Mar 072013
 

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4-channel digital oscilloscope GBS 1204. We can see two analog and two digital signals simultaneously displayed on the screen, each with a different color.

 

An oscilloscope, also called an oscillograph ( Russian), in English technical literature it can be found also as CRO (for cathode-ray oscilloscope) or DSO (for digital storage oscilloscope) is a modern electronic  instrument that allows observation of constantly varying signal voltages, usually as a two-dimensional plot of one or more signals as a function of time. The resulting graph is called oscillogram. Displayed on two axes: X and Y, the oscillogram provides accurate information about the voltage fluctuations for a definite period of time as opposed to the voltmeter, for example, which measures only the momentary value of the voltage.

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Amplitude and period of the waveform displayed on the oscilloscope

 

The expanded image of the measured and processed signal allows us to analyze not only its size but also its form, amplitude and frequency.

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Reading the oscilloscope display

The oscilloscope is one of the most commonly used devices in radio-electronics, both by amateurs and professional scientists in research laboratories. Oscilloscopes are indispensable instruments in areas such as medicine, ECG, engineering, telecommunications. Basic parameter of the oscilloscope is frequency (Hz) – this is the number of cycles of a periodic signal, repeated per second. Another key quantity is the impulse duration – this is the period of time during which the signal is in a position different from zero.

Non-electrical signals (sound, vibration) can be converted into voltage and also be measured with the aid of an oscilloscope.

 

Types of Oscilloscopes
Oscilloscopes can be analog, digital and mixed-signal, each type having its specific characteristics, advantages and disadvantages. According to the number of measuring inputs oscilloscopes are divided into single-channel, dual-channel and multi-channel oscilloscopes. According to their function, oscilloscopes are divided into universal and specialized. Oscilloscopes are not only single devices, they could be an external device plugged into a computer (the so called computer / USB oscilloscope), working with a specialized program installed in it – the monitor of the PC or laptop is used as a display. There are oscilloscopes which are combined with other measuring devices – scopemeters.

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Analog dual-channel oscilloscope GOS-630FC. Although this is an analog oscilloscope, it can visualize the frequency (in the LCD display), the amplitude time base of the two channels as well as the position of the time base.

 

Analog oscilloscopes (CRT) process analog signals. They are massive, heavy and are not suitable for mobile work. They are used to measure and study relatively slow processes. They are not very suitable for fast processes, repeated over a long period of time or for very slow processes. Their main parameters are frequency bandwidth (range), number of input channels, input impedance, trigger, etc.
Digital oscilloscopes digitize the received signal, they have memory and can memorize a series of oscillograms, which can afterwards be opened on the display and tracked over time. They are more compact, they can be moved easily when needed, and there is a variety of miniature oscilloscopes for field work (portable oscilloscopes) powered by batteries with the size of a multimeter.

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Portable oscilloscopes

 

The key parameters here are range (frequency bandwidth); real sampling rate (measured in sampe/ s or kSa / s, MSa / s, GSa / s). There are digital oscilloscopes that have slots for installation of functional generators, logical analyzers

izpolzvane na oscil. s vgraden logic analyser

and other electronic devices that make the digital oscilloscope a convenient and practical device for each laboratory and service-station. Almost all digital oscilloscopes show on their displays the frequency of the measured periodic signal, the amplitude and RMS values. They can record both short impulses and a continuous process.

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Measured quantities: peak to peak voltage, maximum, minimum and average voltage, amplitude, RMS, frequency of the signal, period of the function, time for increasing the voltage, time for reducing the voltage, duty factor.

 

The panel of the oscilloscope consists of the following main sectors: display, LCD, LED, CRT or computer monitor; vertical controllers (vertical amplitude time base) of the signal; horizontal controllers (horizontal time base); trigger control, trigger viewing performs the synchronization and is necessary to stabilize the images of the signal, the signal level and its type; there are also front panel controls such as focus, intensity and beam finder.

 

In addition to the main unit there is one (or more, in multi-channel oscilloscopes) probe with a clip for ground and a resistor equal to 10 times the input resistance (impedance) of the oscilloscope. This leads to a 10-fold attenuation of the input signal, but it helps to insulate the capacitance component of the probe cable in the measured signal. Thus, it is possible to measure a 10-fold greater signal than the range of the oscilloscope. Some probes have a switch that shunts this resistance when it is necessary.

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The measured signal is fed to one of the inputs fitted with connectors. At low frequencies we can also use banana plugs.

When working with an oscilloscope, we initially see a horizontal straight line on the X-axis on the display. This is the zero line, when no source of voltage is connected to the oscilloscope. It is called zero line because it corresponds to a voltage equal to zero. This zero line can move vertically on the oscilloscope: it can go up or down depending on the type of the sampled signal or when there are more signals – for more clarity. If constant voltage is fed to the input of the oscilloscope, the formed oscillogram will be a straight horizontal line shifted vertically, and the difference between the oscillogram and the zero line will be proportional to the voltage value.

 

Of course, most oscillograms differ from a straight line, otherwise we wouldn’t need an oscilloscope. When we measure and study AC voltage, we talk about voltage amplitude. When it goes off the screen, we need vertical adjustment of the displayed oscillogram, i.e. the range increases. A smaller range is required for lower voltage values. The vertical amplification and reduction of the range on the display depends on the voltage amplitude.

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Basic values ​​visualized on the display of the oscilloscope and their names.

 

Signals may be periodical and non-periodical. The latter are repeated at regular intervals, but they are different. There are complex signals with more than one frequency. Signals can be digital or analog. Analog signals are those in which voltage changes over time. Digital signals vary only between two values: on and off or 0 and 1. Digital signals are generated by various electronic components such as transistors, sensors for terminal positions, etc. or by various electromechanical switches such as relays and switches.

The time interval is adjusted horizontally (horizontal time base). If we want to study a longer period of the oscillogram, we need to set a longer time interval through the time switch.

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Change in visualization depending on the time range

The speed ​​determines the time base of the signal. The time base appears on the display from left to right. If we want to examine only a certain impulse or a small fragment, we need to reduce the time interval.

Synchronization is necessary to stabilize the voltage graph on the display. If the oscilloscope has more than one channel, we need to select a synchronization channel and a synchronization level. The synchronization itself is done through a signal edge that is going up or down.

The impulse “drawing“ must always start from the same point. The moment when the drawing on the new screen starts is called trigger viewing. Some oscilloscopes have pre-trigger viewing as well. This is when information, prior to the trigger viewing, is derived from the buffer memory of the oscilloscope. This function is available only in digital oscilloscopes. Synchronization is necessary for obtaining a static image. There are several types of synchronization: automatic, single, external and HOLD-OFF.

 

Automatic synchronization is used for measuring periodically recurring signals.
Single synchronization is used when there are single signals. We again select synchronization level and choose whether to do the synchronization through a signal edge that is going up or down. This type of synchronization may be applied to single signals that pulsate at definite intervals.
External synchronization is used when the oscilloscope has an input (outlet) for external synchronization.
HOLD-OFF, this function is very useful when we want to visualize complex signals which consist of more than one frequency.

In conclusion, we can add that the oscilloscope is a universal, widely used and accurate measuring instrument by which we can explore the nature of electrical and non-electrical signals with extreme precision. Its usage in car-diagnostics, medicine, audio and video engineering and in almost all modern laboratories, makes the oscilloscope invaluable to scientists, engineers and to anyone curious about the nature of the world we live in.

 

 

 

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