Cathode ray tube Essay
Cathode ray tube
An oscilloscope is the standard instrument that is used to examine time dependent voltages in a circuit. They are widely used in the medical professional to examine the electrical output of a heart, and in operating rooms. The voltage can be drawn on graph paper with the voltage on the vertical axis, and the time on the horizontal axis. Adjusting the scope will allow you to analyze part of one cycle or many cycles of the waveform. The objective of this lab is to become familiar with 2 types of oscilloscopes, the model 2530B made by BK Precision, and the Data Studio scope.
The main part of an analogue scope is a cathode ray tube (CRT), which is just a bean of electrons. Electrons are emitted from a hot cathode an accelerated by an electrode, then focused into a thin beam by electrostatic lenses. The “tube,” which is the glass vacuum envelope, is necessary so that the electrons will not become scattered by air molecules and the cathode will not burn up.
The electron beam is then passed through 2 pairs of deflection plates, where one pair of deflection plates deflects the electron beam in the vertical direction, while a voltage across the other pair deflects the electron in a horizontal direction. The electron beam passes through the deflection plates, and then strikes a phosphor material that covers the inside surface of a flat portion of the vacuum tube. Where the electron beam strikes the phosphor and some of this light passes to the outside of the tube is where the light is emitted and can be observed. When the electron beam is suddenly turned off, the light from phosphor does not immediately stop but decays in less than a second.
The electron beam points out of the flat part of the tube called the screen. When the beam is moved around the screen, it creates a pattern of light on the phosphor often called a trace. As the phosphor decays quickly, the trace must be constantly refreshed by the electron beam so the image can continuously be observed. It is refreshed by the electron beam executing
the same path on the screen.
The most simple way a scope is operated is to apply the voltage you want to examine to the vertical deflection plates, and this will allow the trace to proportional to the input voltage. A linear ramp voltage created by the scope is applied to the horizontal deflection plates. This ramp voltage sweeps the electron beam horizontally across the screen at a uniform rate.
The trigger voltage, Vt, is the value of the voltage that tells the electrons beams to start the beginning of the curve. The x-axis is a time scale of the curve and is equal to how fast the electron beam is swept across the phosphor. This is determined by the “time-base” of the scope – if the voltage has a very high frequency the time base needs to sweep the electron beam across the phosphor quickly. A good scope can display voltages with frequency in the MHz range.
A periodic function in time is one that repeats itself over and over again – such as a sine wave or a square wave. A scope can display a voltage that is periodic in exactly the same way. Every scope has a signal generator or time base oscillator. When the ramp voltage across the horizontal deflection plates is VL, the electron beam is at the left of the screen. When the ramp voltage is Vr, the beam is at the right of the screen center of the screen. When the ramp voltage is zero the beam is in the horizontal center of the screen – when the ramp voltage is Vr the beam is at the of the screen. The TIME/DIV controls the electron beam.
If a “trigger” pulse is applied to the circuit the electron beam is turned on and the time base oscillator applies the ramp to the horizontal deflection plates. The input voltage is applied to the vertical deflection plates and the electron beam is swept across the screen from left to right at a constant speed, which will take a time P. At the same time the vertical deflection of the electron beam is proportional to the input voltage. When the speed is slow, you can see a spot of light move across the screen – this spot traces the input voltage for a time P. If the speed is increased,
however, the trace will be too faint to see unless many sweeps executing the same trace are made. When the spot reaches the right end it very quickly returns to VL, and the electron beam is shut off.
When the trigger pulse, Vt, is derived from the input voltage, and can be either negative or positive. We must set the min and max value of Vt to be between the input voltage in order for the trigger to occur.
It takes time for the electron beam to go from the left side of the screen to the rights ide o the screen, and we call this time P. It does this repeatedly and we what we see is a stationary trace of the input voltage on the scope screen. Once a sweep starts it is always completed, even if the trigger conditions are met during the sweep – this allows the use of a low enough sweep speed so that a number of cycles of the input waveform can be displayed.
