The Cathode Ray Oscilloscope
MOTIVATION:
The cathode ray oscilloscope (CRO) is one of the most versatile instruments available in a modern laboratory. The device is readily at hand in every electronics shop, every radio and TV station's engineering department, and every physics, chemistry, and engineering research laboratory. The CRO is the heart of the auto engine analyzer and the patient monitor in a hospital intensive care ward. A fundamental element of this lab is becoming familiar with this instrument and how to use it in making measurements in a variety of situations.
Because the CRO is primarily a visual display instrument, able to display the waveform of virtually any kind of electrical signal, it is usually employed in situations where the waveform is of some interest, often with some kind of AC circuit. In this lab, we will not only be concerned about the shape of some electrical waveforms, but we will also make measurements with this instrument of the frequency, period, and voltage amplitude of electrical signals of an AC nature.
SPECIFIC OBJECTIVES:
When you have completed this laboratory exercise, you should be able to: (1) describe the basic components of an oscilloscope and explain the function of each; (2) use an oscilloscope to measure time intervals and potential differences; (3) use an oscilloscope to display periodic voltage waveforms; (4) use a dual trace oscilloscope to measure phase differences between two signals.
THEORY:
Basic Oscilloscope Theory:
The nucleus of the oscilloscope is the Cathode Ray Tube (CRT) -- a vacuum tube some 30 cm long which you can see in the center of the instrument. At the base (back of the instrument case) of this tube is a small wire filament (F) which is heated to high temperature when an electric current flows through it. Quite near the filament is a special metal oxide surface which is in turn heated by the filament. This surface, called the cathode (K), emits large numbers of electrons when heated. These electrons form a cloud around the cathode much like the cloud of water vapor over a heated pan of water. Some 5 cm from the cathode is a positive anode (A2) maintained at about +500 V potential. The anode attracts the electrons which accelerate across the space between the cathode and anode, acquiring a high velocity -- about 107 cm/s.
In the region on either side of the anode are coils of wire which surround the electron stream. Proper design and adjustment of the current flowing through these coils results in a narrow, well-focused beam of electrons. The beam passes through the cylindrical anode into an essentially field-free region where, some 20 cm away, it strikes a fluorescent screen. The screen glows at the point of impact of the electrons.
Very near the cathode is an open wire mesh called the grid (G). Electrons normally pass freely through the grid. However, when a small negative voltage is applied to the grid, electrons are repelled toward the cathode. This effectively reduces the number of electrons in the beam and thus reduces the intensity of the beam.
In the field-free region beyond the anode are two sets of parallel plates. The first set consists of two horizontal plates. When an electric field is established between these plates (by connecting them to a source of Emf) the beam is deflected up or down. These are called the vertical deflection plates. A similar set of plates can deflect the beam horizontally. The two sets of plates are connected to the vertical input and horizontal input of the scope.
The oscilloscope contains a special circuit which applies a time-varying Emf to the horizontal deflection plates (when the time/div knob is not in the XY position). This time varying voltage allows the beam to sweep across the screen from left to right by sending a "saw-tooth" voltage to the horizontal deflection plates. The steadily increasing Emf causes the spot to sweep across the tube at constant speed, the speed being selected by the knob setting. When the spot reaches the far right side of the screen, the Emf falls suddenly to zero and the spot almost instantaneously returns to the left side of the screen and starts across again. If the beam is moving fast enough, the spot looks like a line because the phosphor of the screen glows for a short time after the beam is gone. (Note: You will not see the saw-tooth pattern displayed on the screen of the oscilloscope. You will observe its effect by watching the beam sweep across the screen.)
EXPERIMENTAL ACTIVITY:
Scope familiarization:
In order to use the oscilloscope effectively, you need to be somewhat familiar with the function of control knobs and switches. In the figure on the next page is the front panel of the Hitachi V-212 Oscilloscope, which will be used in this lab. While this discussion is specific to this scope by knob location and name, all oscilloscopes are very similar in function. You should spend some time before coming to lab to read over the functions of the various controls. You may also want to review this section before the next experiment, which also uses the oscilloscope as a measuring instrument.
