communication systems lab record

8/10/2019 Communication Systems Lab record.

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ECE F311 COMMUNICATION SYSTEMS LAB REPORT

Batch: MONDAY (7-8)

Author : MANINDRA KANDEPU (2011B1A3742H)

Experiment 1: Study of Analog Filters Using Matlab

A. Simple RC Filters:

1,2.

Lowpass:y=tf([0 1000],[1 1000]);bodeplot(y);


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3.

High pass:

y=tf([1 0],[1 1000]);bodeplot(y);


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4.

Bandpass

y=tf([1000 0],[1 2000 1000000]);bodeplot(y);


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B.

Higher order Filters:

Low Pass filter

3dB bandwidth=1.13 rad/s

Pass band ripple=0.97

Stop band ripple=0

Roll off=-19.6 dB/octave

Non-linear.

-150

-100

-50

0

50

Magnitude(dB)

10-2

10-1

100

101

102

-270

-180

-90

0

Phase(deg)

Bode Diagram

Frequency (rad/s)


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Low pass filter

Pass band ripple=0.94

3dB band width=1.01 rad/s

Stop band ripple=220rad/s

Roll off=infinite

Non linear.

-400

-350

-300

-250

-200

-150

-100

-50

0

Magnitude(dB)

10

-2

10

-1

10

0

10

1

10

2

0

90

180

270

360

Phase(deg)

Bode Diagram

Frequency (rad/s)


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C.

Butterworth and Chebyshev Filters:2.

CHEBYSHEV n=1


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Chebyshev n=2

Chebyshev n=4


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Chebyshev n=7

Chebyshev n=10


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2. butterworth

clear allclcnum1=[1];den1=[1 1];sys1= tf(num1,den1);

num2=[1];den2=[1.3827 1.3614 1];sys2= tf(num2,den2);


num42=[1];den42=[1.1509 0.3648 1];sys42= tf(num42,den42);sys4=sys41*sys42;

num71=[1];den71=[4.0211 1];sys71= tf(num71,den71);



num74=[1];den74=[1.0443 0.1156 1];sys74= tf(num74,den74);sys7=sys71*sys72*sys73*sys74;





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sys10=sys101*sys102*sys103*sys104*sys105;bode (sys1)hold onbode(sys2)hold onbode(sys4)hold onbode(sys7)hold onbode(sys10)

3.

n 3dB frequency Max pass band

ripple

Max stop band

ripple

Roll off Phase linearity

1 1.02 0 0 -4.15 Linear

2 0.707 0 0 -4.73 Linear

4 1.01 0 0 -21.4 Linear

7 0.997 0 0 -58.9 Linear

10 1.03 8.57 0 -41.7 Linear


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4. case 1

clear allclc

num1=[1];den1=[1 1];sys1= tf(num1,den1);



num41=[1];

den42=[1.1896 0.2374 1];sys42= tf(num41,den42);sys4=sys41*sys42;



num73=[1];

den73=[1.5927 0.3067 1];sys73= tf(num73,den73);



num102=[1];

den102=[4.6644 0.8947 1];sys102= tf(num102,den102);


num104=[1];den104=[1.2614 0.1233 1];


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sys104= tf(num104,den104);


sys10=sys101*sys102*sys103*sys104*sys105;

bode (sys1)hold onbode(sys2)hold onbode(sys4)hold onbode(sys7)hold onbode(sys10)


ripple

Max stop band

ripple


1 0.988 0 0 -6.058 Linear

2 0.887 2 - -8.08 Linear

4 0.968 1.9082 - -33.23 Linear

7 0.990 -1.8837 - -66.5 Linear

10 0.993 1.94 - -104 Linear


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Case 2

clear allclcnum1=[1];den1=[1 1];

sys1= tf(num1,den1);




sys4=sys41*sys42;










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sys10=sys101*sys102*sys103*sys104*sys105;

bode (sys1)hold onbode(sys2)hold onbode(sys4)hold onbode(sys7)hold onbode(sys10)


ripple

Max stop band

ripple


1 1 0 0 -66.3 Linear

2 0.995 0.495 0 -11.1 Linear4 0.999 0 0 -28.4 Linear

7 0.998 0 0 -64 Linear

10 0.998 0 0 -97 Linear


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Experiment 2: Study of Analog Filters Using RLC components

A. Filter 1.

1.Band pass filter

3dB1=2.86*10^-5 rad/sec, 3dB2=3.5*10^-5 rad/sec

Bandwidth=0.64*10^-5 rad/sec

normalized power gain Vs frequency (semilog scale)


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B.

