CE LAB-04 - CE LAB MANUAL OF KIIT PDF

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CE LAB MANUAL OF KIIT...

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Experiment no. -04 Time division multiplexing Technique and Pulse code Modulation and Demodulation Aim 1. Demonstrations of time division multiplexing technique and the PCM system.The ST 2153TDMPCM transmitter trainer kit used for TDM and Pulse code modulation. The ST 2154 kit is used for PCM receiver. 2. Simulation of TDM technique and PCM system using GNU octave software. Demonstration 1 : A 2kHz sinewave is applied to channel 1 and the sample and hold circuit output is observed.Another sinewave of 4kHz sinewave is applied to channel 2 and the sample and hold circuit output is observed. Now the two input sinusoidal signals are applied simultaneously to the TDM transmitter and the multiplexed signal is observed. Equipments Requried: 1. TDM-PCM Modulation trainer kit ST2153 and PCM receiver trainer kit ST2154 2. CRO 3. Patching Cords 4. CRO Probes Theory: TDM Technique Time Division Multiplexing (TDM) is a technique of transmitting different source signals on the same channel at different time slots. That is several information can be transmitted over a single channel by sending samples from different information sources at different moments. TDM is widely used in digital communication systems to increase the efficiency of the transmitting medium. TDM can be achieved by electronically switching the samples such that they interleave sequentially at correct instant in time without mutual interference. A major problem in any TDM system is the synchronization of the transmitter and receiver timing circuits. The transmitter and receiver must

switch at the same time and frequency.TDM based on analog modulation, the time slots are separated by guard slots to prevent crosstalk between the channels. In PAM, PPM the pulse is present for a short duration and for most of the time between the two pulses no signal is present. This free space between the pulses can be occupied by pulses from other channels. Thus, time division multiplexing makes maximum utilization of the transmission channel. Each channel to be transmitted is passed through the low pass filter. The outputs of the low pass filters are connected to the rotating sampling switch (or) commutator. It takes the sample from each channel per revolution and rotates at the rate of f s. Thusthe sampling frequency becomes fs the single signal composed due to multiplexing of input channels. These channels signals are then passed through low pass reconstruction filters. If the highest signal frequency present in all the channels is fm, then by sampling theorem, the sampling frequency fs must be such that fs≥2fm. Therefore, the time space between successive samples from any one input will be Ts=1/fs, and Ts≤ 1/2fm. Block diagram description: TDM system consists of multiple LPF depending on the number of data inputs. These low pass filters are basically anti-aliasing filters that eliminate the aliasing of the data input signal. The output of the LPF is then fed to the commutator. As per the rotation of the commutator the samples of the data inputs are collected by it. Here, fs is the rate of rotation of the commutator, thus denotes the sampling frequency of the system. Suppose we have n data inputs, then one after the other, according to the rotation, these data inputs after getting multiplexed transmitted over the common channel. Now, at the receiver end, a de-commutator is placed that is synchronized with the commutator at the transmitting end. This de-commutator separates the time division multiplexed signal at the receiving end.The commutator and de-commutator must have same rotational speed so as to have accurate demultiplexing of the signal at the receiving end. According to the rotation performed by the de-commutator, the samples are collected by the LPF and the original data input is recovered at the receiver. Block diagram of TDM system

Connection diagram

Procedure for TDM technique 1. Mode Switch in 320 KHz (FAST mode) position 2. ~ 2 KHz and ~4 KHz control levels set to give 10Vpp. 3. Error check code generator switch A & B in A=0 & B=0 position (OFF Mode) 4. First, connect only the ~ 2 KHz output to CH I. Turn ON the power. 5. Connect CH1(Y) of the oscilloscope to ~ 2 KHz block & CH2(X) of the oscilloscope to input of sample and hold block of the Scientech 2153. Observe both the wave form. 6. Then connect only the ~4 KHz output to CH II. Turn ON the power.

7. Connect CH1(Y) of the oscilloscope to ~ 4 KHz block & CH2(X) of the oscilloscope to input of sample and hold block of the Scientech 2153. Observe both the wave form. 8. Then connect the ~ 2 KHz output to CH I and ~ 4 KHz output to CH II. Turn ON the power. 9. Connect CH1(Y) of the oscilloscope to ~ 4 KHz block & CH2(X) of the oscilloscope to input of sample and hold block of the Scientech 2153. Observe the multiplexed wave form. Expected Waveforms from CRO

2 KHz and 4 KHz sine wave input

Sample and hold output 2 KHz

Sample and hold output 4 KHz

Multiplexed output 2 KHz and 4 KHz

2 KHz and 4 KHz sine wave output

PART II PCM SYSTEM Demonstration II Analog Signal to digital signal conversion will be observed in the PCM transmitter. For different input signals wavefroms at the sampler output are seen.The PCM output is observed at the LEDs and corresponding PCM waveform is traced. Pulse code modulation is known as digital pulse modulation technique. It is the process in which the message signal is sampled and the amplitude of each sample is rounded off to the nearest one of the finite set of allowable values. It consists of three main parts transmitter, transmitter path and receiver. The essential operation in the transmitter of a PCM system are sampling, quantizing and encoding. The band pass filter limits the frequency of the analog input signal. The sample and hold circuit periodically samples the analog input signal and converts those to a multi-level PAM signal. The ADC converts PAM samples to parallel PCM codes which are converted to serial binary data in parallel to serial converter and then outputted on the transmission line as serial digital pulse. The transmission line repeaters are placed at prescribed distance to regenerate the digital pulse. In the receiver serial to parallel converter converts serial pulse received from the transmission line to parallel PCM codes. The DAC converts the parallel PCM codes to multi- level PAM signals. The hold circuit is basically a Low Pass Filter that converts the PAM signal back to its original analog form. PCM Transmitter

Channel:

PCM Receiver

PCM Transmitter 

LPF: Here, the message signal which is in the continuous time form, is allowed to pass through a low pass filter (LPF). This LPF whose cutoff frequency is fm eliminates the high-frequency components of the signal and passes only the frequency components that lie below fm.