For a given input signal, when the electron horizontal sweep speed is increased, more of the input waveform or fewer cycles will be displayed. If the horizontal time/div sweep is decreased, the opposite will happen and less cycles will be displayed.
If the trigger voltage, Vt is changed, the trace of the input voltage can be shifted left to right in a continuous fashion, except in the square wave). If the trigger voltage is kept constant, and the trigger slope is changed, the trace will be shifted either to the right or left.
Remove any input cables to the scope and turn on scope (2). Wait until you see a trace (horizontal line), and leave the trace centered. Turn the time based switch (15) 0.25s and note the light spot move slowly across the screen all the way to the right. Observe the effect of turning the time base knob (15). What effects does increasing the horizontal click one at a time on the moving spot – does it become a solid line. We observed that
there is a range of sweep speeds for which the trace “flickers.”
5.2 Observing Voltage form the Function Generator
This function generator creates a sine, square, and triangular waveforms which are selected by three push buttons. The frequency can be adjusted from .3 Hz to 3 MH by adjusting a knob marked frequency and 7 pushbuttons. The output frequency appears in the middle of the display panel. It is important to check for the k or M to determine whether the stated frequency is in KHz or MHz – if not checked it can throw off calculations.
The amplitude of the output is controlled/varied by a knob at the right marked AMPL – these knobs have “in” and “out” positions. While using this instrument as a simple function generator, be sure all knobs at the bottom of the front panel are in and that the only two green LED’s that are on are one for the waveform and another for frequency.
Connect the 50 output of this oscillator to the CH1 vertical input (9) of the BK scope using the coaxial cable.
Set the AMPL knob full CCW and turn on both the oscillator and scope. Adjust the function generator for about a 1 kHz sine wave and turn the AMPL knob CW until you see a signal. Then adjust the Time/DIV knob until you see a sinewave. Vary all the control listed below leaving voltage input constant, and note any changes. (1) Change Volts/DIV controls
(2) Change Time/DIV controls
(3) Change Trigger Level voltage
(4) Rotate the Trig Level knob both ways.
(5) Change triggering slope from + to -.
(6) Change the positions.
5.3 Other Waveforms and Functions
Observe the square and triangular waveforms from the function generator by pushing the appropriate buttons. The frequency is not critical, but around 1 kHz works well. Then observe what happens to the waveforms when you pull
out the function generator knbo marked DUTY and rotate the knob CW and CCW. Push in the DUTY knob.
Then pull out the function generator OFFSET knob. On the scope, changing the coupling on CH1 from AC to DC, changing the coupling press the CH1 button (1) and a menu option comes up. Next on the function generator, rotate the OFFSET knob – we measured what happened to the trace. Then we observed what happened when we put it back.
5.4 Measuring Waveform Parameters
A useful function of a scope is to measure the peak voltage and period of a voltage waveform. We did this by making the trace so big that it takes up the entire screen. We determined the peak to peak voltage of the triangular wave and then using the calibration of the TIME/DIV control and the vertical grid lines on the screen, we determined the period of the wave. The display of the current time scale will be located in the middle of the bottom of the screen. The amplitude of the output of the function generator is not calibrated. You are using the scope to measure this amplitude – the frequency output of the function generator is calibrated and you should compare the function generator stated frequency with your period measurement.
We did this by setting the function generator to have a triangle wave output at 1 Khz. On the function generator, we turned the amplitude knob fully clockwise. Then using the oscilloscope to determine the peak to peak voltages of the triangle waves. We repeated this for the sine and square waveforms at 1 Khz.
5.5 Scope Probes
We were told not to do this part of the experiment.
Using the Data Studio Oscilloscope
One of the displays available with DataStudio is the oscilloscope – which is not an actual oscilloscope, but rather a software that closely mimics an oscilloscopes.
(1) There are 3 vertical nputs each of which has its own trace – and each trace has a different color. The input for each trace is selected by an input menu button.
