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Laboratory Exercise 4 - Communication System Analysis | 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

Pre 2010

Uploaded on 08/18/2009

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EE3550 - Communications Systems Analysis
Laboratory Exercise 4
This assignment may be done individually or in groups of two or three.
Partners will receive the same grade.
In this exercise, you will use the TIMS system to generate analog signals carrying digital
data.
The goals of the laboratory exercise are:
1. To generate an ASK signal
2. To recover digital data from an ASK signal
3. To generate a FSK signal
4. To generate a PSK signal
5. To recover digital data from a PSK signal
Overview
Digital data is often sent using an analog signal. This allows the signal to be bandlimited,
modulated, and multiplexed using analog techniques. In this lab you will observe the
three basic methods of modulating digital data onto an analog signal.
Amplitude Shift Keying
In ASK, digital data is sent by changing the amplitude of an analog signal. Usually, this
means sending a sine wave with zero amplitude for a logical zero and a nonzero
amplitude for a logical one. See Figure 1 for an example.
Figure 1: Amplitude Shift Keying. The digital signal is shown on top, and the analog signal below.
Generating ASK is about as simple as it gets: The digital signal is either zero or nonzero -
just multiply this by the carrier. That’s it!
To begin this experiment, you will generate ASK by using a sequence generator, an adder
to adjust the DC levels, and a multiplier. You will de-modulate by using a rectifier and
low-pass filter. Install the following modules in TIMS (you might notice a lot of
similarity to the modules you used for the AM lab – that’s no coincidence).
Sequence Generator – Generates a pseudo-random binary sequence.
Adder – Adds two signals together.
Multiplier – Multiplies two signals together.
Utilities Module – Contains several useful parts including a rectifier.
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EE3550 - Communications Systems Analysis

Laboratory Exercise 4

This assignment may be done individually or in groups of two or three. Partners will receive the same grade. In this exercise, you will use the TIMS system to generate analog signals carrying digital data. The goals of the laboratory exercise are:

  1. To generate an ASK signal
  2. To recover digital data from an ASK signal
  3. To generate a FSK signal
  4. To generate a PSK signal
  5. To recover digital data from a PSK signal

Overview

Digital data is often sent using an analog signal. This allows the signal to be bandlimited, modulated, and multiplexed using analog techniques. In this lab you will observe the three basic methods of modulating digital data onto an analog signal.

Amplitude Shift Keying

In ASK, digital data is sent by changing the amplitude of an analog signal. Usually, this means sending a sine wave with zero amplitude for a logical zero and a nonzero amplitude for a logical one. See Figure 1 for an example. Figure 1 : Amplitude Shift Keying. The digital signal is shown on top, and the analog signal below. Generating ASK is about as simple as it gets: The digital signal is either zero or nonzero - just multiply this by the carrier. That’s it! To begin this experiment, you will generate ASK by using a sequence generator, an adder to adjust the DC levels, and a multiplier. You will de-modulate by using a rectifier and low-pass filter. Install the following modules in TIMS (you might notice a lot of similarity to the modules you used for the AM lab – that’s no coincidence).  Sequence Generator – Generates a pseudo-random binary sequence.  Adder – Adds two signals together.  Multiplier – Multiplies two signals together.  Utilities Module – Contains several useful parts including a rectifier.

 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.

ASK Part 1 – ASK Modulation

  1. Set up the sequence generator to be clocked by the 2kHz clock from master signals. Also, trigger the scope using the sync signal from the sequence generator.
  2. The analog output of the sequence generator is at +/-2V levels. It needs to be adjusted so that logical zero is at 0V. Use the adder to do this – add together the analog output of the sequence generator with some Variable DC from master signals. Observe the output of the adder on the scope and adjust the gains (G and g) of the adder until logical zeroes are at exactly 0V.
  3. Multiply the adjusted digital signal (adder output) by a 100kHz carrier from Master Signals. Your setup should like similar to Figure 2.
  4. You should be able to observe a nice ASK signal on the output of the multiplier. Press run/stop to see some details about the signal. Sketch the ASK waveform for a few ones and zeroes using the 100kHz carrier. Figure 2 : ASK Generation in TIMS

ASK Part 2 – ASK Demodulation

You might recall that AM demodulation could be accomplished by a rectifier and low- pass filter. The same is true for ASK demodulation – it couldn’t be much simpler!