Sampler: The output of the LPF is then fed to a sampler where the analog input signal is sampled at regular intervals. The sampling of the signal is done at the rate of fs. This sampling frequency is so selected that it must follow the sampling theorem that is expressed as:

fs ≥ 2fm The output of the sampler is a signal that is discrete time continuous amplitude signal denoted as nTs which is nothing but a PAM signal. 

Quantizer: A quantizer is a unit that rounds off each sample to the nearest discrete level. The sampler provides a continuous range signal and hence still an analog one. The quantizer performs the approximation of each sample thus assigning it a particular discrete level.



Encoder: An encoder performs the conversion of the quantized signal into binary codes. This unit generates a digitally encoded signal which is a sequence of binary pulses that acts as the modulated output.

Channel: The channel introduces distortion in the signal during transmission. This distortion is eliminated by the regenerator in order to provide a distortionless PCM signal. Resultantly, enhancing the transmission ability of the system. 

Regenerator: A regenerative repeater is placed at the receiving end also so as to have an exact PCM transmitted signal. Here, also the regenerator works in a similar manner as that when employed in the transmission path. It eliminates the channel induced noise and reshapes the pulse.

PCM Receiver 

DAC and Sampler: Digital to analog converter performs the conversion of digital signal again into its analog form by making use of the sampler. As the actual message signal was analog thus at the receiver end there is a necessity to again convert it into its original form.



LPF: The sampler generates analog signal but that is not the original message signal. Thus, the output of the sampler is fed to the LPF having cutoff frequency fm. This is sometimes termed as the reconstruction filter that produces the original message signal.

Connection Diagram

Procedure for generating PCM Signal 1. Mode Switch in 320 KHz (FAST mode) position 2. DC signal (I) & DC signal (II) Controls in function generator block fully clockwise. 3. Error check code generator switch A & B in A=0 & B=0 position (OFF Mode) 4. DC signal (I) output to CH I input 5. DC signal (II) output to CH II input 6. Turn ON the power. Turn the DC signal (II) control fully anticlockwise and by varying DC signal (I) control. Check that the digital code for the set voltage value is identical to that of the DC signal (I) setting. 7. Connect CH1(Y) of the oscilloscope to PCM OUTPUT block of the Scientech 2153. Observe the signal. 8. Observe the output on the A/D converter block LED’s (D1 to D7). The LED’s represent the state of the binary PCM word allocated to the PAM sample being processed. An illuminated LED represent a ‘1’ state, while non illuminated LED indicates a ‘0’ state. D7 is the MSB & D1 is the LSB. The LED output looks as follows.

Expected Waveforms from CRO

Octave simulation code for TDM and PCM clear all pkg load signal % generation of TDM-PAM fs = 1000; f0=50; w = 0.005;t=0:1/fs:1; x1=pulstran (0:1/fs:1, 0:1/f0:1, "rectpuls", w); x2=0.5*pulstran (0:1/fs:1, 0.01:1/f0:1, "rectpuls", w); subplot(4,1,1);plot(t,x1,t,x2,'r');title('pulse signals'); y1=20*sin(2*pi*2*t); y2=20*sin(2*pi*4*t);subplot(4,1,2);plot(t,y1,t,y2,'r');title('message signals'); Pam1=x1.*y1; Pam2=x2.*y2; y3=Pam1+Pam2; subplot(4,1,3); plot(t,y3,'r');title('TDM-PAM Signal');

subplot(4,1,4); plot(t,Pam1,t,Pam2,'r');title('PAM signals'); d1=y3.*x1; [b,a]=butter(5,0.02); s1=filter(b,a,d1); figure; subplot(2,1,1);plot(t,s1); title('demodulation1'); d2=y3.*x2; [b,a]=butter(5,0.02); s2=filter(b,a,d2); subplot(2,1,2);plot(t,s2); title('demodulation2') % Generation of PCM % PCM signal generation f=2;a=3; fs=20*f; t= 0:1/fs:2; x=a*sin(2*pi*f*t); subplot(4,1,1),plot(t,x);title ('Analog signal'); subplot(4,1,2), stem(t,x);

% level shifting x1=x+a;l=length(x1); % quantization q_op=round(x1); % encoder enco= dec2bin(q_op); k=1; code=zeros(1,l); for i=1:l for j=1:3 code(k)= enco(i,j)-48; k=k+1; end end subplot(4,1,3),stairs(code);axis([0 243 0 2]); title('encoded signal'); % receiver deco= bin2dec(enco); % shifting amplitude level xr=deco-a; %plotting subplot(4,1,4),plot(t,x,'r',t,xr,'k+-'),title('decoded signal')

VIVA QUESTIONS: 1. What is PCM? 2. How bits are needed to encode N different levels? 3. Define step size? 4. How to calculate Step size in PCM? 5. Define Quantization error. 6. What is the max value of Quantization error? 7. What are the applications of PCM? 8. What are the disadvantages of Pulse code modulation? 9. What is the sampling rate for PCM if the frequency ranges from 1000Hz to 4000Hz? 10. If the interval between two samples in a digital signal is 125 micro seconds. What is the sampling rate?...