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06-27-2010, 12:20 AM
cathode ray oscilloscope
1- INTRODUCTION:
A cathode ray oscilloscope (often called a CRO) is an instrument which can be used to measure voltages and/or frequencies of electrical signals. It is widely used in scientific measurement and research.
Oscilloscopes are have a display screen, and a panel of control knobs and input sockets. The signal to be measured is fed to one of the input connectors, using a coaxial cable, or a 'scope probe'
Because so many physical quantities can be converted to a voltage, a CRO can be used to display and measure such diverse quantities as temperature, sound, velocity and light intensity. It does so by plotting a two-dimensional graph of the voltage of one or more electrical signals (vertical axis) plotted usually as a function of time (horizontal axis).
In this representation of a trace on a CRO, the Potential (voltage) varies from -10V to +10V approx. many times per second.
A grid is drawn on the screen of a CRO to help with measurement.
Each square of the grid is known as a division.
Demonstration (http://www.phy.ntnu.edu.tw/ntnujava/viewtopic.php?t=51)
Functions of an oscilloscope
Trace
In its simplest mode, the oscilloscope repeatedly draws a horizontal line called the trace across the middle of the screen from left to right. The timebase control, sets the speed at which the line is drawn, and is calibrated in seconds per division.
Demonstration (http://www.upscale.utoronto.ca/PVB/Harrison/Oscilloscope/Flash/demo1.html)
Shows the effect of changing the time base control on CRO when there is no input voltage.
If the input voltage is not zero, the trace is deflected either upwards or downwards. The vertical control, sets the scale of the vertical deflection, and is calibrated in volts per division. The resulting trace is a graph of voltage against time obtained just by setting the timebase to match the frequency of the input signal.
Many osilloscopes have two or three channels so that signals can be input on different channels and compared.
Half Wave Rectification produced by a diode, displayed on top of origianl alternating current (shown by dashed lines).
Two channels overlayed on screen of CRO
Timebase
if the input signal is a 20 Hertz sine wave, then its period is 50 ms, so the timebase should be adjusted so that the time between successive horizontal sweeps is 50 ms. Unfortunately, an oscilloscope's timebase is not perfectly accurate, and the frequency of the input signal is not perfectly stable, so the trace will drift across the screen making measurements difficult.
Demonstration (http://www.upscale.utoronto.ca/PVB/Harrison/Oscilloscope/Flash/demo2.html)
Shows the effect of changing the time base control on the display when there is an input voltage varying in time.
If the input voltage is of higher frequency, then the trace may well be a sine wave.
Demonstration (http://www.upscale.utoronto.ca/PVB/Harrison/Oscilloscope/Flash/demo3.html)
Shows the effect of changing the time base control on the display when there is an input voltage varying in time when the frequency of the voltage is high.
Voltage Control
This affects the amplitude of the signal displayed on the screen of the CRO.
Demonstration (http://www.upscale.utoronto.ca/PVB/Harrison/Oscilloscope/Flash/demo4.html)
Shows the effect of changing the voltage control on the display.
Trigger
To provide a more stable trace, modern oscilloscopes have a function called the trigger. When using triggering, the scope will pause each time the sweep reaches the extreme right side of the screen. The scope then waits for a specified event before drawing the next trace. The effect is to prevent horizontal drift of the trace. In this way, triggering allows the display of periodic signals such as sine waves.
Demonstration (http://www.upscale.utoronto.ca/PVB/Harrison/Oscilloscope/Flash/demo6.htm)
Showing the effect of changing the trigger level on the display.
If you wish to review the functions of the CRO, try the demonstration below:
Demonstration (http://www.phy.ntnu.edu.tw/ntnujava/viewtopic.php?t=51)
Functions of an oscilloscope
Internal formed-2
A cathode ray oscilloscope (C.R.O) is an instrument that converts
electronic and electrical signals to a visual display.
The graph produced consist of a horizontal axis which is normally a
function of time, and a vertical axis which is a function of the input
voltage.
