Active probe for oscilloscope. Scheme, description

Free technical library

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
Free library / Schemes of radio-electronic and electrical devices

Active probe for oscilloscope. Encyclopedia of radio electronics and electrical engineering

Free technical library

Encyclopedia of radio electronics and electrical engineering / Measuring technology

Comments on the article

The input capacitance of modern oscilloscopes is about 30...50 pF. When measuring, the capacitance of the connecting cable is added to it, and the total input capacitance reaches 100 ... 150 pF. This can lead to a significant distortion of the measurement results and incorrect settings, for example, notch filters of the output stages of tape recorder recording amplifiers. That is why, when conducting research in circuits that are critical to the introduced capacitance of the measuring device, it is necessary to use special matching devices that have a large input resistance and a small capacitance.

For most practical work, two main types of devices are needed: for low-amplitude harmonic signals (1 ... 50 mV) with a transmission coefficient K> 1 and for high-amplitude signals (up to 10 ... having a transmission coefficient K=20...0,2.

The widespread use in recent years of high-speed analog and digital microcircuits operating at relatively high voltages (op-amps for wide use, microcircuits of the K561 series - up to 15 V) has revealed the need for a device operating in a wide voltage range with the ability to transmit a constant signal component.

Active probe for oscilloscope
Ris.1

A diagram of such a device in the form of a probe is shown in Fig. 1. It is made according to the classical source follower circuit using a MOS transistor and contains a minimum number of parts. The operating frequency range is O...5 MHz. Power is supplied from any current source with a voltage of 7 ... 15 V, for example, a 7D-0,115-U1.1 storage battery or Krona, Korund galvanic batteries. The input capacitance of the probe is not more than 4 pF, the input resistance is not less than 3 MΩ. The output voltage at Uin=0 is 2,5 V. The range of input voltages in the area of negative values (before cutoff) is 7 V, in the area of positive values (before the start of limitation) is 13 V at Upit=9V and 26V at Upit=15V.

The transmission coefficient in the specified frequency range is 0,4.

Resistors R1 and R2 form the input voltage divider, capacitor C1 is used for frequency compensation.

Due to the significant variation in the parameters of specific transistors, the characteristics of probe designs can also differ mainly in cutoff voltage and transfer coefficient. To obtain the maximum operating range in the region of negative input voltages, it is necessary to use transistors with the maximum (in absolute value) cutoff voltage. The author used a transistor with Uzi otc = 4,2 V. Most KP305I transistors have a lower value of Uzi ots, therefore, if necessary, the probe cutoff voltage can be increased by reducing the input divider gain, for example, by increasing the resistance of the resistor R1. However, for many measurements that require adjustment to the maximum or minimum voltage, the cutoff voltage value of the probe is not significant, since the adjustment can be made on the positive half-wave of the signal.

The probe is assembled in a case from a felt-tip pen. The installation is volumetric, without the use of additional structural elements. The conclusions of the radio elements are connected directly to each other. The probe is connected to the oscilloscope with a shielded cable no longer than 30 cm.

The design used resistors type MLT-0,125. Capacitor C1 is constructive, it is made with a PEV wire with a diameter of 0,15 ... 0,35 mm. The wire must be soldered to the left (according to the diagram) terminal of the resistor R1 and wound 12 turns on the right terminal. The capacitance is selected by changing the number of turns. At the end of the tuning on the capacitor thus obtained, clean the track with a fine-grained sandpaper, tin it and solder it in a thin layer (to eliminate parasitic inductance).

When mounting the probe, measures should be taken to prevent the breakdown of the field-effect transistor by static electricity and interference from the network.

Setting up the device consists in calibrating to obtain the required transfer coefficient and selecting the capacitance of capacitor C1. Calibration will require the use of a regulated DC power supply and a voltmeter. By selecting the resistance of the resistor R1, the transfer coefficient is set to K = 0,4 (or 0,5), while taking into account the initial bias voltage at the output.

When selecting the capacitance of the capacitor C1, a rectangular pulse generator with an output signal amplitude of 2 ... 10 V and a repetition rate of 1 ... 10 kHz is required. To provide steep fronts, you can use a trigger frequency divider, for example, on microcircuits of the K155, K176, K561 series. By changing the capacitance of the frequency compensation capacitor C1, rectangular pulses are obtained on the oscilloscope screen without a blockage of the fronts, the amplitude of the surges at the fronts should be no more than 10% of the pulse amplitude. Too much capacitance causes significant surges along the fronts, insufficient - their tightening.

On the body of the manufactured structure, it is necessary to put inscriptions of the device parameters - input capacitance, resistance and transfer coefficient.

When making measurements with a readout of the DC component, the oscilloscope must be corrected for the readout level. To do this, close the probe input and set the oscilloscope beam to zero.

Author: A. Grishin, Moscow; Publication: N. Bolshakov, rf.atnn.ru

See other articles Section Measuring technology.

Read and write useful comments on this article.