(1) POWER Switch: Push switch to turn on. The power lamp (2) is lit when the power is on.
(3) FOCUS Control: This knob can be adjusted to produce a sharp image on the screen. It should be used after the INTENSITY control (5) has been adjusted to the correct brightness level. Brightness is increased by rotating INTENSITY clockwise.
(4) TRACE ROTATION control is used to align the CRT trace with the horizontal graticule, and should be adjusted only by the instructor.
Vertical Deflection System Controls:
(8) CH1 INPUT Connector: The signal input for CH1, causing vertical deflection of the trace when used in either dual trace mode or CH1 single trace mode, is connected with this BNC connector. NOTE: When used in XY mode, CH1 input becomes the X-axis input signal.
(9) CH2 INPUT Connector: This signal input is the same as (8), except that it feeds CH2. NOTE: When used in XY mode, CH2 input become the Y-axis signal input.
(10), (11) Input coupling switches (AC-GND-DC): These switches, one for CH1 and the other for CH2, select the coupling system between the input signal and the vertical axis amplifier section of the oscilloscope.
AC At this setting, the signal is connected through a capacitor. The DC component of the input signal is cut off and only the AC component is displayed.
GND At this setting the input to the vertical axis amplifier is grounded.
DC At this setting the input signal is directly connected to the vertical axis amplifier and displayed unchanged, including the DC component.
(12), (13) VOLTS/DIV select switches: These switches, one for CH1 and the other for CH2, are used to set the gain of the vertical amplifiers. Called step attenuators, the vertical deflection is reduced in 1-2-5 steps as the switch is rotated. Set it to an easily observable range corresponding to the amplitude of the input signal. NOTE: Multiply the reading by 10 when the 10:1 probe (standard item) is used in combination with the oscilloscope.
(14), (15) VAR
PULL x5 GAIN Controls: These are small red knobs concentric with the VOLTS/DIV controls, that are fine tuning devices used to vary the vertical sensitivity (amplification) continuously. Full clockwise rotation of this control is referred to as the CAL position, meaning that the VOLTS/DIV setting is correct. If the VAR knob is not in the CAL position, signal voltage cannot be accurately read from the screen. When the knob is at PULL position, the gain of the vertical axis is magnified 5 times the normal setting, so that a signal that produces a 1 division deflection will show 5 divisions of deflection when the control is PULLed out.
(16) POSITION Control: This knob is used to adjust the vertical location of the CH1 display on the screen. The display rises with a clockwise rotation of the knob and falls with a counterclockwise rotation.
(17) POSITION
PULL INVERT Control: This knob is used to adjust the vertical location of the CH2 display on the screen. When the knob is PULLed out, the polarity of the input signal applied to CH2 is inverted.
(18) MODE Select Switch: This switch is used to select the operation mode of the vertical deflection system.
CH1 Only the signal input to CH1 is displayed on the screen.
CH2 Only the signal input to CH2 is displayed on the screen.
ALT Signals applied respectively to CH1 and CH2 appear on the screen, alternating at each sweep. This is a dual trace mode that can be used when sweep time is short.
CHOP At this setting the input signals applied to CH1 and CH2 are switched at about 250 kHz independent of the sweep and at the same time appear on the screen. This is a dual trace mode used when the sweep time is long.
ADD The algebraic sum of the two input signals appears on the screen as a single trace.
(20), (21) DC
BAL Adjustment Controls: These are used for the attenuator balance adjustment. These controls should not be disturbed by students.
Horizontal Deflection System Controls
(22) TIME/DIV Select Switch: This switch selects the sweep rate for normal oscilloscope usage, and also permits the oscilloscope to be used in XY mode. Sweep time ranges are available in 19 steps from 0.2 *useconds/division to 0.2 second/division. In the XY position, the X (horizontal) signal is connected to CH1 and the Y (vertical) signal is CH2.
