1. To become familiar with the oscilloscope and what it does.
2. To learn how to use the various controls on the
Oscilloscope.
The Oscilloscope is a device for observing and taking measurements of electrical signals and waveforms.
The analog oscilloscope consists of a cathode ray tube (CRT) which displays a graph, primarily voltage versus time. It also has one or more amplifiers to supply voltage signals to the CRT and a time base system for generating the time scale. Some of the modern digital oscilloscopes use a liquid crystal display screen for the same purpose. This is the type of oscilloscope that will be used in this experiment.
This device allows real time graphs of voltage verses time to be drawn on the screen. This allows studying and comparing various voltage waveforms in circuits. Usually two or sometimes up to 4 waveforms may be observed at the same time. The Oscilloscopes used in this lab can display two waveforms as functions of time or it can use one wave form as the, X, variable and the other as the, Y, variable and produce an X-Y plot.
This digital storage oscilloscope also has several built in measuring functions. The oscilloscopes in our lab can display the following measurements on the screen when the measure mode is selected:
Period,
T
frequency, f
and the following voltages:
cycle rms, Vrms
peak to peak, Vp-p
mean, Vmean = Vaverage = VDC
max, Vmax
min, Vmin
If the cursor mode is selected it can be used to measure two specific voltage levels and the difference between them, or two different times.
It can also send the complete picture on the screen to a printer or it can send all the data pointes of the waveforms to a computer for further analysis and /or printing.
Signal Generator or Function generator
Signal connection lead with two alligator clips on one end and a BNC connecter
on the other
DC power supply
Breadboard
Digital Multimeter (DMM)
Fixed resistors: 2 - 3.3 kW
Capacitor: 0.1 µF
Oscilloscope
2 Oscilloscope 10x probes
Construct the following circuit using R1 = 3.3 kW, R2 = 3.3 kW, and C1 = 0.1 µF. Measure the values of these components before construction the circuit.
1. Set the ac signal generator at a frequency of 1
kHZ with an output level of 2 Vrms. Use a coaxial cable with a BNC connector on one end and
two alligator clips
on the other end to connect the signal generator to the circuit with the red lead
connect to Node A and the black lead connected to the circuit ground. Set the DC power supply to 4 VDC
and connect it from Node C to ground as shown in the diagram above.
2. Connect one of the oscilloscope inputs to Node A and the other to Node B using two 10x oscilloscope probes. The ground clip on the side of the probe should be connected to the circuit ground. The end of the probe should be connected to the specified node. Usually it is a good idea to use a short jumper wire plugged into the node on the circuit board with the oscilloscope probe connected to the other end to prevent the possibility of the probe pulling a component out of the circuit board. A 10x probe contains a 9 megOhm resistor in the end of the probe which is placed in series with the one megOhm resistance inside the oscilloscope thus forming a ten to one voltage divider. Therefore the voltage in the circuit seen by the end of the probe is ten times the voltage seen by the oscilloscope itself. Each input channel of the oscilloscope should have the probe setting set to 10X. This setting is found by pressing the ch1 and ch2 menu buttons. Then look at the on screen menu and press the button beside the menu item you want to change. If your scope probes have a switch on the side, make sure it is also in the 10X position. Check the coupling on this same menu. Both channels should be set to DC for the first procedure. Make sure that the ground clips of the oscilloscope probes are connected to the circuit ground.
3. Next set the triggering by pressing the trigger menu button. Select the channel that you connected to node A as the trigger source and turn the trigger level knob to set the trigger level to zero as displayed at the bottom of the screen. Set the slope to positive. Using the position knobs at the top of each channel control section, set the vertical position for zero volts at the center of the screen on both channels by, first pressing the ch1 menu button and turning the vertical position knob for ch1 while observing the vertical position display at the very bottom of the screen until the reading is zero. Note there will also be a small arrow at the left edge of the screen that moves up or down as you make this adjustment. It should be halfway up the edge when you finish. Repeat for ch2. Then adjust the VOLTS/DIV to set the voltage scales to 2.0 volts/DIV. This should give the largest on screen display if you have set your voltages correctly. Next adjust the SEC/DIV to display two or three complete cycles of the waveforms. Record the oscilloscope settings as displayed at the bottom of the screen.
4. Push the MEASURE button and select source with the source/type button (the top button). Set two of the measurements to use ch1 and two to use ch2. Then select TYPE and set one of the ch1 measurements to Vrms (VA) and one to frequency (f). Set the ch2 measurements to Period (T), and Vrms (VB). Record the measurements. Then use the wavestar software on the computer to acquire the two waveforms into the computer. You should first click on the Instrument item in the menu bar at the top of the screen. Then choose select from the dropdown menu. Next scroll down the lins until you find TDS210 and select this model. You should then be able to acquire the waveforms. Print the waveforms from wavestar then change the view to measurements and print the complete set of measurements.
5. Next set both channels to AC coupling and determine the phase shift between the ac waveforms at nodes A and B, using the cursor mode to measure the time difference (delta t) between the adjacent positive slope zero crossings of the two waveforms. To improve the accuracy of this measurement expand horizontal scale (sec/div) to put the zero crossings as wide apart as as possible on the screen. Then increase the slope of the lines on the screen by adjusting the vertical scales (volts/div) of both channels to a higher sensitivity making the slope of the two lines fairly steep so the exact position of the zero crossings are easier to find. See Phase Measurement below. Then calculate the phase difference in degrees, using the following formula: Theta = 360*delta t/T Get the value of the period (T) the measurements obtained on screen from the measure mode or from the printout of measurements from wavestar.
