EE 3302 LAB 1 EQIUPMENT ORIENTATION

Transcription

1 EE 3302 LAB 1 EQIUPMENT ORIENTATION Pre Lab: Calculate the theoretical gain of the 4 th order Butterworth filter (using the formula provided. Record your answers in Table 1 before you come to class. Introduction: The purpose of this lab experiment is to familiarize students with the new equipment obtained from Agilent Technologies. After performing this experiment students should be proficient in the use of this equipment that will enable them to perform the other experiments in this class without any difficulty. All the plots for this experiment must be obtained as a hard copy and signed by the TA before leaving the lab. Obtain the plot as a.jpg file on a floppy disk and then print it out on one of the computers in the lab. This is a very basic lab that teaches you to measure the amplitude, frequency and then view the spectrum, filter roll off and pulse response of square waves. For your convenience, these are the other experiments that you will perform during the course of this semester: Amplitude Modulation Frequency Modulation Analog to Digital sampling Phase Shift Keying Modulation Phase Shift Keying Modulation with noise These experiments may be changed as the semester progresses. Procedures Initial configuration Switch on the Agilent 33250A function generator. At first we need to match the impedance of the function generator and the oscilloscope so that we do not get an attenuated signal to the oscilloscope. For this: Press the Utility button on the function generator (Agilent 33250A). Select the output properties function by clicking the button below it. Select the impedance as High Z. Press done. Now we have set the impedances of the function generator to match that of the oscilloscope.

2 Now we also need to make sure that the probe is to a 1:1 setting to prevent any further attenuation. This can be done by pressing the channel 1 button on the oscilloscope and then viewing the probe setting on the extreme right of the screen. If it is not to a 1:1 setting, click on the button below it and then using the adjustment knob change it to get the desired configuration (1:1). Amplitude and Frequency Measurement OSCILLOSCOPE CHANNEL 1 PROBE FUNCTION GENERATOR Connect the function generator (33250A) to the oscilloscope (54621D). Now switch on the output from the function generator. Adjust the frequency to 10kHz Set amplitude to 200mV p-p. Entering a number from the numeric pad on the function generator can do this. Amplitude can be adjusted by pressing the Amplitude button first before entering the desired value of 200mV. For this experiment we need a square wave so appropriately select this function on the function generator. On the oscilloscope (54621D), press the auto scale button to view the square wave on the oscilloscope. We need to measure amplitude for this square wave. This will be done using the cursor tool. Press the cursor button and study the menu on the screen. The X scale is used to measure on the time scale while the Y scale is used to measure the amplitude. Set the menu to measure the Y scale. Press the Y1 button and then adjust the knob so that the cursor line co-insides with the lower edge of the square wave. Now select the Y2 measurement and move that cursor so that it co-insides with the upper edge of the square wave. The measurement for Y is shown on the scope. Record this value. This is the peak-topeak amplitude of the square wave.

3 To measure the frequency of the square wave, we use the X axes. Select the menu so that we measure the time scale. Using the same procedure to measure the amplitude we find the time for one wavelength. The inverse of the time is the frequency of the square wave. Make sure that the frequency of the square wave calculated from the oscilloscope matches the frequency reading from the function generator. Attach a copy of the plot showing the wave on the oscilloscope and either cursor reading in the lab report. Spectrum Measurement Keeping the same configuration, 10kHz 200mV p-p square wave, we switch on the math button on the oscilloscope and then select FFT from the menu on the screen of the oscilloscope. Press the channel 1 button to close the time display and leave only the FFT on the screen. To adjust the span of the plot, select more FFT on the screen, select span and use the adjustment knob to increase or decrease the span. Make sure that we see at least 11 harmonics on the screen. Now measure the distance between the main signal and the 9 th harmonic. What is the difference in db? (20dBdown) Does it match with the theoretical value? What is the reason for this difference? Filter Roll-off CHANNEL 1 OUTPUT CH1 INPUT CH1 To measure the filter roll-off, we use the filter already provided in the lab. One thing to be careful is to be consistent with the channel that we are using. If the channel is selected to 1 make sure that input channel 1 and output channel 1 is used. For this experiment we are using a 4-pole Butterworth filter which has a cut off frequency of 3.2kHz. The output gain is set to 0dB.

4 To select the filter, on the Krohn-Hite 3364 filter press the type button once to select a Butterworth filter, press mode once and set it to low pass (LP). Adjust the cut-off frequency to 3.2kHz by pressing freq and the entering the required value. Now measure reference values at a frequency of 100Hz sinewave. Now set the frequency from the function generator at 3.2kHz. At this frequency check the amplitude of the wave. The waveform obtained is not triggered correctly and hence does move around a little. Now increase the frequency of the function generator to 4kHz. Observe the amplitude of the output wave on the oscilloscope. How has it changed? Why? Now change the input frequency to 5kHz. How does this change the amplitude of the output wave? Why? Make sure that a copy of all the three different plots and your data table are attached in the report. Frequency A theoretical db theoretical db measured db measured K 4K 5K Table 1. (Hint: equation for a 4 th order Butterworth is 1 1+ f f c 8 ). Pulse response For this response we are going to use a 800Hz square wave of amplitude 200mV p-p. The connection is the same, function generator output to the filter input and filter output to channel 1 of the oscilloscope. View the output on the oscilloscope. To compare a perfect square wave to the wave obtained on the oscilloscope you may set up the other function generator to produce a 200mV p-p, 800Hz square wave and feed that to the channel 2 of the oscilloscope and view both waves together. What is the reason for obtaining the shape that we get? Make sure that a copy of this waveform plot is attached in the report.

5 This concludes the first lab for communications in this course. Make sure that all the questions are answered in the report also that all the plots and the data sheets are initialized by the lab TA before you leave. MAKE SURE THAT ALL THE EQUIPMENT IS DISCONNECTED. TURNED OFF AND THE WIRES STORED IN THE APPROPRIATE PLACE.