ECM3166 - Communications Engineering

Communications Engineering - Laboratory Assignment Baseband Pulse Signaling

This assignment carries 20% of the module mark. Individual reports MUST BE TYPED and submitted as asingle .pdf file via the submission point on ELE by 12 noon on 4 December 2024 at the latest. Please read to the end of these instructions to see the report requirements.

As you work through the exercises, you will need to record your experimental observations and save experimental waveforms using a digital storage oscilloscope (DSO).

1. Introduction

In the classical communications model, information (the message signal) moves from a transmitter to a receiver over a channel. A number of transmission media can be used for the channel including metal  conductors   (such  as  twisted-pair  or  coaxial  cable),  optical  fibre  and  free-space   (radio channels).

Regardless of the medium used, all channels have a bandwidth. That is, the medium lets a range of signal frequencies pass relatively unaffected while frequencies outside the range are made smaller (or attenuated). In this way, the channel acts like a filter.

This issue has important implications. If the medium’s bandwidth is not wide enough, some of the frequency components in the message signal are attenuated and others can be completely lost. In both cases, this causes the demodulated signal (the recovered message) to be distorted.

To  illustrate  this  last  point,  Figure  1(a)  below  shows  what  happens  when  all  but  the  first  two harmonics of a squarewave are removed. As you can see, the signal is distorted.

Making matters worse, like a filter the channel shifts the phase of the frequency components of transmitted signals by different amounts. Again, to illustrate, Figure 1(b) below shows the signal in Figure 1(a) but with one of its harmonic components phase shifted by 40º .

Figure 1 Sketch of (a) A square-wave with all but the first two harmonics (the 1stand 3rd harmonics) removed and (b) the signal of Figure 1(a) with one harmonic shifted in phase by 40o.

Imagine the difficulty a digital baseband receiver circuit would have in interpreting the logic level (i.e. whether a data '1' or a data '0' had been sent) of asignal like Figure 1(b). The receiver would no doubt make errors in its decisions.  The task of the receiver would be made even worse if the signal were also contaminated by noise - which of course in reality it will be.

In this experiment you will use the Emona Telecoms-Trainer 101 to model a digital baseband pulse communications system, and investigate the effects of channel-induced distortions and noise on the received and recovered signals.

The  'Emona  101  Telecommunications  Trainer   User   Manual'   is  provided  and  contains  useful information on the specifications and operation of each module in the system.

Instructions in italics refer to the setting-up of the various modules in the E101 kit.

2. Non-ideal Channels

We are going to investigate the effects of non-ideal channel characteristics on digital baseband signals.

The signal is generated by a digital sequence generator and can be line-coded using a variety of options.

Connect up the circuit shown in Figure 2(a). If you are using an analogue oscilloscope you will need to use and external sync signal from the sequence generator, as shown in Figure 2(a), to 'freeze'the data pattern on the'scope.  If you are using a digital storage oscilloscope (DSO), then you should be able to capture segments of the signal quite easily using the storage functions of the DSO.

The set-up in Figure 2(a) can be represented by the block diagram in Figure 2(b). Initially we will use NRZ-L signalling:

- Set the dip-switches to 00 on the sequence generator to select the NRZ-L line code.

- Set the Tuneable Low-pass Filter module’s Gain control to about the middle of its travel.

- Turn the Tuneable Low-pass Filter module’s Cut-off Frequency Adjust control fully clockwise.

Set the scope’s Timebase control so that you can see a reasonable number of bits on the screen (twenty or so)

Now compare the transmitted and  received signals  -  i.e. the signal directly from the sequence generator to that from the output of the LPF.

Note the effects of making the channel’s bandwidth narrower by turning the LPF module’s Cut-off Frequency Adjust control anti-clockwise.

Q1. What two things are happening to cause the digital signal to change shape as you reduce the LFP cut-off frequency?

Figure 2 (a) (top) Initial set-up and (b) (bottom) block diagram representation

An obvious solution to the problem of bandwidth limiting of the channel is to use a transmission medium (i.e. a channel) that has a sufficiently wide bandwidth for the digital data. In principle, this is a good idea that is also used in practice. Certain types of cable design have better bandwidths than others. However, as digital technology spreads, there are demands to push more and more data down existing channels. To do so without slowing things down requires that the transmission bitrate be increased. This ends up having the same basic effect as reducing the channel’sbandwidth. The next part of the experiment demonstrates this.