6. Switch the Scope to x-y by pressing the DISPLAY menu button and then pressing the format button. This graph can be printed from Wavestar without having to reacquire the data since the voltages are still the same and you acquired them in the previous step. Select view XY and then print the display. Then on the oscilloscope select the ch1 menu and change the VOLT/DIV setting from course to fine. You will then be able to adjust the x-scale in small increments until the elliptical pattern just fills the full ten divisions of the screen. You may have to adjust the x-position(ch1) slightly to center the ellipse. This will maximize the accuracy of the this second phase measurement. Once this is done your XTotal will be exactly 10 DIV. Now you must determine XZero as accurately as possible. To do this increase the sensitivity of the y scale (reduce the volts/div on ch2) until the sides of the ellipse cross the zero at a steep angle making it easy to see where it crosses. Estimate the distance between the toe zero crossings as accurately as possible.
7. Measure both the DC voltage and the AC voltage at each of the three circuit nodes with the DMM.
Calculations and comparisons:
1. Compare the ac voltage measurements made with the DMM with the cycle rms measurements from the scope measure mode and with the Wavestar printout.
2. Using the measured DC voltage at node C and the measured resistance values calculate the theoretical DC voltage at node B. Remember no DC current flows through the capacitor so it looks like an open circuit to the DC voltage. Compare this value with the measured value at node B.
3. Check your printout for DC coupling and estimate how far the node B voltage is shifted from being centered at zero volts. Is this about the same as the DC voltage?
4. Compare the DC voltages measured with the DMM to the Cycle mean from the Wavestar printout.
The picture below shows two waveforms at the same frequency, but with a difference in phase angle. The period, T, of the signal can be measured by the Oscilloscope using the measure mode, as long as there is at least slightly more than a complete cycle on the screen (at least three zero crossings). Use AC coupling and carefully set the zero of both signals at the center of the screen. Then the difference in time between the adjacent zero crossings of the same slope can be measured using the cursor mode.
The sign of the angle depends on which of the two waveforms is used
as the reference for zero phase. If the blue waveform (the one crossing
zero at the center of the screen) in the picture above is used as the reference,
then since the other waveform reaches zero before the reference reaches
zero it is leading the reference waveform. This is a positive phase
shift. A negative phase shift would move the waveform to the right
of the reference waveform.
On the picture the difference in time (or phase) of the two negative slope zero crossings is labeled as, t. The total width of the waveform, T, is the length of time the wave takes to repeat. It can be most accurately measured between two successive zero crossings of the same slope on the same wave. Never try to determine the period by measuring from one peak to the next. The results will be less accurate due to the difficulty in finding the exact center of the peaks. The second waveform shown in the illustration above leads the reference waveform by 45 degrees. It is leading because it crosses zero at an earlier time than the reference waveform. In this case the angle is in the first quadrant since the zero crossing is between 0 and 90 degrees ahead of the reference.
This waveform leads the reference by 135 degrees(second quadrant).
It crosses zero between 1/4 and 1/2 a period before the reference waveform.
This angle could also be expressed as -225 degrees.
The phase angle of this waveform is in the third quadrant. It
leads by 1/2 to 3/4 of a period or it lags by 1/4 to 1/2 of a period.
the actual value in this case is an angle of -135 degrees or 135 degrees lagging
or 225 degrees leading.
In this picture the phase angle is -45 degrees or +315 degrees or 45
degrees lagging. This angle is in the fourth quadrant between 0 and
1/4 period lagging behind the reference waveform.
An older alternate method for determining phase angle difference between two signals is as follows:
This method has the disadvantage of not being able to tell you which quadrant
the angle is in, but it can be used on older oscilloscopes that do not have a
cursor measurement mode. The accuracy of this method is low for angles
near 90º or 270º,
but is quite good near 0º or 180º.
It is a very good way to tell when two signals are exactly in phase 0º,
or exactly out of phase 180º. Switch the display mode from Voltage-time mode to X-Y mode. The display
on the oscilloscope will show an elliptical pattern similar to the following.
Always adjust the oscilloscope horizontal scale so that the ellipse, X total, is
the full width of the screen. this will maximize the accuracy of the
measurements. Before measuring the, X zero, distance increase the vertical
gain to make the zero crossings steep for more accurate measurements.
Having the top and bottom of the figure off the screen will not effect the
measurement since the distance measured is along the horizontal axis. The
horizontal scale does not matter since both measurements are measured with the
same scale. Therefore the scale factor cancels out when the two values are
divided. Also
check to make sure the vertical zero is exactly in the center of the
screen.
This pattern corresponds to a phase difference of 45 degrees, or -45
degrees, and if the ellipse were sloping in the other direction as shown
below it would correspond to 135 degrees, or -135 degrees. The big problem
with this method is that you cannot determine if the angle is leading or lagging
from this figure. You will have to go back to the voltage-time scale to
determine this.
The angle is then calculated as
This page last updated 02/23/2006