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.