Lab 2 Oscillosocpe and Funtion Generator

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California State University, Sacramento *

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117L

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Electrical Engineering

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Dec 6, 2023

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ECE 117L Lab 2, Oscilloscope and Function Generator PURPOSE: The purpose of this experiment is to provide students with an introduction to the oscilloscope. In addition, the students will have an introduction to the function generator and the DMM. Introduction: The oscilloscope is the most versatile measurement instrument used in the electrical engineering laboratory. It allows the user to, not only view waveforms, but also show the relative shape and positions of several waveforms. COMMENT: You will be using the oscilloscope extensively during your career as an electrical engineer. It is important that you become very familiar with its operation. Modern oscilloscopes with integrated signal generators, like the one implemented with the Analog Discovery 2 kit (AD2), are highly automated and require minimal set-up. It is good to take the time to explore the various menu functions on the AD2 implementation. This will provide some insights, in the case that later in your career you are tasked to work with an older oscilloscope that requires a more manual set-up. The topics outlined for this experiment in this laboratory merely demonstrate the basic operation. As you work through this experiment you may discover many other lines of investigation. Take time to satisfy your curiosity and explore the other capabilities. Digilent provides tutorial videos at https://www.digilentinc.com/analog- discovery-2-100msps-usb-oscilloscope-logic-analyzer-and-variable-power-supply/ and also on Youtube PRE-LAB : We will build the circuit in Figure 1 on proto boards to test and become familiar with the operation of the signal generator and oscilloscope. This is a simple RC low pass filter. Prior to the laboratory, calculate the value of R1 to produce a -3dB cut-off at 1 kHz. Recall that the -3dB cut-off frequency can be calculated as follows: 𝑓𝑐 = 1 2𝜋𝑅𝐶 In this case, R= R1 || R2 (parallel combination of the two resistors). Also recall that at the -3dB is also called the half power point. Power, P= V 2 /R . Half power occurs at P= V 2 /2R, or when the output voltage is equal to V in / √2 , or 0.707 V in . Also, recall that at the -3dB cut off frequency, network theory says the phase shift will be 45 ° Figure 1, RC Low Lass Filter

ECE 117L Lab 2, Oscilloscope and Function Generator Part 1: Oscilloscope Set-up and Operation . Connect the AD2 module to your computer and open the Waveforms program. Before we build the circuit in figure 1, we should check the operation of the oscilloscope probes and assure they are functional and configured in agreement with the oscilloscope function. Figure 2 shows the AD2 kit. When using the oscilloscope connector board with the AD2, the signal generator ports and oscilloscope ports are routed to the 50 Ohm BNC connectors on the connector board and not routed through to the AD2 connector pin interface. We will also use a 50 Ohm cable with BNC connector to connect to the signal generator port on the connector board to our circuit. Using the pull-down menus, configure the Wavegen to provide a 1 kHz sine wave signal with 1V peak (2Vpeak-to-peak) amplitude. Press the run button to activate the signal source. Figure 2, AD2 set-up Figure 3, Waveforms Welcome Page and WaveGen Page Now connect the oscilloscope channel 1 to the connector from the signal source. As shown in Figure 4, make sure you connect the red signal clip to the oscilloscope probe tip and the oscilloscope ground to the signal ground. Also, make sure the red switch on the probe is switched to 1x. Open the Scope tab from the Wavefroms Welcome page and activate channel 1