The components in a cathode ray tube consists of a vacuum glass tube
with an electron gun, a deflection system for deflecting the electron
beam and a fluorescent coated screen.
3-type
1-Digital Sampling Oscilloscopes
These oscilloscopes are used for analyzing very high frequency signals. They are used for looking at repetitive signals which are higher than the sample rate of the scope. They collect the samples by assembling samples from several successive waveforms, and by assembling them during the processing, they are able to build up a picture of the waveform. In this way these oscilloscopes may be able to view signals at frequencies up to 50 GHz and more.
The design of these scopes is optimised for very high frequency operation, and to achieve this the vertical amplifier topology is somewhat different. On entering the vertical amplifier chain, the signal is sampled prior to any amplification to ensure the maximum bandwidth is achieved. After the signal is sampled a lower frequency amplifier / attenuator combination can be used because the signal is effectively at a lower frequency at this stage. However this methodology does reduce the dynamic range of the instrument. Typically the maximum voltage that can be handled is around 3 volts peak to peak and it is not possible to place protection diodes ahead of the sampling diode ring as this would limit the frequency response.
2-Digital storage oscilloscope
The digital storage oscilloscope (DSO) is the conventional form of digital oscilloscope. It uses a raster type screen like that used on a computer monitor or television and in this way displays an image that fills the screen and may include other elements in addition to the waveform. These additional items may include text on the screen and the like.
To understand more about a digital storage oscilloscope it is necessary to understand what is inside the unit. The first stage the signal enters within the scope is the vertical amplifier where some analogue signal conditioning is undertaken to scale and position the waveform. Next this signal is applied to an analogue to digital converter (ADC). This takes samples are regular time intervals or sample points. The actual rate at which the samples are taken is important because this determines the time resolution to which the signal can be analysed later. Scope specifications quote the sample rate as a number of samples per second, or more usually mega samples per second (MS/s) or Giga samples per second GS/s)
The samples are stored in the memory within the oscilloscope as what are termed waveform points, and a single waveform point may be made up of several samples. The overall waveform is stored as a waveform record and its start is governed by the trigger, its finish being determined by the horizontal timebase time.
Being digital in format there is naturally a signal processor. This enables the signal to be processed in a variety of ways, before passing to the display memory and the display itself.
DSOs are widely used for many applications in view of their flexibility and performance. They excel when used as a single shot mode as the image can be captured, stored and manipulated as required. As a result they can be used for capturing transient conditions that may not be as easy to examine when using other forms of scope.
3-Analogue oscilloscope
The analogue oscilloscope of the original type of oscilloscope. As the name implies it uses analogue techniques throughout to create the pattern on the display. Typically they use a cathode ray tube where the voltages on the x and y plats cause a dot on the screen to move. In the horizontal direction this is controlled by the time base, whereas in the vertical direction the deflection is proportional to the signal input. Essentially the signal is amplified and applied to the Y plates of the cathode ray tube.
A cathode ray tube consists of a number of elements. There is an electron gun that generates an electron beam that is fired along the length of the tube. This beam passes by deflection plates that are used to deflect the beam, as a result of electrostatic attraction and repulsion, and finally the beam hits a phosphor coating on the "screen" creating a small dot of light.
To assist in making the trace as clear as possible, intensity and focus controls are included. The focus ensures that the dot that scans the screen remains as sharp as possible and in this way it can deliver a clear trace. The intensity control is required because the intensity of the dot or trace varies according to the speed at which the scan is made. Controlling the intensity enables a clear trace to be obtained.
When the scan is very slow the dot is seen to traverse the screen and it is difficult to visualize the waveform. As the speed increases, it ceases to be seen as a dot, but instead it traces out a line and the signal waveform, which when triggered correctly remains static on the screen. The trace may be scanned across the screen many times a second. In many instances it my traverse the screen 100 000, 500 000 or more times a second.