  1. Set the ASK carrier to 100kHz.
  2. Hook up the ASK signal (from the multiplier output) through a rectifier in the utilities module and through a tunable LPF. See Figure 3.
  3. Observe the output of the LPF as you adjust the tuning – try both Normal and Wide modes. Sketch the recovered signal that looks the best to you.
  1. Hook the TTL data of the sequence generator (either X or Y) to the Data input of the VCO.
  2. Set the VCO’s external switch to HI. Your setup should look like Figure 5.
  3. Observe the output of the VCO on the scope. Look at a transition from zero to one (use run/stop to freeze the output). Sketch the transition for your report.
  4. Hook up the spectrum analyzer to look at the FSK output. Set the range for about 25-200KHz. Sketch what you see on the spectrum analyzer. Report the values of the two frequencies being used.
  5. Remove the connection from the spectrum analyzer before proceeding. Figure 5 : FSK Generation Because FSK demodulation is more complicated and requires a lot of hardware, we will not be performing demodulation experiments.

Phase Shift Keying

By now, you’ve got the idea how analog keying of digital data works. The last of the primary methods for keying is phase shifting. The method we will use is Binary Phase Shift Keying (BPSK) where we use two phases, 180 degrees apart, to represent logic one or zero, as shown in Figure 6. (Note that this method is slightly different than the differential phase shift keying described in the class lectures.) Figure 6 : Binary Phase Shift Keying BPSK can be generated quite easily from a bipolar digital signal (-1 to +1) and a carrier – all we have to do it multiply them together. When the digital signal is -1, the carrier will be inverted, effectively shifting its phase by 180 degrees. This is shown in Figure 7.

Figure 7 : BPSK Generation One important detail makes this model a little more complicated – if you look closely at Figure 6, you’ll see that all phase reversals occur when the carrier is at a zero crossing. This makes the signal easier to detect – however, it requires that the carrier frequency be an exact multiple of the digital bit clock. Thus, we need to base the digital clock and the carrier on the same master frequency. To begin this experiment, you will generate BPSK by multiplying a bipolar digital sequence by a carrier. Install the following modules, which also support demodulation.  Audio Oscillator – Provides a clocking frequency.  Phase Shifter – Makes small adjustments in the phase of the carrier in order to align the carrier with the digital sequence.  Sequence Generator – Generates a pseudo-random binary sequence.  Line Code Encoder – Converts the digital signal to a bipolar NRZ signal appropriate for multiplication.  Multiplier – Multiplies two signals together.  Utilities Module – Contains several useful parts including a rectifier.  Quadrature Utilities – Contains a multiplier to use for demodulation.  Tunable LPF – Filters out the high frequency carrier so we can restore the original signal. Please see the TIMS Reference Pages on the course web page for detailed descriptions of these modules.

Part 1 – BPSK Generation

  1. Our first step is to set up the clocks we need. We will use the audio oscillator to generate the carrier clock, and then divide this clock by four (using the Line Code Encoder) to create the digital bit clock. (See Figure 8 for a diagram.) a. Set the audio oscillator to produce an output of about 8kHz. b. Hook the TTL output (8kHz square wave) to the Master Clock (M.Clk) input of the Line Code Encoder. This signal will be divided by 4 by the encoder to produce a synchronized 2kHz square wave on the Bit Clock output. c. Hook the Bit Clock (B. Clk) output of the Line Code Encoder (2kHz square wave) to the TTL Clock input of the Sequence Generator. d. The result of this is that the Sequence Generator is clocked at 2kHz, and the output is precisely synchronized to the 8kHz output of the Audio Oscillator. We will use the 8kHz signal for our synchronized carrier in the step 3.

Figure 9 : BPSK Demodulation Waveforms. Top - BPSK signal for 0110. Middle - Carrier. Bottom - Draw the product of these two waves.

  1. Take the BPSK signal from Part 1 and multiply it by the 8kHz carrier (analog output of the Audio Oscillator). Note that you are already using the regular multiplier for the BPSK generation. Use the multiplier portion of the Quadrature Utilities module instead.
  2. Observe the output of the demodulating multiplier on the scope. Compare it to the source digital signal from the sequence generator – other than a delay, can you see how it corresponds to the source?
  3. In order to demodulate correctly, the carrier and BPSK signal need to be perfectly in phase. Adjust the phase of the BPSK signal by turning the Phase Shifter knob. What do you see? Sketch three waveforms – the “best” position, the “worst” position, and the result of flipping the 180 degrees toggle switch on the phase shifter.
  4. Your signal should look similar to the source digital signal, except that it has this annoying high frequency component built in to it. How do we fix that? Hopefully you guessed: “Low Pass Filter.” Do that – take the output of the multiplier and filter it with the Tunable LPF. Adjust the filter (use “wide” mode) to find the best recovery of the digital waveform. Sketch three waveforms: the “best” LPF setting, LPF set too low, LPF set too high.

To Turn In

  1. Turn in a combined lab report that includes your observations, sketches, and comments, as described above. Note that required portions are underlined. 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.