ECE 117L Lab 2, Oscilloscope and Function Generator Figure 4, Oscilloscope probe connected directly to the WaveGen output You should get a waveform similar to that show in Figure 4. Note, that if the signal amplitude does not agree with the amplitude set- up in the WaveGen page, you can select the “gear” icon and adjust the signal attenuation (amplification) to adjust the amplitude of the measured signal. Now repeat and connect the Wavegen output to the channel 2 (CH2) oscilloscope probe. Verify that the measured signal amplitude agrees with the signal amplitude set up in the WaveGen page. Figure 5, Channel 2 Measurement of a 1 kHz sine wave The oscilloscope Trigger function synchronizes the horizontal sweep time with the input signals. Generally, we want the trigger level to be at the midpoint within the waveform, or at least within the amplitude of the waveform. When you open the scope function in Waveforms , the trigger level will automatically be set at 0V. Experiment with setting the trigger level at +0.8V and then +2V. When the trigger level is outside the amplitude of the waveform, the oscilloscope will lose synchronization with the waveform and the display will scroll unsynchronized. You can also experiment with the options

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ECE 117L Lab 2, Oscilloscope and Function Generator provided in the Trigger Modes , Sources and Conditions pull-down menus. Generally, it is sufficient to Auto Trigger on the Input Signal ( Channel 1 ) and the Rising Edge of the signal. Before moving to part 2, connect the DMM to the WaveGen signal source and set the DMM to AC mode. Record the measured value. Most likely the DMM measurement is different than the peak-to-peak voltage displayed on the oscilloscope. Can you explain why they would be different. Part 2: Testing the RC Low Pass Filter: Now build the circuit shown in the Figure 1 schematic. Calculate the value of R1 so that the -3dB cut-off frequency (fc) is at 1 kHz. This should provide a 45 ° phase shift between the input signal (CH1) and output (CH2). If the input amplitude is 1V peak, at the -3dB cut-off the amplitude at the output (CH2) should be around 0.707 Vin. The period (T) of the signal is T=1/f. You can add cursers (Figure 6) to the display to measure the period, amplitude and delay of the output signal on CH2. Record the values in a table Channel 2 Measurements Parameter Calculated Measured % difference Frequency 1 KHz Period 1 ms Output Amplitude at Fc (when Vin =1Vpeak) 0.707 Output Delay 125 us Output Phase Shift 45 ° Note that the phase can be calculated by the equation: Phase shift = 360 * time delay/ period Take a screen shot of your display (use the Microsoft “ Snip ” tool) and include it in your report. Figure 6, Adding cursers to your display

ECE 117L Lab 2, Oscilloscope and Function Generator You can go back into the WaveGen page and adjust the signal frequency. Set the frequency of the input signal one decade ahead of fc (100Hz) and one decade beyond fc (10kHz). Record the amplitude at each of those frequencies. Now that you have observed how the filter affects the sine waveform, you can experiment with other waveform types. Go into Wavgen, reset the signal frequency to 1 kHz and use the Type pull down menu to select other waveforms; triangle, square, ramp , etc. Observe with the oscilloscope how the simple RC filter affects each of the waveforms. Snip and paste some screenshot examples in your lab report Part 3: DC Measurements . You can also add a DC offset to the input signal. Set Wavgen back into Sine mode and configure the waveform so that it has a +1V offset. To observe the DC offset, you will need to configure the AD2 adapter card for DC measurement. Figure 7 shows where to move the jumper to allow for DC measurements. Most likely the signal display will start scrolling and you will have to adjust the Trigger level mid-way within the input signal to re-synchronize the display. Figure 2, Configure the AD2 Adapter Board for DC measurements DISCUSSION TOPICS 1. What is the difference between voltage measurements made with the DMM and the oscilloscope? 2. Show the input/output signals found in Part 2. Are they about 45° apart? Show your phase calculation 3. At what frequency does the output signal amplitude begin to exhibit attenuation 4. Between 0.1x Fc(100Hz) and 10xFc (10kHz), what is the attenuation difference, in units of dB? 5. What happened when the triggering level was changed from + to -? 6. What happened when the coupling was changed from AC to DC in part 3? Explain the difference between DC coupling and AC coupling on the vertical channel. 7. Show a screen shot of the input and output signals found in part 3. Make sure the oscilloscope channels are configured for DC. Add cursers to show the maximum and minimum voltage values at the output.