comms report lab 2

8/2/2019 Comms Report Lab 2

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Swinburne University of Technology

Faculty of Information and Communication Technologies

ASSIGNMENT AND PROJECT COVER SHEET

Unit Code: HET314 Unit Title: Communications Principles

Lab number and title: DSB Modulation and Detection Due date:

Lab group Day: Time: Demonstrator:

Family name: Student ID:

Other names:

To be completed if this is an individual assignmentI declare that this assignment is my individual work. I have not worked collaboratively norhave I copied from any other students work or from any other source except where dueacknowledgment is made explicitly in the text, nor has any part been written for me byanother person.

Signature:

To be completed if this is a group assignmentWe declare that this is a group assignment and that no part of this submission has beencopied from any other student's work or from any other source except where dueacknowledgment is made explicitly in the text, nor has any part been written for us by anotherperson.

ID Number Name Signature

Marker's comments:

Total Mark:

Extension certification:

This assignment has been given an extension and is now due on

Signature of Convenor:


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Student id: Name: Group:

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EXPERIMENT 2 DSBMODULATION AND DETECTIONPreliminariesThe preliminary work is worth 20% of the report mark and must be completed before

the laboratory session.

Enter all your results in the next section, the background and methodology can befound in page 10.

Results

1. DSB-SC (Suppressed Carrier) Generation1.1. Preliminary Work

Using mathematical analysis, predict the frequency spectrum (frequency domain)

for y(t) in Error! Reference source not found.. Sketch both the time domain view

and the spectrum in the graphs provided below (Predicted) noting all important

amplitudes, strengths and frequencies.

Predicted (time domain)


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Observed (time domain)

Results 1-1 Time Domain representation of a DSB-SC modulation

Predicted (frequency domain)


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Observed (frequency domain)

Results 1-2 Frequency Domain representation DSB-SC modulation

1.1. For the DSB-SC case compare your predictions with the experimental resultscommenting on differences.

The results closely followed the predicted results with only slight variations in the

amplitude of both the frequency spectrum and time domain.

2. DSB-LC (Large Carrier) - Generation2.1. Preliminary WorkSketch the frequency domain pictures at points A, B, C and D in Error! Reference

source not found..

2.2. For the DSB-LC case, why must the Coupling on the Multiplier be set to DC?

Because it allows the DC offset from the variable dc supply to be multiplied along

with the message so that the message becomes 1+xm(t) which then multiplies that with

the carrier giving the modulated message as well as the carrier.


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Observed (Trapezoidal Pattern)

Observed (Time Domain)

Results 2-1 Trapezoidal Display and Time Domain representation of DSB-LC


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Results 2-2 Frequency Domain representation of DSB-LC2.3. From your time domain observations what is the value of the modulation

index m, show your calculations? Compare this value with from the frequency

domain observations, again show calculations.

.78/1.6 = 0.4875(time) (2*0.145)/0.824 = 0.3519(frequency)

The differences could from misinterpretation of the graphs, obtaining the wrong

values to put into the equations

3. DSB-LC (Large Carrier) - Detection

3.1.

Preliminary workExplain the operation of the envelope detector in Figure 3-1. Sketch indicative signal

waveforms for the output of this envelope detector. The envelope detector used in the

lab is as shown in Figure 3-2. Predict the output of this detector for the three values of

capacitance. You may assume that the 1.5nF capacitor leads to a time constant which

is very short, and that the 100nF capacitor gives a time constant which is very long.


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Observed (time domain) C=0 nF

Observed (time domain) C=1.5 nF


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Results 3-3 Envelope Detected signal for different values of capacitance


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3.2. Compare your predictions with the experimental results commenting ondifferences, What is the optimum value of capacitance for the detector? Is

there any distortion? Discuss.

The optimum value was 10 nF as the 1.5nF dropped too quickly and the 100 nF

dropped and raised too slowly. There was however slight distortion in the 10 nF,however it was small and the message still most likely would have been decoded and

understood

4. DSB-SC Product Detection4.1. Preliminary work

Derive an expression for the output of the product detector from the block diagram in

Error! Reference source not found. assuming that the input is a single tone

modulated DSB-SC signal. Also show the effect on the output if the local carrier is out

of phase from the modulated carrier by 45.

Observed (time domain) C= Optimum

Results 4-1 Envelope Detection of DSB-SC


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4.2. Was the message signal correctly recovered? State the frequency of the signalat the output of the envelope detector and describe its shape compared to the

message?

Yes the message was recovered, it was however 180 degrees out of phase and the

amplitude which was predicted. The frequency was also, as predicted the same as the

message as well as the shape of the signal.



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Results 4-2 Product Detector Time and Frequency Domain


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Results 4-3 Filtered output of Product Detector Time and Frequency Domain


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4.3. Comment on the results from the product detection section.

The detected signal was very close to the original message signal, the only differences

were the amplitude and the phase which could easily be corrected with an amplifier

and phase shifter at the decoded end, which was done with the gain in the LPF and

phase shifter module, which made the signal very close to the original.

4.4. Optional Challenge

Phase shifted 180(time Domain)The phase shifter in our TIMS unit did not work, the phase was shifted but the signal

remained the same, it was expected that the signal would change amplitude by a factor

of cos(phase angle) however nothing had happened, the 180 phase change howeverdid work the final signal was matched up.

5. Conclusion

In conclusion it was learnt that AM can be a very effective method for encoding a

message to a much higher frequency to be broadcast and very effectively recovered

using a variety of techniques, each with their own advantage.


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comms report lab 2

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