Modify your circuit to that shown in Figure 3(a) (the dotted lines in the figure refer to previous connections that can remain in place).  The setup of Figure 3(a) can be represented in block diagram form. as in Figure 3(b). Notice that the Sequence Generator module’s clock is now provided by the VCO module’s DIGITAL output and so it is variable.

- Turn the Tuneable Low-pass Filter module’s Cut-off Frequency Adjust control to midway (i.e. 12 'o' clock position)

- Set the VCO module’s Range control to the LO position.

- Turn the VCO module’s Frequency Adjust control fully anti-clockwise.

Now increase the bit rate for the signals transmitted over the channel by turning the VCO module’s Frequency Adjust control clockwise while watching the scope’s display (as you do this, you’ll need to turn the scope’s Timebase control clockwise as well so that you can see the signals properly).

Q2. From your observations of the distortion of the received signal, make a very rough estimate of what bitrate you think the LPF channel can reasonably sustain.

Q3. Measure the cut-off frequency of the LPF and compare this to your estimate of the maximum bit rate for the channel.  The cut-off frequency of the LPF can be measured by taking an output from the (fC x 100) terminal - this is a square wave with a frequency 100 times fC.

Figure 3 (a) (top) VCO driven set-up and (b) (bottom) block diagram representation

3. Eye Patterns

Estimating the  maximum  bit-rate  sustainable  by  the  channel from  a simple observation of the received waveform. is not a very reliable approach. However, it would be nice to have a simple way to inspect and test the performance of the channel in digital transmission systems, to see if it is likely to function correctly.

So-called'eye patterns'are ideal for this purpose and are surprisingly easy to generate.

Modify your set up to that of Figure 4(a). This can be represented in block-diagram form. by Figure 4(b).

The scope is now being triggered by the system clock instead of the Sequence Generator module’s SYNC signal. This forces the scope to draw the Sequence Generator module’s data bits over each other and should look something like the sketch in Figure 5. Triggering the scope this way is normally something that we want to avoid. However, when used to inspect digital data out of the channel it can give us an excellent idea about the signal’s quality and hence the channel’s bandwidth. If the digital signal is not greatly affected by bandwidth limiting, the spaces between the traces (called the “eyes”) are wide-open. As bandwidth limiting degrades the signal’s quality, the eyes begin to close.

NOTE: see appendix for more information on eye-diagrams.

Figure 4 (a) (top) Eye-diagram set-up and (b) (bottom) block diagram representation

Figure 5 A typical eye diagram

See the eye pattern for yourself by following the next few steps:

- Return the VCO module’s Frequency Adjust control fully anticlockwise.

- Set the LFP bandwidth to midway (turn fC control to the 12 'o' clock position).

Q4. While watching the eye diagram on the scope, slowly turn the VCO module’sFrequency Adjust control clockwise to increase the clock frequency and hence the data bit-rate (as you do this, you may need to change the scope’s time-base control to see the eye pattern properly). You should see that the eye begins to close as the bitrate increases. Why is this?

Q5. Estimate the maximum bitrate for the channel from the eye-diagram. How does this compare to the measurement made directly from observing the received waveform? Which estimate do you think is most reliable?

4. Noise Effects

The measurements so far have been made without the presence of significant amounts of noise.  In a real communications system the effects of noise can be quite severe.  We can simulate this by adding white noise to the message signal after it goes through the channel.

Locate the Noise Generator Module and the (unity gain) Adder Module. Take the signal output from the LPF module and the -20dB output from the noise generator module and add them together.  Use the output of the adder to produce an eye-pattern as before.

Q6. What happens to the'eye-opening' now that we have added some noise? Make an estimate of the maximum bitrate sustainable by the LPF channel (with fC in the 12 'o'clock position again) now that you have added noise.

Q7. How does this estimate compare to the value measured without noise, and why are they different?

5. Signal Recovery

As you have seen, the fact that real communications channels are non-ideal and invariably bandlimited means that received signals are distorted. Specifically, they suffer from ISI and noise.

To manage this problem, the received digital signal must be cleaned-up or “restored” before it is decoded (i.e. before the original data is recovered).  In modern communication systems the signal recovery process usually entails some form. of equalisation (to reduce the effects of ISI), filtering (to reduce the  effects  of  noise),  and  finally  decision-making  (was  a  '1'  sent  or  a  '0').  Often  these processes are now carried out using digital signal processing techniques - so the received signal is often sampled before the equalisation, filtering and decision-making processes.

