To study and perform experiment on CRO demonstration kit.
Aim: To study and perform experiment on CRO demonstration kit.
Apparatus: CRO kit, connecting lead, power supply.
Theory:- The CRO ( cathode ray oscilloscope), often referred to as a “scope”, is the most powerful tool available for measuring electrical quantities associated with electronic circuitry. It is such an important instrument that a thorough understanding of its operations is absolutely essential for any electrical engineer. Although we will be using a Digital Storage Oscilloscope (DSO), it is instructive to cover the basics of the Cathode Ray tube technology since there are many still in industry. The display on the CRO screen is created by an electron beam exciting a phosphor coating on the inside face of the CRT (cathode ray tube).Fig.3.1(a) shows the major parts of a Cathode Ray Tube (CRT).
The electrons generated by the cathode are accelerated and focused into an electron beam. When the beam strikes the fluorescent screen, it emits a tiny spot of visible light. The position of the electron beam (hence the spot on the screen) generated by the electron gun is controlled by the vertical and horizontal deflection plates. The spot on the screen changes its position depending on the voltages applied to the vertical and horizontal deflection plates. Continuously changing signals on vertical and horizontal deflection plates causes the beam to trace out a path on the screen. Even though only one spot is highlighted on the screen by the electron beam at any given moment, the persistence of the phosphor makes the path appear to be continuous. The path must be retraced frequently in order to render a steady display.
Figure 3.1(a):- Explanation of a cathode ray tube (CRT) - oscilloscope...
Figure 3.1(b):- Like a television screen, the screen of an oscilloscope consists of a cathode ray tube.
Figure 3.1(c):-The basic structure 0f a CRO
Based on the operations of the CRT discussed above, there are four major electronic subsystems needed to generate control signals for the CRT: Display control – for controlling the intensity and focus of the electron beam. Vertical control—for controlling the up-down deflection of the electron beam by external signals. These signals are amplified through a series of electronic circuits controlled by the front panel knob whose scale is calibrated in VOLTS/DIV on the older scopes. Horizontal control – for controlling the right-left deflection of the electron beam by either internal or external signals. If the external input is selected for the horizontal deflection system, the scope is operating in the x-y mode and the horizontal scale is calibrated in VOLTS/DIV. If the internal source is chosen, the scope is operating in the y-t (or time-base) mode and the horizontal scale is given in SEC/DIV. There is an internal circuit (known as the sweep circuit) producing a time-calibrated signal (sweep signal) for the horizontal deflection plates. Trigger control – for controlling the starting point of each trace when operating in the y-t mode. In order to obtain a steady display pattern, each trace (and there could be thousands of traces per second) must duplicate exactly the previous trace. Thus, each trace must begin from precisely the same point of a periodic waveform in order for traces to fall on top of each other. The trigger subsystem permits one to select the exact starting point on each trace.
Figure: - 3.2, Digital Storage type Oscilloscope
In this lab we will be using a Digital Storage type Oscilloscope (DSO). The technology is radically different from the conventional CRT type oscilloscope, but its purpose is the same and in many ways it is much easier to use. In the normal acquire mode of the DSO, it constantly samples the data and updates the screen. The DSO software acquires (samples) the input every 5 ns. It compresses 2 million acquisition points per channel into a 1,000-point display record. Because the input information is digitized, measurements and calculations are readily available at the push of a button. Study Appendix G on operations of the scope carefully.
Figure: - 3.3, Sinusoidal waveform
Sometimes it is difficult to tell whether a waveform is a true sinusoidal waveform. However, qualitative inspections of its attributes are still possible using the scope. Obtain a full screen display of exactly one cycle of the waveform. Switch the scope INPUT COUPLING switch to AC to block any possible dc offset. Let’s define the “starting point” of a sine wave as the positive-going zero-crossing point (not the starting point of the trace). Adjust the TRIGGER LEVEL on the scope so that the “starting point” of the waveform is precisely at the centre of the screen.
The first thing you can do is to visually inspect if the waveform on the scope is “smooth” and follows the general sinusoidal pattern. The next thing you can do is to check its symmetry. A sine wave should exhibit an odd symmetry. That is, f (t) =-f (-t). Another thing you can do is to check the voltage values at some selected points. A convenient way of measuring the voltage is to adjust the scope trigger level so that the “starting point” of the waveform is shifted either to the right or left (on the centre horizontal gratitude line) by an integral number of divisions. If the trigger level is adjusted so that the “starting point” is shifted to the left by one division, the waveform should cross the centre vertical axis at v=Vpsin (36).
(With one cycle covering 10 divisions, each division corresponds to 36 in angle.) Similar measurements can be made for values at other angles.
The quality of a square wave can be measured by the “flatness” of the top and bottom flat areas of the waveform, the symmetry of the waveform and the speed of transitions from one level to the other known as the “rise” and “fall” times as illustrated in fig. 3.4. The smaller the rise and fall times, the better the square wave.
Figure:-3.4, Rise and fall time of square wave
To make accurate rise and fall time measurements, use the Quick Meas button on the oscilloscope and follow the on-screen menu to locate rise time and fall time. An ideal square wave has zero rise and fall times. This is impossible to achieve in real world. The rise and fall times of the square wave from your function generator are usually very small (in nano seconds); you may have to use a high frequency waveform and a fast time base setting on the scope. Note that the observed rise and fall times are not all due to the waveform itself. The limited bandwidth of the scope also contributes to the readings you obtain from the scope.