(23) SWP VARiable Control: This is a small red knob beside the TIME/DIV Control that permits the sweep time to be continuously varied. In full clockwise position (CAL), the sweep time is calibrated to the steps indicated on the TIME/DIV control. Counterclockwise rotation to the full slows the sweep by 2.5 times or more.
(24) POSITION
PULL x10 MAG Control: This knob is used to move the display in the horizontal direction on the screen. Display is moved toward the right when the knob is rotated clockwise and toward the left with counterclockwise rotation. Sweep is magnified 10 times by PULLing out the POSITION knob. In this case the sweep time is 1/10 of the value indicated by the TIME/DIV control. If you wish to magnify a waveform, bring it to the center of the display with the POSITION control, then switch the x10 MAG knob to the PULLed out position. The waveform paced at the center is magnified in right and left directions.
Synchronization (Trigger) System Controls
(25) SOURCE Select switch (INT-LINE-EXT): This switch is used the select the signal source that the horizontal deflection is synchronized with. The synchronization signal is usually referred to as a trigger signal.
INT The input signal applied to CH1 or CH2 becomes the triggering signal.
LINE This setting is used when observing a signal with power supply line frequency.
EXT External triggering signal applied to TRIG INPUT (27) connector becomes the synchronization source. This is used when a signal independent of the vertical axis signal is needed to trigger the display.
(26) INT TRIGger Select Switch: This switch is used to choose which of the available vertical axis signals is to be used as the trigger signal.
CH1 The input signal applied to CH1 is used as the trigger signal.
CH2 The input signal applied to CH2 is used as the trigger signal.
VERT MODE For observing two waveforms, the sync signal changes alternately, corresponding to the signals on CH1 and CH2 to trigger the signal.
(27) TRIG INput Connector: Input terminal used to external triggering signal only.
(28) Trigger LEVEL Control
PULL (-) SLOPE: This knob is used to decide at which portion of the waveform that the sweep will be triggered. This knob also determines whether to trigger the sweep on the upward moving waveform (+ slope) or the downward waveform (- slope). See the figure for further explanation of trigger level and polarity.
(29) Trigger MODE Select Switch: Of the four positions, two are used when analyzing television signals, and will not be of normal use in the lab. On most occasions, either AUTO or NORM is the correct setting.
AUTO The sweep is automatically triggered, even if no input signal is present. A line should be present in this mode. In the presence of a signal on the channel selected for triggering, a triggered sweep is obtained and the waveform stands still. This is the setting most commonly used.
NORM A triggering signal must be present for the beam to sweep the screen. No sweep will occur if there is no trigger signal. This setting is best for use with very low frequency signals (25 Hz or less).
Miscellaneous Connectors
(31) CAL 0.5 V Terminal: The oscilloscope has a built in calibration signal that outputs a square wave of about 1 kHz and 0.5 V. This can be used to check the voltage calibration of the oscilloscope and probe.
(32) Grounding Terminal: The ground or "Earth" terminal of the oscilloscope. This terminal can be used in place of the ground wire on the probe.
Initial Switch Settings:
Before turning the power switch on, set the following switches:
INTENSITY (5): Full Counterclockwise (off)
FOCUS (3): Midrange
AC-GND-DC (10,11): GND
Vert POSITION (16,17): Midrange (and 17 pushed IN)
MODE (18): CH1
Trigger MODE (29): AUTO
Trigger SOURCE (25): INT
INT TRIG (26): CH1
TIME/DIV (22): 0.5 ms/div
Horiz POSITION (24): Midrange.
After finishing the settings above, push the POWER switch ON, and wait about 15 seconds before rotating the INTENSITY knob clockwise. The display should appear as a line across the screen. Adjust the FOCUS to obtain a sharp display image.
Sweep Display
Set the TIME/DIV switch (22) to 0.2 s, and notice what happens. How long does it take the dot to sweep across the screen? Try to get an accurate timing.