For binary signalling systems we only need to determine if a '1' was sent or a '0' was sent.  A simple way of doing this is to use a'threshold-detector' - if the received signal is above a certain threshold, then a '1' is assumed to have been transmitted; if it is below the threshold a '0' is assumed to have been sent (this is somewhat of a simplification - the details depend on the line code used and how this represents ones and zeros).

A simple form. of threshold detector is the comparator. Recall that the comparator amplifies the difference  between  the voltages  on  its  two  inputs  by  an  extremely  large  amount.  This  always produces a heavily clipped or “squared-up” version of any AC signal connected to one input if it swings above and below a DC voltage on the other input.

In the communications system the received signal is connected to one of the comparator’s inputs and a variable DC voltage (the threshold) is connected to the other. The received signalswings above and below the DC voltage to produce a digital signal at the comparator’s output. By proper choice of the threshold voltage, the comparator’s output should be a copy of the original digital signal. Unfortunately, because of the effects of ISI and noise this'copy' may not be a perfect one; in other words, the decision-making threshold detector will make errors.

Setup the system shown in Figure 6.

Figure 6 (a) (top) Receiver set-up and (b) (bottom) block diagram representation

- Use NRZ-L line coding

- Set the Variable DCV module’s Variable DC control fully anti-clockwise

- Set the LPFfC control to 12 'o' clock.

Q8. Compare the transmitted and recovered signals (i.e. those from the sequence generator and the comparator output).  Are they identical? If not what are the differences?

Q9. Now, slowly turn the Variable DC module’s DC Voltage control clockwise and observe the effects. Why do some DC voltages cause the comparator to output the wrong information?

Q10. Estimate the optimum value of the DC threshold voltage that should be used in the comparator.  How does this compare with what might be expected?

- Return the DC voltage control to a value that generates the 'best' recovered signal.

-     Now, slowly make the channel’s bandwidth narrower by turning the Tuneable Low-pass Filter module’s Cut-off Frequency Adjust control anti-clockwise.

Q11. Why does the comparator begin to output the wrong information when this control is turned far enough?

We can also look at the effects of noise on the comparator output (and hence the error rate).

- Reset the LPFfC control to maximum (fully clockwise)

Again, you need to use the Noise Generator Module and the (unity gain) Adder Module.

- Take the signal output from the LPF module and the -20dB output from the noise generator module and add them together.

Use the output of the adder to feed into the comparator and compare the transmitted and recovered signals on the oscilloscope as before.

Q12. Has the noise introduced any errors into your recovered signal? Why is this the case? Q13. Vary the channel bandwidth and note the effects on your recovered signal.

6. Line code spectra

The digital oscilloscopes also have a 'math module'that, amongst other things, allows you to perform. a Fast Fourier Transform. (FFT) of signals to observe their spectra.

Q14. As a final task you should use this FFT capability to compare and contrast the 4 line codes available on the Emona 101 system.  In other words plot the spectra for the 4 line codes that are available and comment on them. Make sure you use a reasonable frequency scale on your plots (think about the frequencies at 1/Tb, 2/Tb etc)  - and compare the experimental spectra to what you would expect from theory.

Your Report

Your report should be a professionally produced (typed not handwritten) record of your findings in carrying out this experimental  investigation, answering all the questions  posed  in the text  and providing diagrams, plots and waveforms captured from the DSO (or sketches when DSO not used) where they are appropriate. Please remember to put labels on all axes and refer to the figures in your text when you discuss your results.

The report should consist of the following sections:

1. Introduction [15 marks]

The  introduction  should  provide  a  brief  overview  of  the  background  (e.g.  why  are  we interested in low pass channels? What applications are they used in etc.) and summarise the main objectives of the laboratory.

2. Method [10 marks]

In this section, you should outline what you have done in the laboratory. What equipment have you used? How have you recorded your findings etc.

3.   Experimental Results and Discussion [70 marks]

Here you should describe each experiment and present the necessary evidence and data to answer the 14 questions posed in the lab instructions. You should include the questions as subheadings.

4. Conclusions [5 marks]

Finally, you should conclude the report with a summary of the laboratory. Have you met the objectives? What have you learnt?