Figure: - 3.5, Triangular Wave
Before coming to class, you will need to find the RMS value of the triangular wave in Fig. 4. Use the formula provided in your textbook.
The quality of a triangular wave is measured by it symmetry and the “linearity” of its ramps. The increasing ramp is also known as the positive ramp while the decreasing ramp is also referred to as the negative ramp.
1) Using Quick Guide to Using the Electronics Laboratory Equipment, as your guide, create a 1 Volt peak-to-peak (1 Vpp) sine wave (0 volts DC offset) at 1 kHz with the Waveform Generator and connect the output of the Waveform Generator to the Channel 1 input of the oscilloscope. Be aware that waveforms can be referred to as peak, rms, or peak-to-peak. (Note: a 1Vpp sine wave means that the peak is 0.5V). What is the calculated RMS value of a 1 Vpp sine wave?
2) DSO setup procedure
a) Make sure that the Run/Stop button of the DSO is green for continuous scanning.
b) Make sure that the Channel 1 button is green, indicating that Channel 1 will be
Used. If channel 2 is not being used, press the Channel 2 button until the green light goes out.
c) Press the Auto-scale button to automatically configure the DSO for the waveform.
3) Horizontal controls
4) Vertical controls
a) Practice adjusting the Horizontal controls on the DSO and watch what it does to the waveform. Notice that at the top of display screen that some numbers change when the controls are adjusted. What do those numbers signify?
a) Use Auto-scale if needed to restore the waveform.
b) Press the Quick Means button on the DSO to enable the on-screen menu of measurements.
c) Press the on-screen menu button for Frequency, and Period. Notice that frequency and period are now displayed on the screen.
d) Press the Next Screen arrow and continue until you see the RMS menu. Press that button to display the RMS value. What is the RMS value? Does this value correspond to your calculation in part 1? Print out the waveform (using “Quick Print”)
6) Cursors: rise time and fall time
a) Use the cursor function by pressing “cursors” on the oscilloscope. Cursors will be used when performing rise-time and fall-time measurements.
b) The rise-time of a signal is the time difference between the crossing of the lower threshold cursor and the crossing of the upper threshold cursor for a positive going edge. The X cursor shows the edge being measured. For maximum measurement accuracy, adjust the time base as much as possible while leaving the complete rising edge of the waveform on the display.
c) The fall-time of a signal is the time difference between the crossing of the upper threshold cursor and the crossing of the lower threshold cursor for a negative going edge. The X cursor shows the edge being measured. For maximum measurement accuracy, set the sweep speed as fast as possible while leaving the complete rising edge of the waveform on the display.
7) Adjust the function generator to output a sinusoidal waveform with Vp=2.5 V, f=1 kHz, dc offset =0 V. Adjust the scope settings so that between one and two cycles of the sinusoidal function is displayed on the screen. Make sure that the waveform is centred on the screen with DC input coupling (use “Mode Coupling” on oscilloscope). What is the AC VOLT reading on the DMM? Does it agree with the calculated rms value?
8) adjust the display so that exactly one cycle of the waveform covers the entire screen with the positive-going zero-crossing point (i.e., the “starting point”) of the waveform coincides with the center origin. Is the waveform symmetric? Print waveform.
Using the cursors (one complete cycle is 360), read voltage values at 36`, 54`,-36`, and -54`. Do they agree with expected values? What can you conclude about the sine wave provided by the function generator? Is it close to an ideal sine wave?
9) Adjust the scope and the function generator to produce on the screen about one complete cycle of the square wave shown in Fig. 2. Make sure that the offset is zero. What is the DMM AC VOLT reading? Does it agree with the calculated rms value of the square wave? If not, why? Is the square wave symmetrical? (The square wave shown in fig. 2 is also an odd function.) Examine the top and bottom flat parts of the waveform using as a sensitive vertical scale as possible. You may have to use the vertical POSITION knob to bring the waveform back to the viewing area. Are these parts really “flat”? Print waveform.
10) Set the sweep speed as fast as possible while leaving the complete rising edge of the waveform on the display. Is there a clear rise time and fall time of the square wave? If so, measure using the Quick Meas function. If not, print out the results and explain what is happening.
11) Change the scope settings and the function generator to produce on the screen about one complete cycle of the triangular waveform shown in Fig. 4. Is the triangular wave symmetrical? (The triangular waveform shown in Fig. 4 is also an odd function.) What is the DMM AC VOLT reading? Does it agree with the calculated rms value of the triangular wave? If not, why? Are the positive and the negative going lines (ramps) linear as far as you can tell from the display? For better accuracy, adjust the scope so that the ramp of interest covers the entire screen with as sensitive vertical and horizontal scales as possible. Print waveform.
12) Using cursors, measure the rise and fall times of vo. Is vo a “good” square wave?
Why or why not? Print out both vi and vo plus cursor data.
13) Let vi be a triangular wave. Are the positive and negative ramps of vo linear? Is vo a “good” triangular wave? Why or why not out both vi and vo plus cursor data.
We have studied CRO demonstration kit.