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To use the oscilloscope as an AC voltmeter, apply the signal to the vertical input terminals, andadjust the calibrated vertical sensitivity (or volt/div) so the amplitude is of suitable magnitude onthe graticule. The peak-to-peak value is then the distance indicated multiplied by the verticalcalibration. For example, assume that a sine wave generator is set to 1000 Hz and adjusted formaximum output voltage. For any periodic signals, we calculate the RMS value by performing theintegration.

As in Figure 1, a peak-to-peak value of 60V is observed on the oscilloscope. The RMS output ofthe generator at 1000 Hz is therefore 21.2 Vrms. For a purely sinusoidal waveform, the RMS valuecan be calculated by:

or

Figure 1: Vp, Vp-p and Vrms measurement of AC signal

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For DC (Direct Current) measurements, apply the voltage to the vertical input terminals, againsuitably adjusting the vertical sensitivity. A straight line is produced with the horizontal sweepfunctioning. With no horizontal voltage applied, a spot will appear on the screen. In measuring DCvoltages, it is necessary to remember where the trace was with 0V applied to the vertical input.

Figure 2: Voltage measurement of DC signal

Please bear in mind that RMS is the value measured using digital/analog multimeter. Meanwhilepeak and peak to peak are values displayed on the oscilloscope. With advanced technology,some new digital oscilloscope may also display the RMS value on the screen.

EQUIPMENTS

Resistors 100 X 1, 1k X 1, 4.7k X 1, 10k X 1, 100k X 1, 1M X 2, 2 M X 1

Potentiometer 5k

Breadboard

Digital multimeter

Analog DC Miliammeter GW GPR3030 DC power supply

DSO-X 2002A Digital Oscilloscopes

Crocodile clips, BNC Connectors, Connection wires (single core)

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Experiment: Part ISmall AC voltage measurement (Vp, Vp-p and Vrms ) using oscilloscope

1. As we just learned in class, the RMS value of a periodic signal is related to the energycontent of the signal. Forsinusoidal signals the RMS value is 1/2 of the amplitude Vp.For AC circuit, set channel 1 setting on oscilloscope to AC coupling mode.

2. RMS is a way of expressing an AC voltage or current in terms functionally equivalent to aDC voltage or current. For example, 10 volts AC RMS is the AC voltage that wouldproduce the same amount of heat dissipation in a resistor of given value as would 10volts DC. The RMS value is also known as the equivalent or DCequivalent value of anAC voltage or current. The labs digital multimeters (DMM) display the true RMS values ofmeasured voltage and current, but to see what the actual voltage signals (V p and Vp-p)look like, we turn to the oscilloscope.

3. Set up the following circuit. Use the built-in waveform generator from the oscilloscope togenerate AC source. Connect the generator output to the channel 1 probe (CH1) of thescope and the reference point is connected to the reference lead of the scope using anyextra BNC cable obtained from lab technician.

Figure 1a: Measuring AC circuit

4. Turn on your oscilloscope and waveform generator. Select sinusoidal waveform.Gradually increase the output amplitude until the oscilloscope indicates the output is5 Vp-p. Record the exact value of AC digital voltmeter.AC digital voltmeter output = __________ Vrms

5. Observe the waveform on the oscilloscope and compare the digital voltmeter reading withthe peak voltage (that is the peak-to-peak voltage) measured with the oscilloscope. Fora sine wave, the peak voltage, Vp, as seen on the oscilloscope, should be 2 times thevoltmeter reading, Vrms.

Vp= 2 Vrms and Vp-p= 22 Vrms

6. On the oscilloscope, set up an automated measurement for the AC RMS - N Cycles.(You have learned this during Lab 3). Compare the values for V rms reported by the digitalvoltmeter reading and the oscilloscope. Are they almost the same?Oscilloscope output = __________ Vrms

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7. Measure, record and draw the input and output signals using oscilloscope for thefollowing circuit. Use channel 1 for input and channel 2 for output.

Figure 1b: AC circuit with load

Table 1a: Oscilloscope readings

Vin, p-p (Channel 1) Vout, p-p (Channel 2) Vout, rms (DMM)

1

2

3

4

5

Figure 1c: Input and output signals

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Experiment: Part IIDC voltage measurement using oscilloscope

1. Construct the circuit on a breadboard such as in Figure 2a. A 4.7k resistor isconnected in series with a 5k potentiometer (variable resistor) and a DC powersupply. For DC circuit, set channel 1 setting on oscilloscope to DC coupling mode.Configure the oscilloscope to display DC RMS FS.