However as the writing speed increases, the trace becomes steadily more dim, and ultimately becomes difficult to see despite the intensity control. For higher frequency signals faster writing speeds are required, and as a result analogue oscilloscopes have a limited frequency range. Typically the maximum frequency that can be seen by an analogue oscilloscope is around 1 GHz. Above this other types of oscilloscope are required.
4-Digital oscilloscopes
The concept behind the digital oscilloscope is somewhat different to an analogue scope. Rather than processing the signals in an analogue fashion, the scope converts them into a digital format using an analogue to digital converter and then processes the signals digitally and then may convert them into an analogue format again for them to be displayed. With digital signal processing hardware and software becoming more powerful, this enables the processing of the signals to be undertaken in a far more flexible manner, and enables many additional features to be added.
Digital oscilloscopes can be put into three main categories: the digital storage oscilloscope; digital phosphor oscilloscope, and the digital sampling oscilloscope
5-Digital Phosphor Oscilloscope
For any oscilloscope there is a time delay between the end of one scan and when the trigger is ready to initiate the next one. During this period the scope does not see any activity that may occur on the signal line For a DSO this time can be relatively long because the scope processes information serially and this can form a bottleneck. However the DPO uses a separate parallel processor and this enables it to capture and store waveforms despite the fact that the display may be acting much slower. By using the parallel processing the DPO is not limited by the speed of the display, signals may be captured independently of the activity of the display.
Although the name of the DPO may indicate that it relies on a chemical phosphor, this is not necessarily the case as more modern displays are used. However it possesses many of the aspects of a phosphor oscilloscope, displaying a more intense image the more often the waveform passes a certain point. Each time a waveform is captured it is mapped into the DPO memory. Each cell represents a screen location. The more times data is stored into a location, the greater the intensity attached to it. In this way intensity information builds up in cells where the waveform passes most often. The overall result is that the display reveals intensified waveform areas, in proportion to the frequency of occurrence of the signal at each point. This has the same appearance as those displayed on an analogue phosphor oscilloscope, and this gives rise to the name.
1- INTRODUCTION:
A cathode ray oscilloscope (often called a CRO) is an instrument which can be used to measure voltages and/or frequencies of electrical signals. It is widely used in scientific measurement and research.
Oscilloscopes are have a display screen, and a panel of control knobs and input sockets. The signal to be measured is fed to one of the input connectors, using a coaxial cable, or a 'scope probe'
Because so many physical quantities can be converted to a voltage, a CRO can be used to display and measure such diverse quantities as temperature, sound, velocity and light intensity. It does so by plotting a two-dimensional graph of the voltage of one or more electrical signals (vertical axis) plotted usually as a function of time (horizontal axis).
In this representation of a trace on a CRO, the Potential (voltage) varies from -10V to +10V approx. many times per second.
A grid is drawn on the screen of a CRO to help with measurement.
Each square of the grid is known as a division.
Demonstration (http://www.phy.ntnu.edu.tw/ntnujava/viewtopic.php?t=51)
Functions of an oscilloscope
Trace
In its simplest mode, the oscilloscope repeatedly draws a horizontal line called the trace across the middle of the screen from left to right. The timebase control, sets the speed at which the line is drawn, and is calibrated in seconds per division.
Demonstration (http://www.upscale.utoronto.ca/PVB/Harrison/Oscilloscope/Flash/demo1.html)
Shows the effect of changing the time base control on CRO when there is no input voltage.
If the input voltage is not zero, the trace is deflected either upwards or downwards. The vertical control, sets the scale of the vertical deflection, and is calibrated in volts per division. The resulting trace is a graph of voltage against time obtained just by setting the timebase to match the frequency of the input signal.
Many osilloscopes have two or three channels so that signals can be input on different channels and compared.
Half Wave Rectification produced by a diode, displayed on top of origianl alternating current (shown by dashed lines).