Filter 2.

Band pass filter

3dB1=2.86*10^4 rad/sec, 3dB2=3.6*10^4 rad/sec

Bandwidth=0.64*10^4 rad/sec


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C. Filter 3.

Band stop filter

3dB1=31.3 rad/sec, 3dB2=31.9 rad/sec

Bandwidth=0.6 rad/sec


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D. Filter 4.

Band pass filter

3dB1=1.9*10^4 rad/sec, 3dB2=7.61*10^4 rad/sec

Bandwidth=5.7*10^4 rad/sec


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Experiment 3: Signal and Noise Experiments

With Emona Telecom Trainer Kit

A) GENERATION OF SIGNALS

a) sine signal of 100khz,pk-pk voltage 4.12

b) Table 1: Time and Frequency Domain Measurements with DSO

Sl.

No

SignalTime Domain Frequency Domain Level of other spectral

components

Amplitude Peak

to Peak

Period

(us)

Frequency

(KHz)

Amplitude

(dB)

Frequency

(KHz)

1. 4 10 100 64.8 100 8

2. 2.2 10.2 100 64 100 8.8

3. 5.8 10 100 68 100 8

4. 8.24 10 100 64 100 -


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c)FFT spectra of sine signal

B. Generation of Noise:

a) noise at 0db


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b)fft spectra of noise

c) NO, PSD of noise depends on frequency.

d) & e) Table 2: Noise Generation and Measurements with DSO

Sl.

No

Time Domain Frequency Domain

Maximum

Amplitude

(in V)

Minimum

Amplitude

(in V)

Maximum

Amplitude

(in dB)

Average

Amplitude

(in dB)

1. 10 -10.2 40 24

2. 2.8 -2.6 39.2 19.2

3. 1.6 -1.76 42.4 20

4. 1.1 -1.2 41 195. 2.1 -2 41 19.5


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C. Studies on Signal Plus Noise:

a)Signal and noise:

c)FFT spectra of signal plus noise:

Table 3: Effect of Noise on Intelligibility of Audio


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Sl.

No

Signal *

(in dB)

Signal Noise*

(in dB)

Noise S/N Very

Clear

Just

Intelligible

Not

Intelligible

1. 64 2.51X10^6 22 158.48 1.583X10^4 yes2. 64 2.51X10^6 32 1583.2 1.583X10^3 yes

3. 64 2.51X10^6 40 10^4 2.51X10^2 yes

D) Filtering of Noise:

Sl.No

Type of Filter Noise beforeFilter) * (dB)

Pass bandNoise* (dB)

Stop bandNoise* (dB)

3 dBfrequencies

(kHz)

Roll off (dB /Octave)

1 Baseband LPF 62.8 8.6 2.2 2.8 6.4

2 Bandpass 62.8 39.8 28 5 15.4

3 RC Low pass 62.8 14 10 160 11.2

E) Conclusions:

Gained experience on usage of Emona Kit.


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Experiment 4: Amplitude Modulation and Demodulation

A) Generation of AM with Carrier:

A1) AM with carrier

A2) FFT spectra

A3) Table 1: Modulation indices and Efficiencies


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A6) OVERMODULATION

Sl. No Message

Amplitude(Peak-peak)

Carrier

Amplitude(Peak-peak)

DC Bias

Value

Modulation

Index

Efficiency Power in

Carrier (FromSpectra)

Power in Side

Bands (FromSpectra)

Efficiency

1. 1 4 2 0.2421 0.0284 7.26 0.085 0.0201

2. 2 4 2 0.4670 0.095 7.26 0.458 0.112

3. 560m 4 2 0.101 5.13x10^-3 7.32 0.019 5.16x10^-3


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B Demodulation of AM with Carrier:

B1)&B2)

B3)FFT SPECTRA OF DEMODULATED SIGNAL


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B4) SPECTRA OF ORIGINAL SIGNAL

B5)No distortion in the demodulated AM signal output compared to the original Message

Signal.