Figure 2a: Measuring DC circuit and potentiometer layout

2. On channel 1, press the Push to Zero button to set the DC level at the center of thescreen. This will indicate the zero volt has been achieved on the oscilloscope screen.

The output voltage signal across the 5k potentiometer is observed using theoscilloscope. The potentiometer works as a voltage divider circuit. Connect the outputto the channel 1 probe of the scope and the ground point is connected to thereference lead of the scope.

3. Power up the circuit by turning on the DC power supply. Set Vdc to 10 V. Vary (turn)the potentiometer and measure the voltage across B and C using digital voltmeter.Record and measure the DC level seen on the oscilloscope for the followingmultimeter readings as in Table 2a. You need to configure the oscilloscope so that itwill display DC RMS FS signal.

Table 2a: Multimeter and Oscilloscope readings

Digital Multimeter Reading

VBC, VChannel 1 Oscilloscope Output, V

1

2

3

4

4. Explain the differences between the DC voltage recorded using DMM and the DCvoltage displayed in DC RMS-FS by oscilloscope.

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Experiment: Part IIILoading effect of the Oscilloscope Probe

1. Construct the following circuit, using the DC power supply for the power source. Set theDC power supplys output to about 10V. The measurement is attempted with a scope thatpresents a load of say, RSCOPE to the circuit. Then the 1M internal resistance and thescope resistance R

SCOPEform a voltage divider and the oscilloscopes reading will be V

R

volts.

Figure 3a: Input resistance of oscilloscopes probe

2. Measure VR with the oscilloscope for the following values of RSCOPE: 10k, 100k, and1M.VR (10k) = __________ VVR (100k) = __________ VVR (1M) = __________ VVR (2M) = __________ VVR (3M) = __________ V

3. Expect/predict the value of VR with RSCOPE = infinity (ideal for the scope).VR () = __________ V (expected)

4. Measure the actual result of infinite VR by removing RSCOPE (open or remove RSCOPE).VR () = __________ V (measured)

5. From results in step 2, plot log10RSCOPE vs. VR measured on the oscilloscope. What is thesignificance of the slope? Slope = __________

6. From step 3 and 4, calculate the oscilloscope probe input resistance.Oscilloscope probe input resistance = __________

Figure 3b: log10RSCOPE vs. VR

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Experiment: Part IVEffect of Internal Resistance on the Analog DC Milliammeter

1. Construct the following circuit, using the DC power supply for the power source in serieswith a 1k resistor, a 100 resistor and an analogue DC milliammeter provided insideyour workbench (black case) as shown in Figure 4a.

2. Note that the ammeter is properly connected in series between points B and C.Scale the ammeter to 50mA at first.Any group who fails to do this will lead to thedamage/broken of the ammeter, causing the group to get 75% marks deduction.This is considered as a highly mandatory mistake. You should be alert with thiscaution all the time especially when measuring current.

Figure 4a: Input resistance of analog DC miliammeter

3. Set the DC power supplys output to about 15V. The measurement is attempted with adigital multimeter (DMM) that measures voltage between A, B and C. Measure thevoltages VAB and VAC using DMM. Why is there a difference between VAB and VAC?Explain.VAB = __________ VVAC = __________ V

4. The analogue DC ammeter is put in series between point B and C in measuring I.I = __________ mA

5. From the results obtained in step 3 and 4, calculate the internal resistance of DCammeter.Rammeter= __________

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REVIEW QUESTIONS

1. Compare between RMS, Peak and Peak to Peak voltage in AC, DC and pulse signals.

2. Which instrument is more accurate when measuring at 500 Hz and 5000 Hz? Explain thereason.

3. Are digital and analogue voltmeters used in the experiment True RMS Voltmeter?

4. Why it is important to set a specific coupling mode for DC and AC measurement ofoscilloscope?

5. Generally, can the normal oscilloscope measure current? Explain. How can we use thenormal scope to measure current?

6. Discuss the loading effect of the oscilloscope by referring to results obtained from Part IVof the experiment.

7. Discuss the effect of resistance in an ammeter.

TURN IN ONE REPORT PER GROUP AT THE END OF YOUR LAB SESSION.THERE IS NO TAKE HOME REPORT.