Now set the TIME/DIV switch to 20 ms, and report what you observe. How long does it take the dot to sweep across the screen now? Do you begin to see a "line" where the dot is sweeping? Try other positions, such as 2 ms, .2 ms, and 20 m s. How can you use the sweep to determine time intervals?
Voltage Display
Set the TIME/DIV switch to 1 ms, and move the AC-GND-DC switch of CH1 (10) to DC. Connect a probe to the CH1 input terminal (8), and set the VOLTS/DIV switch of CH1 (12) to 0.1 V. Use the probe to measure a battery. What happens to the trace when the probe touches the battery's terminals? How far does it deflect? Does it stay on the screen? Adjust the VOLTS/DIV if necessary to keep the line on the screen. Can you measure the voltage of the battery with the oscilloscope? What value do you find for the voltage? Does the probe have a 10:1 reduction?
Observe a signal on CH2 by attaching a probe to the CH2 input terminal (9), setting the AC-GND-DC switch (11) to DC, and changing the MODE setting (18) to CH2. You should also set INT TRIG (26) to CH2, and the CH2 VOLTS/DIV switch (13) to 0.1 V. Touch the CH2 probe to the battery terminals. What do you observe? Do you notice anything different from your measurements in the previous paragraph? The two channels should be identical. Now pull the PULL INVERT knob and touch the probe to the battery terminals again. Do you notice any change? What does PULL INVERT do?
Push the PULL INVERT knob back in, and set the AC-GND-DC lever to AC. Touch the probe to the battery terminals again. What do you observe? Can you measure the voltage of the battery with this setting? Remember that the purpose of the capacitor in this setting is to block any DC component of voltage, and permit only the time-varying AC component to pass into the amplifier and be displayed. Set the AC-GND-DC lever back to DC.
Dual Trace Mode
Set the MODE switch (18) to CHOP, and set both AC-GND-DC switches to DC. Adjust the vertical POSITION switches until you see two lines across the screen, one for CH1 and the other for CH2. How can you tell which trace is CH1? Touch the CH1 probe to the battery and see what happens to the trace. Do the same for the CH2 probe. Be sure you understand how to distinguish which trace on the screen is the display for each input signal.
Since DC signals do not vary with time, usually other instruments such as digital multimeters are used to measure them. However, AC signals are more interesting when displayed on an oscilloscope, and certain measurements which are easy to make with an oscilloscope are difficult with other instruments. Thus, the most common usage of the oscilloscope is to measure AC signals.
You will use the Function Generator as a source of AC signals. The Function Generator allows you to vary the waveshape of the AC signal, the frequency of the AC signal, and the amplitude of the AC signal. It therefore provides great flexibility as a signal source. Be sure to use the output terminal marked HI on the function generator.
The large dial, in conjunction with the pushbuttons for the frequency range, allows you to select the frequency of the AC signal. Since this gives only an approximate value for the frequency, the
Meterman multimeter is used to read the frequency precisely. To use the Meterman to measure frequency, turn switch to Hz; and touch probes to the circuit to be measured. The meter changes ranges automatically, so be sure to watch the units shown on the display, whether Hz, or kHz, or MHz, to get an accurate frequency value. Use the Meterman to set frequency precisely.Set the MODE switch back to CH1, set INT TRIG to CH1, and use the CH1 probe to connect with the output leads from the Function Generator. Set the Function Generator to 500 Hz, and use the Sine wave output. Adjust the amplitude setting of the function generator to approximately midrange, and observe the display on the oscilloscope. Adjust the VOLTS/DIV and vertical POSITION so that the signal does not go off the top and bottom of the screen. Sketch the display in your report. Label it to avoid confusing it with other sketches you will make later. How many horizontal divisions are required for one complete cycle? How much time is required for one complete cycle? (Time = [(number of divisions) * (TIME/DIV setting)]. The time for one complete cycle is called the Period of the wave. What is the period of this wave?
The frequency of a wave can be found as the reciprocal of the period. What is the frequency of this wave, as measured by its displayed period on the oscilloscope? What is the percent difference between this computed value of frequency and the value measured with the Meterman?
