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Digital Sequence, Eye Patterns and Line Coding - Lab 2 | EE 3550, Lab Reports of Electrical and Electronics Engineering

Seattle Pacific University (SPU)Electrical and Electronics Engineering
Prof. Kevin Bolding

Material Type: Lab; Professor: Bolding; Class: Communication System Analysis; Subject: Electrical Engineering; University: Seattle Pacific University; Term: Unknown 1989;

Typology: Lab Reports

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EE3550 - Communications Systems Analysis
Laboratory Exercise 2
This experiment may be done in groups of 1-3 people.
You may turn in a combined report.
In this exercise, you will use the TIMS system to generate signals used for digital
communications. You will examine the signals using an oscilloscope and spectrum
analyzer.
The goals of the laboratory exercise are:
1. You will become familiar with the TIMS system.
2. You will gain practice looking at digital signals using an oscilloscope.
3. You will gain understanding of band limited digital signals.
4. You will become familiar with eye patterns.
5. You will observe common digital encoding schemes.
It is very important to read the following notes before attempting each laboratory
exercise.
To turn in
Since this laboratory exercise is more qualitative than quantitative, you will not be
reporting measurements. Instead, you will respond to the questions and discussion
portions listed under “conclusions” for each subsection. Please see the expectations for
laboratory reports in the course syllabus (under “Grading”).
Important Oscilloscope Notes
The TIMS manual is written with the assumption that you are using an analog scope.
We’re using a more advanced digital storage scope instead. Please keep these in mind.
The previous person to use the scope may have left it in an unusual configuration.
Press Default Setup to reset the scope to the default settings.
To select an external trigger, press the Trig Menu button for the trigger setup
menu and select EXT for source. Connect the triggering signal to the Ext Trig
BNC. To see the trigger waveform (for adjustments) press and hold Trig View.
Use the trigger level adjustment as needed to get a stable signal.
Make sure that all of the inputs are set to X1 (not X10). Use the individual
channel menu buttons for this.
DC coupling is preferred for most parts of this lab. However, you may select AC
or DC coupling using the individual channel menu buttons.
Part 0 – TIMS Intro
Read the Introduction to TIMS lab sheet found on the course web site under the
“Assignments” section. Although there is nothing to turn in for this part, it is very
important to familiarize yourself with the equipment before beginning.
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EE3550 - Communications Systems Analysis

Laboratory Exercise 2

This experiment may be done in groups of 1-3 people. You may turn in a combined report. In this exercise, you will use the TIMS system to generate signals used for digital communications. You will examine the signals using an oscilloscope and spectrum analyzer. The goals of the laboratory exercise are:

  1. You will become familiar with the TIMS system.
  2. You will gain practice looking at digital signals using an oscilloscope.
  3. You will gain understanding of band limited digital signals.
  4. You will become familiar with eye patterns.
  5. You will observe common digital encoding schemes. It is very important to read the following notes before attempting each laboratory exercise.

To turn in

Since this laboratory exercise is more qualitative than quantitative, you will not be reporting measurements. Instead, you will respond to the questions and discussion portions listed under “conclusions” for each subsection. Please see the expectations for laboratory reports in the course syllabus (under “Grading”).

Important Oscilloscope Notes

The TIMS manual is written with the assumption that you are using an analog scope. We’re using a more advanced digital storage scope instead. Please keep these in mind.  The previous person to use the scope may have left it in an unusual configuration. Press Default Setup to reset the scope to the default settings.  To select an external trigger, press the Trig Menu button for the trigger setup menu and select EXT for source. Connect the triggering signal to the Ext Trig BNC. To see the trigger waveform (for adjustments) press and hold Trig View. Use the trigger level adjustment as needed to get a stable signal.  Make sure that all of the inputs are set to X1 (not X10). Use the individual channel menu buttons for this.  DC coupling is preferred for most parts of this lab. However, you may select AC or DC coupling using the individual channel menu buttons.

Part 0 – TIMS Intro

Read the Introduction to TIMS lab sheet found on the course web site under the “Assignments” section. Although there is nothing to turn in for this part, it is very important to familiarize yourself with the equipment before beginning.

