What is an oscilloscope for: characteristics and principle of operation of the device, how to use the device correctly

You can measure simple electrical quantities such as current, resistance, voltage using a multimeter. But they will not be able to investigate the shape of the signal or its behavior in time. Therefore, an oscilloscope is needed to measure, check and fine-tune instruments. This universal device was previously used only in laboratories and service centers, but today it has become quite affordable for use by radio amateurs.

Types and characteristics

Various studies in the field of electricity required an instrument capable of making a series of measurements of the behavior of a given parameter over a period of time. The founder of such a device was André Blondel, who was born in 1863 in France. Studying electrical engineering, he founded a laboratory in the city of Levallupe. In it, based on the theory of Alfred Cornu, the scientist invented and constructed a magnetoelectric device with a bifilar suspension. It happened in 1893.

This device made it possible to register the intensity of alternating currents by recording the oscillations of a pendulum with ink connected to an inductor. The meter had low accuracy due to mechanical parts. And its passband was in the range of 10-19 kHz.

Further evolution of the device led to the appearance in 1897 of an oscilloscope with a cathode-ray tube (CRT). Physicist from Germany Karl Braun became its designer. But the first industrial copy was released only in 1932 by the British company A. C. Cossor Ltd. In November, the American firm Allen B. DuMont Laboratories introduced an oscilloscope consisting of two parts: a CRT and a housing. The latter housed beam focusing units, a power source and a sweep unit. But the screen production technology allowed it to be used for no more than one thousand hours.

The Second World War stopped the development of the device, but after its end the engineers Wollum and Murdoch, the founders of Tektronix, a waiting sweep is introduced into the device device, that is, one that is launched only during the occurrence of an electromagnetic signal. This device worked with a 10 MHz bandwidth.

The development of semiconductor technology led to the development of a digital device by LeCroy in 1980. After that, digital devices began to be mass-produced in Europe, not only of a professional level, but also of an amateur radio. All kinds of devices that differ in accuracy and functionality have appeared on the markets.

At the beginning of 2000, digital technology almost completely replaced analog devices, this was facilitated by the development of personal computers and the possibility of interfacing a meter with them. However, whichever method of signal processing is used, the principle of operation of various oscilloscopes remains the same.

Analog device

Today, analog oscilloscopes are less and less common in research laboratories or service centers. But radio amateurs still have enough outdated, but still quite efficient such devices. Any analog device consists of one or more vertical channels, a horizontal channel, a trigger circuit, and a cathode ray tube (CRT).

CRT is the main part of the device. It displays the shape of the signal being investigated. It is carried out from a vacuum flask into which electrodes for various purposes are soldered. The first group forms an electron gun that forms a beam. The signal under investigation is sent to it. And the second one consists of contacts of vertically and horizontally deflecting plates and the voltage of the sweep generator is supplied to it.

Thus, the device consists of the following parts:

• attenuator - input voltage divider;
• preamplifier;
• delay block;
• synchronization and sweep triggering scheme;
• generator;
• final amplifier.

The measured signal is fed to the vertical plates, and then to the attenuator, which allows you to adjust the sensitivity of the device. A regulating device is made in the form of a rotary knob. The switching scale is indicated in volts per division. Dividers are used when measuring a powerful signal. These are special devices that work on the principle of attenuators, but at the same time they reduce the signal to a safe level for the input circuits of the oscilloscope.

The signal from the divider or attenuator is branched off at the preamplifier and enters the delay and synchronization unit. The last node creates conditions for starting the generator when electromagnetic oscillations appear. The sawtooth signal from the generator enters the horizontal channel X, where it is amplified and fed to the screen.

The second part of the signal goes through the delay line to the Y channel, and then to the CRT. As a result, the pulse position is displayed on the screen in the XY coordinate system. The lower frequency limit is around 10 Hz, and the upper one depends on the capacitance of the plates and the quality of the amplifiers.

Therefore, if a measured voltage is applied to the plates, then the beam begins to deviate vertically and horizontally. These movements occur synchronously, and as a result, the signal "unrolls" in time. The resulting image on the screen is called an oscillogram.

Digital instrument

The digital device combines an analog oscilloscope and a mini-computer. Using it, you can not only visually see the shape, but also perform a number of operations, such as addition and subtraction of signals, Fourier transform, and determination of the spectrum. The device includes:

• scaling node;
• analog-to-digital converter (ADC);
• random access memory (RAM);
• microcontroller;
• storage cells;
• screen;
• control elements (buttons, knobs).

The signal goes to the input of the scaling node, where it is reduced to a safe value for the internal circuits of the device. Then it is fed through an amplifier to the ADC. It converts an analog form into a series of a discrete sequence of a logical code. For this, a microcontroller is used that works on the principle of pulse width modulation (PWM).