Two channels overlayed on screen of CRO
Timebase
if the input signal is a 20 Hertz sine wave, then its period is 50 ms, so the timebase should be adjusted so that the time between successive horizontal sweeps is 50 ms. Unfortunately, an oscilloscope's timebase is not perfectly accurate, and the frequency of the input signal is not perfectly stable, so the trace will drift across the screen making measurements difficult.
Demonstration (http://www.upscale.utoronto.ca/PVB/Harrison/Oscilloscope/Flash/demo2.html)
Shows the effect of changing the time base control on the display when there is an input voltage varying in time.
If the input voltage is of higher frequency, then the trace may well be a sine wave.
Demonstration (http://www.upscale.utoronto.ca/PVB/Harrison/Oscilloscope/Flash/demo3.html)
Shows the effect of changing the time base control on the display when there is an input voltage varying in time when the frequency of the voltage is high.
Voltage Control
This affects the amplitude of the signal displayed on the screen of the CRO.
Demonstration (http://www.upscale.utoronto.ca/PVB/Harrison/Oscilloscope/Flash/demo4.html)
Shows the effect of changing the voltage control on the display.
Trigger
To provide a more stable trace, modern oscilloscopes have a function called the trigger. When using triggering, the scope will pause each time the sweep reaches the extreme right side of the screen. The scope then waits for a specified event before drawing the next trace. The effect is to prevent horizontal drift of the trace. In this way, triggering allows the display of periodic signals such as sine waves.
Demonstration (http://www.upscale.utoronto.ca/PVB/Harrison/Oscilloscope/Flash/demo6.htm)
Showing the effect of changing the trigger level on the display.
If you wish to review the functions of the CRO, try the demonstration below:
Demonstration (http://www.phy.ntnu.edu.tw/ntnujava/viewtopic.php?t=51)
Functions of an oscilloscope
Internal formed-2
A cathode ray oscilloscope (C.R.O) is an instrument that converts
electronic and electrical signals to a visual display.
The graph produced consist of a horizontal axis which is normally a
function of time, and a vertical axis which is a function of the input
voltage.
The components in a cathode ray tube consists of a vacuum glass tube
with an electron gun, a deflection system for deflecting the electron
beam and a fluorescent coated screen.
3-type
1-Digital Sampling Oscilloscopes
These oscilloscopes are used for analyzing very high frequency signals. They are used for looking at repetitive signals which are higher than the sample rate of the scope. They collect the samples by assembling samples from several successive waveforms, and by assembling them during the processing, they are able to build up a picture of the waveform. In this way these oscilloscopes may be able to view signals at frequencies up to 50 GHz and more.
The design of these scopes is optimised for very high frequency operation, and to achieve this the vertical amplifier topology is somewhat different. On entering the vertical amplifier chain, the signal is sampled prior to any amplification to ensure the maximum bandwidth is achieved. After the signal is sampled a lower frequency amplifier / attenuator combination can be used because the signal is effectively at a lower frequency at this stage. However this methodology does reduce the dynamic range of the instrument. Typically the maximum voltage that can be handled is around 3 volts peak to peak and it is not possible to place protection diodes ahead of the sampling diode ring as this would limit the frequency response.
2-Digital storage oscilloscope
The digital storage oscilloscope (DSO) is the conventional form of digital oscilloscope. It uses a raster type screen like that used on a computer monitor or television and in this way displays an image that fills the screen and may include other elements in addition to the waveform. These additional items may include text on the screen and the like.
To understand more about a digital storage oscilloscope it is necessary to understand what is inside the unit. The first stage the signal enters within the scope is the vertical amplifier where some analogue signal conditioning is undertaken to scale and position the waveform. Next this signal is applied to an analogue to digital converter (ADC). This takes samples are regular time intervals or sample points. The actual rate at which the samples are taken is important because this determines the time resolution to which the signal can be analysed later. Scope specifications quote the sample rate as a number of samples per second, or more usually mega samples per second (MS/s) or Giga samples per second GS/s)
The samples are stored in the memory within the oscilloscope as what are termed waveform points, and a single waveform point may be made up of several samples. The overall waveform is stored as a waveform record and its start is governed by the trigger, its finish being determined by the horizontal timebase time.