(2) The scope is a storage scope – when you stop monitoring data the last trace is stored for further use.
(3) There is a smart cursor which is used in a similar way as in the graph display, it’s located on the top left of the DataStudio scope.
In comparison to the DataStudio oscilloscope, the BK is a much faster scope. The highest frequency you can examine with the BK scope is 25 MHz. There are only 2 trigger options in the Data Studio scope – the default is triggering the signal on the rising level. The other option is the falling level – which can be adjusted by selecting the little down arrow next to the triangle by the smart cursor.
6.1 Voltage From 750 Interface
In this section voltages from the DataStudio signal generator will be examined with the Data Studio scope. In the set up experiment window click on the output terminals of the 750 interface. When the signal generator will come up, drag the scope icon from the display window to the Output icon in Data window to open the scope display. Set the signal generator to 100 hz to click start and observe the different signals from the signal generator with the scope display and thoroughly familiarize yourself with the controls of the Data Studio scope.
6.2 DataStudio Scope and the Voltage Sensor
Using the voltage senor, repeate the steps in section 5.4 of measuring the waveforms. We observed the different peak to peak voltages for all three waveforms.
7.0 Analog Versus Digital Scopes
In an analog scope the input voltage, after amplification or attenuation, is
applied to the vertical deflection plates of the scope, the scope trace is smooth. You may have noticed this fact when looking at the DataStudio scope trace. In reality, the actual sampling rate is lower and is usually adjusted so that there are both a few hundred to a thousand samples per sweep. To get a reasonable portrayal of the signal the sampling rate should be at least ten the frequency of the signal. We determined the sampling rate for a sweep speed of 1 ms/div and then calculated the samples per sweep.
V. DATA & CALCULATIONS
When we turned the scope to (2), within a few seconds we saw a trace which was a horizontal line. Leaving the trace centered, we turned the time base switch (15) to .25s and noted the light spot move across the screen until it got to the right hand side of the screen. We noticed that the flicker of the trace got faster as we decreased time.
5.2 Observing Voltage From the Function Generator
1. The VOLTS/DIV controls are changed?:
This changed the values on the y-axis. When the Volts/Div is increased, the y-axis condenses the amplitidue making the amplitude appear smaller, and vice versa. It also allowed us to see more periods and cycles.
2. The TIME/DIV controls are changed?
Each wavelength is 1000 s when you increase the time/div, it condenses the x-axis making it appear to get smaller, when really it is not changing. Adding greater values to each square allows more cycles to be shown, while period stays the same.
3. When The trigger level voltage is changed?
Decreasing trigger level voltage causes the picture to move to the right & vice versa. 4. You rotate the TRIG LEVEL knob both ways? Can you make the trace disappear? Explain. Adjusting Trig level cannot make it disappear, rather it stops moving at the origin. And above the max we can’t make it stop so location affects the trace.
5. Changing the triggering slope changed from + to – ?
6. The position controls are changed?
This changes the placement of the graph relative to the y-axis.
5.3 Other Waveforms and Functions
Observe square and triangular waveforms. (Draw in pictures)
5.4 Measuring Waveform Parameters
The peak to voltages are 6.4 volts. This is the same for the sine and the square waveforms at 1 Khz.
5.5 Scope Probes
We were told not to do this.
6.2 Data Studio and the Voltage Sensor
For all three waveforms, we got 6.4 volts, same as with the Oscilloscope.
7.0 Analog versus Digital Scopes
We determined the sampling rate for a sweep speed of 1 ms/div and then calculated the samples per sweep.
1 ms/div = n cycles
2 ms/div = n/2 cycles
VI. ERROR ANALYSIS
In this experiment we were not calculating any data so much as were just getting used to the using the two machines. Therefore, there isn’t much error analysis to be made.
Both the BK Oscilliscope and the Data Studio Oscilliscope were very useful in analyzing time dependent voltages. They both gave us very similar results when we adjusted various functions on the machine.