The code is written into RAM, from which, after a certain condition is met, it is transferred to the storage cells. Each block corresponds to a pixel that is overexposed. The X coordinate is determined by the cell number, and the Y coordinate by the code written in it. The memory cell can contain several code symbols, which form a line of continuously lit pixels.

Digital oscilloscopes are divided into several subtypes and can be:

• Virtual - having different input and output ports. They are designed to work with external software installed on a PC.
• Stroboscopic - using sequential sampling of instantaneous values ​​and their temporal transformation using short pulses (strobes).
• Phosphorous - displaying the signal in the time and amplitude planes, as well as its intensity. Such devices are characterized by high sample density and accuracy.

The use of the LCD screen improves the convenience of the oscilloscope. It becomes possible to visually display any data on it, and the use of memory in the device allows you to compare any changes in the waveform over time.

Fixture parameters

An oscilloscope, like any electrical device, has a number of technical parameters. It is they who determine its functionality and degree of use. Requirements for the class of accuracy, stability of operation, noise characteristics are imposed on its work.

The most important parameters of the device are:

• Frequency bandwidth. Specifies the accuracy of measurements. The larger it is, the more detailed you can study the waveform. In this case, the value of this parameter should exceed the frequency of the signal under investigation by several times.
• Sampling. Determines the resolution of the instrument.
• The number of channels. Their value determines the number of simultaneously independent measurements that can be performed on the device. This makes it possible to display several graphs on the screen at once and compare them with each other. The radio amateur class has 2-4 channels, and the professional class has up to 16.
• Memory size. Its value affects the response speed of the device.
• Type of food. There are devices that operate on a 220 volt alternating voltage network or rechargeable batteries.
• Rise time of the input signal. Less is better. This means that the less the beginning of the first signal on the screen is “gnawed off” during internal synchronization, the better the frequency properties of the oscilloscope.
• Screen characteristics. This includes: detail, inertia, sweep frequency. Moreover, the higher the resolution, the greater the degree of detail.
• Segmented memory mode. Some digital instruments have a segmented memory mode. That is, they have the ability to selectively capture signals with the desired (high) sampling rate.
• Equivalent mode availability. It is used to study a periodic signal. Allows you to raise the sampling rate several times.

Oscilloscope Applications

The oscilloscope is designed to study various relationships between several quantities. The waveform displayed on the screen shows how the voltage waveform changes over time. So, by it you can easily determine the polarity, amplitude, duration, duty cycle and frequency of the signal.

In a rough approximation, an oscilloscope works like a graphical voltmeter. It measures the signal and displays its shape on the display. Even high frequency voltages can be measured with the device. Its main purpose is to use troubleshooting in complex electronic circuits or research measurements. For example, using it it is possible:

• determine time parameters;
• study phase shift;
• fix the signal frequency;
• observe the alternating and constant voltage components;
• note the presence of harmonics and their parameters;
• find out the processes taking place in time.

Thus, the oscilloscope is needed in order to be able to visually observe the fluctuations of the electrical signal, and also see interference and distortion, thereby identifying the faulty element in various nodes by the shape of the input and output pulse. In addition, the oscilloscope is widely used in the diagnostics of electric motors. Studying the generations arising during the operation of the engine, it is possible to calculate the malfunction of the catalyst, identify increased air leakage, and track signals from various sensors.

Working with the meter

Calibration is performed before using the oscilloscope. To do this, the test leads are connected to the amplifier input (vertical deflection of the beam) and a common terminal, designated ground. If a CRT is used, after switching on, you must wait a while for the screen to warm up. Then you need to go through the following steps:

1. The time adjustment knob is set to a division corresponding to 1 ms / div.
2. The Volt / Div knob is switched to the 0.5 V / Div position.
3. Sync pulse control is switched to "auto" mode. If this position is not provided, then the internal synchronization is selected and the signal type is set - variable.
4. Rotating the beam position controls (up / down and right / left), set the "Auto" mode or simply achieve the appearance of the beam on the screen.
5. The signal type switch is set to the GND position.
6. The common probe is connected to a special ground contact of the device case. If there is no such contact in the oscilloscope, then the probe clamp is put on any uninsulated metal part of the case.
7. The Signal Type switch is moved to neutral to connect the pin to ground. If there is no such switch, then the probes are closed with each other.
8. Use the vertical and horizontal adjustment knobs to set the beam to the middle of the screen.
9. If the device has a "Signal type" switch, then it is set to the measurement position of a constant waveform, or the probe is simply disconnected from the ground socket.
10. Switching the scale "Volts / division" achieve the unfolding of the signal to the full screen, which increases the accuracy of observations.
11. With the help of the measuring wires, the necessary studies are started, adjusting the "Volt / division" scale, if necessary.

Thus, the use of an oscilloscope makes it possible to carry out operations for setting up and repairing complex instruments that cannot be performed using a tester. Working on a modern device is not much more difficult than measuring with a multimeter.