Being digital in format there is naturally a signal processor. This enables the signal to be processed in a variety of ways, before passing to the display memory and the display itself.
DSOs are widely used for many applications in view of their flexibility and performance. They excel when used as a single shot mode as the image can be captured, stored and manipulated as required. As a result they can be used for capturing transient conditions that may not be as easy to examine when using other forms of scope.
3-Analogue oscilloscope
The analogue oscilloscope of the original type of oscilloscope. As the name implies it uses analogue techniques throughout to create the pattern on the display. Typically they use a cathode ray tube where the voltages on the x and y plats cause a dot on the screen to move. In the horizontal direction this is controlled by the time base, whereas in the vertical direction the deflection is proportional to the signal input. Essentially the signal is amplified and applied to the Y plates of the cathode ray tube.
A cathode ray tube consists of a number of elements. There is an electron gun that generates an electron beam that is fired along the length of the tube. This beam passes by deflection plates that are used to deflect the beam, as a result of electrostatic attraction and repulsion, and finally the beam hits a phosphor coating on the "screen" creating a small dot of light.
To assist in making the trace as clear as possible, intensity and focus controls are included. The focus ensures that the dot that scans the screen remains as sharp as possible and in this way it can deliver a clear trace. The intensity control is required because the intensity of the dot or trace varies according to the speed at which the scan is made. Controlling the intensity enables a clear trace to be obtained.
When the scan is very slow the dot is seen to traverse the screen and it is difficult to visualize the waveform. As the speed increases, it ceases to be seen as a dot, but instead it traces out a line and the signal waveform, which when triggered correctly remains static on the screen. The trace may be scanned across the screen many times a second. In many instances it my traverse the screen 100 000, 500 000 or more times a second.
However as the writing speed increases, the trace becomes steadily more dim, and ultimately becomes difficult to see despite the intensity control. For higher frequency signals faster writing speeds are required, and as a result analogue oscilloscopes have a limited frequency range. Typically the maximum frequency that can be seen by an analogue oscilloscope is around 1 GHz. Above this other types of oscilloscope are required.
4-Digital oscilloscopes
The concept behind the digital oscilloscope is somewhat different to an analogue scope. Rather than processing the signals in an analogue fashion, the scope converts them into a digital format using an analogue to digital converter and then processes the signals digitally and then may convert them into an analogue format again for them to be displayed. With digital signal processing hardware and software becoming more powerful, this enables the processing of the signals to be undertaken in a far more flexible manner, and enables many additional features to be added.
Digital oscilloscopes can be put into three main categories: the digital storage oscilloscope; digital phosphor oscilloscope, and the digital sampling oscilloscope
5-Digital Phosphor Oscilloscope
For any oscilloscope there is a time delay between the end of one scan and when the trigger is ready to initiate the next one. During this period the scope does not see any activity that may occur on the signal line For a DSO this time can be relatively long because the scope processes information serially and this can form a bottleneck. However the DPO uses a separate parallel processor and this enables it to capture and store waveforms despite the fact that the display may be acting much slower. By using the parallel processing the DPO is not limited by the speed of the display, signals may be captured independently of the activity of the display.
Although the name of the DPO may indicate that it relies on a chemical phosphor, this is not necessarily the case as more modern displays are used. However it possesses many of the aspects of a phosphor oscilloscope, displaying a more intense image the more often the waveform passes a certain point. Each time a waveform is captured it is mapped into the DPO memory. Each cell represents a screen location. The more times data is stored into a location, the greater the intensity attached to it. In this way intensity information builds up in cells where the waveform passes most often. The overall result is that the display reveals intensified waveform areas, in proportion to the frequency of occurrence of the signal at each point. This has the same appearance as those displayed on an analogue phosphor oscilloscope, and this gives rise to the name.