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Communication Systems Lab (10B17EC574)
List of Experiments
1) To study Amplitude modulation and Demodulation. Design a circuit which performs amplitude modulation for a modulating signal with a specific frequency and amplitude. Then find the modulation index to see whether the signal is under modulated or over modulated. Trace the modulated signal on the trace paper. Then employ demodulation circuit to obtain the original modulating signal. Compare and see whether the two signals are identical or not? 2) To study Frequency Modulation and Demodulation. Implement the circuit for frequency modulation using modulating signal and the carrier wave. Calculate the frequency deviation in the modulated signal. Trace the modulated signal. Find the modulation index. Now employ demodulation circuit and obtain the original modulating signal. Also study phase modulation corresponding to frequency modulation. 3) To study Pulse Amplitude Modulation (PAM) Generate a message signal and carrier signal. Use appropriate circuit for pulse amplitude modulated signal generation. Observe and trace the modulated signal. 4) To study Pulse Width Modulation (PWM) Generate a message signal and carrier signal. Use appropriate circuit using 555 timer for pulse width modulated signal generation. Observe and trace the modulated signal 5) To study Pulse Position Modulation (PPM) Generate a message signal and carrier signal. Use appropriate circuit using 555 timer for pulse position modulated signal generation. Observe and trace the modulated signal 6) To study Amplitude Shift Keying (ASK) Construct the ASK generation circuit using BJT. Observe the ASK waveform. Employ demodulation circuit to obtain the original signal. 7) To study Frequency Shift Keying (FSK) Construct the FSK generation circuit using BJTs.
Observe the FSK waveform. 8) To study Delta Modulation Construct the circuit using Op-amp and 7474 IC. Observe the delta modulated signal. 9) To study various line coding techniques Polar Manchester 10) To design a phase locked loop (PLL) circuit Study the IC characteristics using PLL IC.
One input is a single, high frequency carrier signal of constant amplitude and second input is compromised of relatively low frequency information signal which may be a single frequency or a complex waveform made up of many frequencies. AM modulators are non linear devices with two inputs and one output.
Procedure:
- Connect the circuit as per the given circuit diagram.
- Apply fixed frequency carrier signal of 1 V (^) p-p of 100 kHz.
- Apply modulating signal from function generator of 0.5 Vp-p of 500 Hz.
- Note down and trace the modulated signal envelop on the CRO screen.
- Find the modulation index by measuring V max and V min from the modulated (detected/ traced)
envelope.
- Repeat the steps 3, 4 & 5 by changing the frequency or/& amplitude of the modulating signal so as to observe over modulation, under modulating and perfect modulation.
- For demodulation, apply the modulated signal (AM) as an input to the demodulator and verify the demodulated output with respect to the applied modulating signals and their respective outputs.
- For demodulation circuit, the value of R and C should satisfy the following condition: f m<1/RC< f c.
- Observe the fidelity of the demodulated signal and calculate the distortion using the distortion meter.
Waveforms:
Important Formulae:
The normalized total side band powers = (^2) mE c^2
Where Ec is the carrier amplitude, Ec V max^2 V^ min
EXPERIMENT NO. 2
Aim: Frequency Modulation & Demodulation techniques and measurements. a) Measurement of modulation index. b) Draw the spectral characteristics. c) Verify Carson’s rule. d) Use any demodulation technique to retrieve the original signal. Apparatus Required: Bread Board, CRO, DC Power Supply, Function Generator, IC8038, Resistors, Capacitors, Connecting probes and wires. Theory: Frequency modulation is the process of varying the frequency of a constant-amplitude carrier directly proportional to the amplitude of the modulating signal at a rate equal to the frequency of the modulating signal. Frequency modulation consists of varying the frequency of the carrier voltage in accordance with the instantaneous value of the modulating voltage. Thus the amplitude of the carrier does not change due to frequency modulation. Let the modulating voltage be given by expression:
V m vm cos m t
where ω m is angular frequency of the signal and v m is the amplitude. Let the carrier voltage be given by expression:
V c vc sin( ct )
On frequency modulation, the instantaneous value of modulated carrier voltage is given by:
Vc vc sin
where ct
Hence, the frequency modulated carrier voltage is given by:
V V sin[ t k V sin mt ] m
m c f m^
The modulation index is defined as the ratio of frequency deviation to frequency of modulating signal
f m
f
where Δ f = ( f max - f min)/
Circuit Diagram:
Fig 1 : Pin diagram of IC 8038
Fig 2 : For Observing Carrier Signal
Fig 4 : Frequency Demodulation Procedure:
- Connect the circuit as per the given circuit diagram.
- Check the carrier generated internally by the IC 8038 at pin 2.
- Apply modulating signal of 5V (^) p-p of 10 kHz frequency from function generator.
- Observe the frequency modulated signal.
- Calculate f max and f min.
- Vary the amplitude of the message signal and observe the changes in the frequency modulated signal.
- Carson’s rule: Bandwidth = 2(Δ f + f m)
- Now apply the demodulation circuit and obtain the demodulated waveform. Observation :
EXPERIMENT NO. 3
Aim: Pulse Amplitude Modulation & Demodulation techniques and measurements. a) Generate a message signal and carrier signal. b) Use appropriate circuit for pulse amplitude modulated signal generation. c) Observe and trace the modulated signal.
