 mA), the non-linearity of the follower in the high frequency region is reduced. The temperature stability of the device is ensured by deep negative feedback and the introduction of a current source on the transistor VT3.

The main characteristics of the voltage follower are shown in fig. 4. Curves 1-4 show the frequency response of the device for various values ​​of the load capacitance. As the capacitance increases from 15 to 100 pF, the repeater bandwidth, measured at 3 dB, narrows from 25 to 10 MHz. The load capacitance above is the sum of the cable capacitance and the oscilloscope input capacitance.

Rice. 3. A variant of the voltage follower circuit - a probe to the oscilloscope

It must be borne in mind that modern RF cables with polyethylene insulation have a linear capacitance that increases with decreasing wave impedance. So, for example, the typical value of the linear capacitance of a cable with a wave impedance of 50 Ohm is PO ... 125 pF, with a wave resistance of 75 Ohm - within 60 ... 80 pF. High-resistance cables and cables with semi-air insulation may have lower capacitance per unit length, but they are relatively inaccessible.

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