The vertical axis of the display measures the voltage of the signal. Determine the peak-to-peak voltage of the wave by measuring the number of vertical divisions between top and bottom, and multiplying by the VOLTS/DIV setting. What is Vp-p for this signal? Remember the effect of the probe, if using the 10:1 probe. Any AC signal can be measured in this same way.
Push the triangle waveshape button on the function generator, and notice how the display changes. Sketch the display and label it. Measure the period and Vp-p of this signal. How has the wave changed, if at all?
Black Box Measurement
Ask the instructor for the little "black box" and determine the waveshape, period, frequency, and voltages of the signal, using only the oscilloscope. Sketch and label the output display, and record the values in your report.
AC Dual Trace Mode
Set the MODE switch to ALT, and connect the function generator across the Resistor-Capacitor pair on the little circuit board (for connections, see figure 4). Set the function generator frequency to 5000 Hz. Connect the CH1 probe to measure the signal applied to the circuit, and use the CH2 probe to measure the signal across only the resistor (R). Sketch the display and label it carefully. Identify the two traces in the sketch. Measure the period (T) of the CH1 waveform, and measure the time (horizontal) difference (t) between the two waves.
The difference between the waves is that the formula for the applied signal is yCH1 = A1 sin (2p ft) and the description for the signal across the resistor is yCH2 = A2 sin (2p ft - d ). The d is the phase difference between the two waves, and can be measured from the oscilloscope display as
(1)
What is the phase difference for this circuit? Record it along with the frequency in a data table.
Sweep the frequency dial of the function generator from 1000 Hz to 10,000 Hz. Is the phase constant or frequency dependent?
Set the MODE switch to ADD and notice what happens. What has happened to the display? PULL the PULL INVERT knob and observe the display. Explain what is happening. What is the function of the ADD mode? Push the PULL INVERT knob back in.
XY Mode Operation
Set the TIME/DIV switch to the XY position (full clockwise). Adjust the frequency of the function generator to the 5000 Hz value as above. Be sure that both VOLTS/DIV settings are the same. What does the screen display look like now?
Before you sketch this display, center it by setting both AC-GND-DC levers to the GND position and center the resulting dot on the screen using the POSITION controls. Then set the AC-GND-DC levers to AC and sketch the display. This figure is referred to as a Lissajous pattern, and can be used to determine the phase difference between two waves of the same frequency. Other Lissajous patterns are used to determine frequency ratios between signals.
When two waves have the same frequency and are in phase, the Lissajous figure is a straight line at a 45o angle upward toward the right. When the signals are out of phase, the Lissajous figure is an ellipse, that broadens into a circle when the phase difference is 90o, and narrows back down to a straight line at a 45o angle downward when the phase difference is 180o. By measuring the parameters of the ellipse, the phase difference can be determined from the display.
To determine the phase difference, measure the maximum voltage in the horizontal direction (Xm), and the voltage at which the trace crosses the x-axis (Xo). The phase difference d is
(2)
Calculate the phase difference for this circuit and record it in the data table where you found the phase difference by time displacement. You can also find the phase difference by making similar measurements in the vertical direction. Calculate a value of d by measuring Yo and Ym, and record the measurements in the data table.
Average the three values of phase difference. If you are careful about making the measurements, the three values should be close. Use a percent difference to compare each of the three values with the average value. What is the largest percent difference of the three?
Computation of Capacitance
The final step is to calculate the capacitance value and compare it with the measured value of the capacitor. Recall that the phase angle of an RC circuit is given by
(3)
and that Xc = 1/( w C). Solving these two relations for the value of C gives
(4)
Measure the resistance of R, and calculate the value of C using the average value of d . Then measure the capacitance of C with the Fluke 83, and compare your measured value with the computed value by using percent difference.
FINAL SUMMARY:
You should be sure to mention the value of the phase difference and the capacitor as measured with the oscilloscope, and should also mention the values of the black box measurements. You should comment on the capabilities of the oscilloscope for making measurements, and make special mention of the features which seem to be unique to it.