<< Back

Latest news of science and technology, new electronics:

The world's tallest astronomical observatory opened 04.05.2024

Exploring space and its mysteries is a task that attracts the attention of astronomers from all over the world. In the fresh air of the high mountains, far from city light pollution, the stars and planets reveal their secrets with greater clarity. A new page is opening in the history of astronomy with the opening of the world's highest astronomical observatory - the Atacama Observatory of the University of Tokyo. The Atacama Observatory, located at an altitude of 5640 meters above sea level, opens up new opportunities for astronomers in the study of space. This site has become the highest location for a ground-based telescope, providing researchers with a unique tool for studying infrared waves in the Universe. Although the high altitude location provides clearer skies and less interference from the atmosphere, building an observatory on a high mountain poses enormous difficulties and challenges. However, despite the difficulties, the new observatory opens up broad research prospects for astronomers. ... >>

Controlling objects using air currents 04.05.2024

The development of robotics continues to open up new prospects for us in the field of automation and control of various objects. Recently, Finnish scientists presented an innovative approach to controlling humanoid robots using air currents. This method promises to revolutionize the way objects are manipulated and open new horizons in the field of robotics. The idea of controlling objects using air currents is not new, but until recently, implementing such concepts remained a challenge. Finnish researchers have developed an innovative method that allows robots to manipulate objects using special air jets as "air fingers". The air flow control algorithm, developed by a team of specialists, is based on a thorough study of the movement of objects in the air flow. The air jet control system, carried out using special motors, allows you to direct objects without resorting to physical ... >>

Purebred dogs get sick no more often than purebred dogs 03.05.2024

Caring for the health of our pets is an important aspect of the life of every dog owner. However, there is a common assumption that purebred dogs are more susceptible to diseases compared to mixed dogs. New research led by researchers at the Texas School of Veterinary Medicine and Biomedical Sciences brings new perspective to this question. A study conducted by the Dog Aging Project (DAP) of more than 27 companion dogs found that purebred and mixed dogs were generally equally likely to experience various diseases. Although some breeds may be more susceptible to certain diseases, the overall diagnosis rate is virtually the same between both groups. The Dog Aging Project's chief veterinarian, Dr. Keith Creevy, notes that there are several well-known diseases that are more common in certain breeds of dogs, which supports the notion that purebred dogs are more susceptible to disease. ... >>

Random news from the Archive

192-core processor for smartphones and tablets 12.01.2014

The American company Nvidia announced the "192-core superchip" Nvidia Tegra K1 based on the ARM Cortex A15 architecture for smartphones and tablets.

In fact, calling the new processor 192-core is not entirely correct, since we are talking about graphics cores, and not cores of the main processor, which are five in Tegra K1 (four cores plus an additional core to save power).

What makes the Tegra K1 truly unique is Nvidia's Kepler architecture, which up until now Nvidia has used exclusively in PC processors. Thus, Tegra K1 becomes the first mobile processor to use technologies originally created for desktop gaming systems, such as DirectX 11, OpenGL 4.4 and tessellation (a method of breaking polygons into smaller parts). As a result, Tegra K1 allows you to run, for example, the Unreal Engine 4 game engine, which is the basis of hundreds of games.

"What's more, the Tegra K1 is the first mobile processor to offer the same functionality as the Xbox One and PlayStation 4 game consoles, delivering higher levels of performance than the Xbox 360 and PlayStation 3 in the palm of your hand," Nvidia said. . "This allows us to take a game made for PC or console and run it on a mobile device," said Tim Sweeney, founder of Unreal Engine, Epic Games.

Tegra K1 supports screens with Ultra HD (3840 x 2160 pixels), the maximum supported resolution is 4096 x 2160 pixels. The maximum amount of supported RAM is 8 GB (DDR3L, LPDDR3). The maximum clock frequency is 2,3 GHz. The chip will be manufactured on the basis of a 28-nm process technology.

Tegra K1 is also the first mobile gadget processor to support the CUDA parallel computing architecture.

Nvidia expects the first 1-bit Tegra K32-equipped devices to be available in the first half of 2014, while 64-bit devices will be available in the second half of the year.

Other interesting news:

▪ Paper against coronavirus

▪ Neurons evaluate the benefit of a habit

▪ Intel retires Direct Rambus DRAM (DR DRAM)

▪ Chemists fight global warming

▪ Slug stats

News feed of science and technology, new electronics

Interesting materials of the Free Technical Library:

▪ section of the site Medicine. Selection of articles

▪ article Frog-traveler. Popular expression

▪ article What is the function of the ears in addition to hearing? Detailed answer

▪ article Anti-terrorism security and protection of children. Standard instruction on labor protection

▪ article DC/DC converter that generates two voltages. Encyclopedia of radio electronics and electrical engineering

▪ article Focus with a folding box. Focus Secret

Leave your comment on this article:

All languages of this page

Home page | Library | Articles | Website map | Site Reviews