Part 1 – Digital Sequences

The simplest type of digital signal is a two-level NRZ waveform – a classic modified square wave signal. In this experiment, you will generate a digital sequence and examine its characteristics when it is transmitted through a limited bandwidth channel. To begin this experiment, you will generate a pseudo-random digital sequence and examine it using an oscilloscope. You will need three TIMS modules:  Sequence generator – Generates a pseudo-random binary sequence synchronized to an input clock.  Audio oscillator – Generates an adjustable-frequency clock in the audio range.  Tunable Low-pass Filter – Filters the input to allow only low frequencies to pass. The cutoff frequency is adjustable. Please see the TIMS Reference Pages on the course web page for detailed descriptions of these modules.

1-A A digital sequence and triggering

In this exercise, you will create a digital sequence and experiment with oscilloscope triggering.

  1. Set up the Audio Oscillator to establish a clock signal with a frequency around 2KHz. a. Observe the TTL output of the Audio Oscillator using an oscilloscope. Make sure to reset the scope and then set the scope probe type to 1X first. b. Adjust the frequency to around 2KHz by turning the knob on the Audio Oscillator.
  2. Before plugging in a sequence generator, set it to produce a sequence with length 32 by adjusting the DIP switches on the card. See the reference page for the Sequence Generator for details.
  3. Plug in the Sequence Generator and connect its TTL Clock to the 2KHz clock signal from the Audio Oscillator. This will cause the Sequence Generator to output a sequence with each bit synchronized to the 2Khz clock. The output sequence is sent to both the ANALOG (-1V to +1V) and TTL (0V to 5V) outputs. There are two forms of the sequence, called X and Y. Just use the X sequences for this exercise.
  4. Observe the TTL X sequence on the oscilloscope, along with the clock signal. Try triggering on the clock signal and on the sequence signal. (Use the scope’s trigger menu to select the source for triggering.) You will probably have a difficult time seeing a stable signal.
  5. The sequence generator provides a synchronization signal that indicates the beginning of a sequence. This can be used to trigger the scope at the beginning of each sequence. Connect the SYNC output of the sequence generator to the EXT TRIG input of the scope. Use the trigger menu to select external triggering. You should now see a stable version of the pseudo-random sequence. If not, try adjusting the trigger level on the scope.
  6. Switch the sequence output from the TTL to the Analog output and observe the difference in the signal. The analog signal is centered around 0V and is more suitable for analog processing later on.

Counter (on the main unit). Set the Frequency Counter to a GATE TIME of 0.1s. The value displayed on the red LED display will be the LPF’s cutoff frequency divided by ten. For example, if the cutoff frequency is 2000Hz, the display will show “200.000”. Adjust the cutoff frequency (in both normal and wide modes) and observe how the displayed value changes.

  1. Now, “play around” a bit by adjusting the cutoff frequency of the filter. Observe the shape of the sequence as you lower the cutoff frequency. Try both the Normal and Wide options to see how the filter behaves differently. Part 1-B Conclusions. In this section, you observed the effect of band-limiting a digital signal in both the time and frequency domains. Draw sketches describing what you saw with the cutoff frequency set at several different points. Be sure to include some sketches of the interesting area near the fundamental frequency of 2000Hz. Comment on what you saw as the filter frequency was decreased – other than the basic shape of the wave, what other aspects of the waveform changed?

Part 2 – Eye Patterns

As you have seen in part 1, digital waveforms lose their shape as the bandwidth allocated to them is reduced. It is obvious that the original digital data can be recovered from waveforms that are given “a lot” of bandwidth; likewise, it is obvious that there is no hope of recovering data from waveforms that are given “too little” bandwidth. However, the engineer is interested in finding out the minimum amount of bandwidth required to successfully transmit a digital signal at a given speed (or, alternatively, the maximum digital signal speed that can be transmitted using a given bandwidth). Nyquist’s law gives us C = 2Blog 2 M as a theoretical result – but what about practical results? For digital signals with excess bandwidth, the signal is at one extreme or another (high or low) almost all the time and spends very little time in transition. If we set up an experiment to observe several transmitted bits overlapped on one another (the bits could be 1’s or 0’s) we expect to see a pattern where the signal is almost always fully high or low and all of the transitions are nearly vertical and lined up together. Essentially, we see rectangular boxes. We will perform this experiment and then reduce the bandwidth to see what happens.

