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