Apparatus Required: Bread Board, CRO, DC Power Supply, Function Generator, BC 547, Resistors, Capacitors, Connecting probes and wires. Theory: Pulse amplitude modulation is a scheme, which alters the amplitude of regularly spaced rectangular pulses in accordance with the instantaneous values of a continuous message signal. Then amplitude of the modulated pulses represents the amplitude of the intelligence. A train of very short pulses of constant amplitude and fast repetition rate is chosen the amplitude of these pulse is made to vary in accordance with that of a slower modulating signal the result is that of multiplying the train by the modulating signal the envelope of the pulse height corresponds to the modulating wave .The Pam wave contain upper and lower side band frequencies .besides the modulating and pulse signals. The demodulated PAM waves, the signal is passed through a low pass filter having a cut – off frequencies equal to the highest frequency in the modulating signal. At the output of the filter is available the modulating signal along with the DC component. PAM has the same signal to noise ratio as AM and so it is not employed in practical circuits Circuit Diagram:
Fig 1 : Pulse Amplitude Modulation
EXPERIMENT NO. 4
Aim: Pulse Width Modulation. a) Generate a message signal and carrier signal. b) Use appropriate circuit using 555 timer for pulse width modulated signal generation. c) Observe and trace the modulated signal Apparatus Required: Bread Board, CRO, DC Power Supply, Function Generator, IC 555, Resistors, Capacitors, Connecting probes and wires. Theory: In pulse width modulation, the width of pulses of carrier pulse train is varied according to the instantaneous value of the message signal. Amplitude of the pulses is made constant. PWM signal may be generated using 555 timer in monostable mode. 555 acts as monostable multivibrator if the signal at pin number 5 is removed. Depending on the values of R and C, it will produce the pulses whenever it will get the trigger pulse. The width of the pulse can be adjusted by applying the modulating signal at the control pin.
Circuit Diagram:
Fig 1: Pin diagram of 555 IC
Fig 2 : Pulse Width Modulation
Procedure:
- Connect the circuit as per the given circuit diagram.
- Apply a pulse of carrier signal at the trigger input i.e. pin 2.
- Apply modulating signal from function generator.
- Observe the pulse width modulated signal. Observation :
-Vm
Carrier Signal
Time
Time
Time
Vm
Amplit ude
Amplit ude
Observation : PPM signal is further modification of a PWM signal. It has positive thin pulses (zero time or width) corresponding to the starting edge of a PWM pulse and negative thin pulses corresponding to the ending edge of a pulse.
EXPERIMENT NO. 6
Aim: Amplitude Shift Keying Modulation & Demodulation techniques and measurements (a) Generate a message signal and carrier signal. (b) Use appropriate circuit for Amplitude shift keying modulated signal generation. (c) Observe and trace the modulated signal. (d) Observe the fidelity after demodulation. Apparatus Required: Bread Board, CRO, DC Power Supply, Function Generator, IC 741, Transistor, Diodes, Resistors, Capacitors, Connecting probes and wires. Circuit Diagram:
Fig 1 : Amplitude Shift Keying modulation circuit
Fig 2 : Amplitude Shift Keying demodulation circuit
Theory: Amplitude shift keying (ASK) is the simplest digital modulation technique. In this, the binary information signal directly modulates the amplitude of an analog carrier. ASK is similar to standard amplitude modulation except there are only two output amplitudes possible. ASK is sometimes called digital amplitude modulation (DAM). Mathematically, amplitude shift keying is
EXPERIMENT NO. 7
Aim: Frequency Shift Keying techniques and measurements. (a) Generate a message signal and carrier signal. (b) Use appropriate circuit for Frequency shift keying modulated signal generation. (c) Observe and trace the modulated signal Apparatus Required: Bread Board, CRO, DC Power Supply, Function Generator, IC 741, Transistor, Resistors, Connecting probes and wires. Circuit Diagram:
Fig 1 : Frequency Shift Keying modulation circuit Theory: Frequency shift keying (FSK) is another relatively simple, low performance type of digital modulation scheme. FSK is a form of constant-amplitude angle modulation similar to standard frequency modulation except the modulating signal is a binary signal that varies between two discrete voltage levels rather than a continuously changing analog waveform. FSK is sometimes also called binary FSK (BFSK). The general expression for FSK is
V ( fsk )( t ) Vc cos[ 2 [ fc vm ( t ) f ] t ]
Where V fsk (t) = frequency shift keying wave V c = peak analog carrier amplitude (volts) f c = analog carrier centre frequency (hertz) Δ f = peak change (shift) in analog carrier frequency (hertz)
vm(t) = binary input(modulating ) signal (volts) The modulating signal is a normalized binary waveform where a logic 1 = +1V and a logic 0 = - 1V. Thus, for a logic 1 input, v m(t) = +1, above equation can be rewritten as
V ( fsk )( t ) Vc cos[ 2 ( fc f ) t ]
For a logic 0 input, vm(t) = - 1, above equation becomess V ( (^) fsk )( t ) Vc cos[ 2 ( fc f ) t ]
With binary FSK, the carrier centre frequency ( f c) is shifted (deviated) up and down in the frequency domain by the input signal. As the binary input signal changes from logic 0 to logic 1 and vice versa, the output frequency shifts between two frequencies: a mark, or logic 1 frequency ( f m) and a space, or logic 0 frequency ( f s). The mark and space frequencies are separated from the carrier frequency by the peak deviation (Δ f ) and from each other by 2Δ f. With FSK, frequency deviation is the defined as the difference between either the mark or space frequency and the centre frequency, or half the difference between the mark and space frequencies. Frequency deviation is expressed as
2 f |^ fm^ fs |
Where Δ f = frequency deviation (hertz) | f m- f s| = absolute difference between mark and space frequencies (hertz). Observation :