2-A Eye Patterns Experiment

In this exercise, you will use “eye pattern” experiments to gauge the quality of a band- limited signal and determine the maximum data rate for an NRZ signal with a given bandwidth.

  1. Set up the Sequence Generator (32-bit sequence) with the output filtered by a Tunable LPF. Use a 2KHz ANALOG output from an Audio Oscillator to clock the Sequence Generator. Set up the oscilloscope to observe the output of the Sequence Generator before and after filtering. Initially, set the filter to WIDE and turn the knob to maximize bandwidth. (Note: Connect the ANALOG output of the Audio Oscillator to the ANALOG CLK signal on the Sequence Generator.)
  2. If you connect the Sequence Generator’s SYNC signal to the scope’s trigger, you will see a nice pseudo-random sequence, as in part 1-A. Make any adjustments needed to display this signal nicely before proceeding. You may play with the

filter a bit if you like to re-familiarize yourself with how the signal changes. Reset the filter to maximum bandwidth before proceeding. Now, change the Sequence Generator to a 2048-bit sequence. This may not synchronize well because it is so long – that’s OK.

  1. Because the scope is triggered using the start-of-sequence SYNC signal, the signal is always lined up on top of itself and it is easy to see the entire signal. What would you expect if you instead triggered the scope on the bit clock – re- triggering the scope for each bit? Try it out! (Make sure the filter is set to maximum bandwidth first.) Move the bit clock (the ANALOG output of the Audio Oscillator) to the TRIG signal for the scope. a. Sketch what you see for your report – does this match the prediction from the introduction to Part 2? b. Imagine that you were a receiver trying to decode the sequence of bits sent to you. When you sample, you would prefer to see a solid ‘1’ or ‘0’, with nothing in between. Where would you be able to sample this waveform and recover the signal? Where would you have difficulty? c. What do you predict will happen to this display if the bandwidth is reduced?
  2. Now, see what really happens as the bandwidth is reduced. Switch the filter to NORM and slowly turn down the TUNE knob. Try different levels of persistence (DISPLAY menu on the scope). You should see the open space between transitions begin to shrink – as it bandwidth is reduced it becomes shaped like an eye, and eventually closes altogether. a. Sketch a few waveforms that you see show “eyes” of various degrees of openness. b. Imagine that you were a receiver trying to decode the sequence of bits sent to you. When you sample, you would prefer to see a solid ‘1’ or ‘0’, with nothing in between. Draw a line on the diagrams from question (a) indicating the best area to sample the waveform and recover the signal. Comment on the ease of sampling for each eye diagram.
  3. Re-check the data rate of your original signal. (It should be 2Kbps if the clock is 2Kbps, but check anyway.) Adjust it to exactly 2Kbps.
  4. Now, find the minimum acceptable bandwidth for this 2Kbps signal. Set the filter to the lowest value that gives an acceptably open eye (that is, you would be able to determine if it represents a ‘1’ or ‘0’ with 100% accuracy). Set the frequency counter to display the cutoff frequency (don’t forget that the display is off by a factor of 10!).
  5. Use Nyquist’s law to compute the theoretical minimum bandwidth needed for a 2Kbps signal. Compare to your results from (6). If your experimental results did not reach the theoretical minimum, propose some reasons for the difference. Part 2-A Conclusions. You have now seen how to use eye patterns to find the minimum acceptable bandwidth for a given digital waveform. Alternatively, you could use the same technique to find the maximum data rate for a given bandwidth. In your report, include the sketches and answers to the questions for steps 3,4,6, and 7.
  1. “Play Around” with the various codes to see each in action. Try predicting the patterns you will see for each code. Try mis-matching the encoder and decoder scheme and see if any combinations still work correctly. Optionally, insert a Tunable LPF in between the encoder and decoder and observe the results as the bandwidth is limited. Figure 2 : Lab setup for encoder/decoder. Do not use the Buffer Amplifiers. Part 3-A Conclusions. The main purpose of this part is for you to gain experience with real digital codes. “Play around” for a minimum of fifteen minutes. In your report, describe the most interesting observations you make as you play around. When you are finished, please unplug all of the patch cords on the TIMS system. Please leave the coax lines (BNC connectors) connected to the scope and spectrum analyzer.