DE-1. Analogue and Digital Signals DE-1. Analogue and Digital Signals Objectives To understand the concepts of time and frequency domains To appreciate the concept of the spectrum of a waveform and the bandwidth it occupies To examine the effects of filtering on waveshape and bandwidth restriction To appreciate the waveform and spectrum of a noise signal and the effect of filtering on the noise To understand the concepts of return to zero (RZ) and non-return to zero (NRZ) coding of digital signals and to examine the spectra and bandwidth of each form To generate both unipolar and bipolar RZ and NRZ signals and examine their waveforms and spectra Preparation: login the PC with username: lab and password: lab Switch on the “Feedback Rat 92-200” box. Double-clicking the “Modulation and Coding...” icon: Click on the “Signals in the Time and Frequency Domain” block Click on Practical 1 on the right panel. Practical 1: Waveshape and Spectrum of Sine, Triangle and Square Wave Signals In this practical you will investigate how the waveshape in the time domain affects the spectrum in the frequency domain. This is an important relationship to understand in order to be able to adjust how much frequency spectrum is occupied by a signal. Ya Bao Page 1 Procedure 1. Click Make Connections, follow it to complete all connections on the board. 2. In the Function Generator block, set the frequency range switch to fast and the Frequency control to full scale. Set the Signal Level Control to half scale. 3. From the test equipment available, open the frequency counter and reduce the Frequency control in the Function Generator block until a frequency of approximately 50 kHz is shown. 4. From the test equipment available, open the oscilloscope. Select a sine wave using the waveform selector in the Function Generator block. 5. From the test equipment available, open the spectrum analyser. 6. Using the oscilloscope cursor, measure the time for one cycle and, from this, calculate the frequency of the waveform. (Frequency is the reciprocal of the time). Compare this calculated frequency with a direct measurement made using the frequency counter. 7. In the Function Generator block, change the waveform to triangle. Note the spectrum on the analyser and compare with before. Use the cursor to find the frequencies of all harmonics. What are the relationships between the fundamental frequency and harmonics’ frequencies? 8. Now change to a square waveform. How many harmonics you counted? Compare with previous TWO readings. 9. Adjust the Signal Level Control and note that on the frequency spectrum all the signals change amplitude by the same amount. Practical 2: Effect of Filtering on Waveshape and Spectrum Procedures 1. Click Make Connections, follow it to complete all connections on the board. 2. Select the sine waveform and set the frequency range switch to Fast on the Function Generator block on the workboard. Set the Signal Level Control to half full scale. 3. Open the Oscilloscope and adjust the Frequency control on the Function Generator block so that two complete cycles of the signal are shown on the oscilloscope Channel 1 when the time-base is set to its default setting. 4. Change the signal to square-wave. Use the two channels of the oscilloscope to compare the input and output of the Pre-modulation Filter (a low-pass filter). Record your observations and explain the effects of a low-pass filter. 5. Repeat the observations using the triangle wave and then the sine-wave. Practical 3: Noise Signals in the Time and Frequency Domain Procedures 1. Click Make Connections, follow it to complete all connections on the board. 2. Set the Noise Generator Amplitude control to half full scale. 3. Open the oscilloscope and notice that the signal on Channel 1 is a random waveform. Such a signal is referred to as noise. 4. Open the spectrum analyser and examine the spectrum. It contains random noise at approximately the same amplitude at all frequencies up to a certain frequency. This upper frequency limit is determined internally by the noise generator. 5. Adjust the noise Amplitude control and notice how much easier it is to measure noise amplitude differences on the analyser. 6. Check the Ch2 Show box on the spectrum analyser and examine the spectrum of the output of the filter. Note that the upper frequency limit of the noise has reduced significantly. The oscilloscope shows that faster transitions have been removed but, again, it is easier to see what has happened on the analyser. Practical 4: Comparing NRZ and RZ in both Bipolar and Unipolar Forms Close the previous window. Go back to follow window and click on Uncoded Binary Data Formats. Then click on Practical 1button on the right pannel. Procedure: 1. Click Make Connections, follow it to complete all connections on the board. Dont’ connect any connections labelled “later connect” initially. 2. Set the Signal Level Control to minimum. 3. Open the oscilloscope. The upper trace (blue) is NRZ data and the lower trace (yellow) is the synchronisation signal. Notice that, without any synchronising signal, the data is continuously moving in time. 4. Increase the Signal Level Control until the synchronisation is achieved. 5. Note the two signals. What is the sequence of ‘1’s and ‘0’s, starting at the sync pulse? (You may need to increase the size of the oscilloscope and decrease its timebase to see all the data clearly) 6. Now, use the button at the bottom of the block diagram to switch to Bipolar NRZ. Explain the difference with previous observation. 7. Now select Unipolar RZ and Bipolar RZ data button. Record and explain your observations. References: Feedback software help files.