B6) MESSAGE SIGNAL AFTER PASSING THROUGH LOW PASS FILTER


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OVERMODULATED SIGNAL

C Conclusions:

Learnings from the experiment:

The experiment helped us to get a clearer picture of modulation of signal using DSB-SC.

And also that, over and under modulation distort the message signal.

We also learnt how to calculate spectra power, useful power, sideband power, concepts of modulation

index and efficiency.


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B- DEMODULATION OF DSB-SC SIGNAL:

B1 B2,B3

Sl.

No

Message

Amplitude

(Peak-

peak) (V)

Message

Power

(From

peak-to-

peak)

(W)

Power in

the

Message

(From

Spectra)

(dB)

Carrier

Amplitude

(Peak-

peak) (V)

Carrier

Power

(From

peak-

to-

peak)

(W)

Power

in the

Carrier

(From

Spectra)

(dB)

Power in

USB (From

Modulated

Spectra)

(dB)

Power in

LSB (From

Modulated

Spectra)

(dB)

Power in

Carrier

(From

Modulated

Spectra)

(dB)

Powe

Demo

Me

(F

Spect

1 4 2 1.85 4 2 1.86 3.01 3.01 -10.2 7

2 4.2 2.2 2.25 4 2 1.85 2.21 2.61 -21.8 7

3 4.2 2.2 2.25 4.3 2.31 3.05 3.01 2.61 -21.8 9

4 4.2 2.2 2.25 4.5 2.53 3.45 3.01 3.41 -20.6 1


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C.

Demodulation of DSB-SC Signal: Effect of LO Phase Errors

C1

Effect of Phase Offset of LO on DSB-SC Modulation

Sl.

No

Phase Shift

(degrees)

Power in the Message

(From Spectra) (dB)

Power in the

Demodulated

Message

(From Spectra) (dB)

Audio Quality of the

demodulated messag

(Good / Moderate/

Very bad )

1 0 1.85 7.41 good

2 30 1.85 2.95 moderate

3 120 1.85 3.85 bad

4 155 1.85 6.65 bad

C2

Effect of Frequency Offset of LO on DSB-SC Modulation

Sl. No LO Frequency

Deviation

Hz

Power in the

Message (From

Spectra)

Power in the

Demodulated Message

(From Spectra)

Audio Quality of the

demodulated message (Good /

Moderate/ Very bad )

1 0 2.23dB -6.95dB moderate

2 400 2.23dB -4.55dB good

3 1200 2.23dB -32.1dB bad


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D Generation and Demodulation of SSB-SC Signals

Let the Message Signal be a 2 KHz Sinusoid and the Carrier be a 100 KHz Sinusoid. Let the peak to peak

amplitudes be ~ 4V, respectively. Use the following connection diagram.

D1. Adjust the phase shifter to provide 90 degree phase shift to the message. Use DSO features to

measure the phase difference between the input and output signal of the phase shifter. Can we use the

XY feature of the DSO to make sure that the phase difference between signals at X and Y is 90 degrees?

Yes. If the display is a perfect circle, the phase difference is 90

D2. How does the spectrum at the output of adder module look like? Is it truly and SSB spectrum? If not

what could be the reasons? Check for the amplitudes of the signals at B and A, the adder inputs. Work

with the gains of the adder to get a nearly perfect SSB signal.

The LSB is not perfectly suppressed.

D3. Measure the power of the SSB-SC signal in spectral domain. How does it compare with the either

USB or LSB power in Part A this experiment?

Power in SSB is 7.41dB

D4. Adjust the phase shifter slightly and note the effect on the spectrum.

Power in the suppressed sideband is getting increased. For phase value of 180, we get two sidebands.

D5. Demodulate the SSB-SC signal, generated in D1 & D2 using the following circuit. Capture the time

domain and spectra and comment on the nature of the demodulated message signal. Listen to thedemodulated message and comment on the quality of the audio.

Conclusions:

We have learnt the techniques of demodulation of DSB-SC and SSB-SC modulated signals

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