Indian Institute of Technology Kanpur
EE 381 Electrical Engineering Lab-II
2003 – 04; Semester II
Experiment #3
Baseband Digital Data Transmission
Goal:
Study of Inter Symbol Interference due to band limited channels in baseband digital data transmission.
Introduction:
The transmission of digital data serially over a communication link requires a number of considerations. First, the digital
data is usually in the form of a line coded pulse train such as NRZ (non-return-to-zero), AMI (alternate mark inversion)
etc. Second, the communication channel is analog, has 1imited bandwidth and is equivalent to some low pass filter. Third,
the goal of data transmission is to transmit data at as high a rate as possible subject to distortion and noise limitations.
Fourth, there should be suitable schemes in the design of the transmitted signa1 to ensure ease of equalization, clock
recovery and samp1ing synchronization so that data can be recovered re1iab1y. The first three considerations are met in the
transmitter design.
To understand the baseband pulse transmission, consider the response of a low pass filter (representing the communication
medium) to a stream of impulses of different heights as shown in Fig. 1. The digital data is a finite alphabet (binary, 4
levels or 2 bits per sample, 8 levels or 3 bits per sample, etc.) and can be represented by a set of impulses with discrete and
distinct energies for each character of the alphabet. The impulse train  ak  (t  kT ) represents the digital data.
Band Limited Channel
0
T
3T 4T 5T
Fig.1 Transmission of impulse train through a band limited channel
The output of the LPF (or the channel) is given by the following equation:
y (t )   ak g (t  kT   )
In the above equation,  is the constant propagation delay and g (t ) is the impulse response of the LPF. Since g (t ) is
spread out in time due to finite bandwidth, the response of each impulse in the input train spills over to those adjoining it
(particularly at high baud rates), thereby corrupting the estimates of ak we can get from the output waveform y (t ) at the
samp1ing instants t  kT   . This problem is referred to as Inter Symbol Interference (or ISI) and can be serious if T is
small. Nyquist was the first to study the ISI problem seriously and state some properties of equiva1ent low pas response of
the channe1, which enab1es ISI free transmission of pulse amplitude modulation signals. He showed that a class of low
pass transfer-functions called "raised cosine filters" having the following frequency response
1 
0  f  2T
T


1 
1 
H ( f )  (T 2) 1  sin T ( f  21T 
 f  2T
2T



1 

0
f  2T
These functions have both smooth roll-off and impu1se response with zero crossings at multiples of sampling instants
1
except at t  0 as shown in Fig. 2. Baseband channels with the above response have approximate -3dB frequency of 2T


Hz and are ISI free for 0    1 up to baud rates of 1/T samples per second.
Fig. 2 Pulses having raised cosine spectrum
Practical implementations try to approximate the above filter characteristics using realizable filter sections. Since impulse
train excitations are not practical, the input is usually a PAM version of rectangu1ar pulses and the"impu1se response" g(t)
of LPF corresponds to the response to the rectangular pulse input (Fig. 3).
Third Order Low
Pass Filter
Fig. 3 Impulse response of a low pass filter
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Indian Institute of Technology Kanpur
EE 381 Electrical Engineering Lab-II
2003 – 04; Semester II
In this experiment the basic idea that PAM baud rate cannot exceed twice the bandwidth without ISI is being illustrated for
binary and 4 level digital PAM transmission system.
Pre-lab exercise:

Design a third order low pass filter with cut off frequency of approximately 1kHz and pass band voltage gain of
approximately 3dB. (Attenuation slope is 60dB/decade) using the following circuit. Use ua741 opamp.
R4
R5
R1
R2
R3
OUTPUT
+
+
INPUT
C2
C3
C1


Using PSPICE simulations:
o Plot the frequency response of the filter.
o Plot the impulse response of the filter by applying a rectangular pulse train of amplitude 0 to 3V,
repetition rate of 400Hz and a period of 0.5ms.
Design a 4-bit Pseudo Random Bit Sequence generator using a shift register (7495) and XOR gate (7486/CD7040)
and a two-bit Digital to Analog converter using R-2R ladder network (see the following figure).
2R
D1 Q1
D2 Q2
D3 Q3
D4 Q4
4 Level
PAM
R
2R
+
Ck
2R
PRBS
Using PSPICE simulations:
o Plot the output of the PRBS generator and write down the data sequence.
o Plot the output of the D/A converter, which gives 4 level PAM.
The Experiment:

Setup the 3rd order low pass filter circuit and measure and plot its frequency and impulse responses.

Complete the wiring of the PRBS sequence generator and observe and sketch its output by applying a clock of
1kHz. Write down the data sequence.

Feed the PRBS sequence to the LPF. Connect the LPF output to CRO Ch1 and clock to Ch. 2 of the CRO. Trigger
the CRO sweep with respect to Ch. 2. Display both the CRO channels.

For the clock set at 1kHz adjust the CRO sweep rate to get a full eye pattern on the CRO as shown in the following
figure for the binary case. Sketch the eye pattern.

Increase the clock frequency gradually and stop just when the eye opening starts to close due to ISI. Adjust the
sweep rate to show at least one full clock period. Measure the corresponding clock frequency and compare it to
the Nyquist rate. Measure the peak distortion (see the following figure) and distortion of zero crossings if any.
Measure  , the time difference between the clock transition and the instant where the eye has maximum opening.

Sensitivity to timing error


Peak Distortion
Optimum
sampling
time
Distortion of
zero crossing
Noise Margin
Increase the clock frequency further and observe that the eye is so much closed and it is difficult to get a proper
sampling instant in each clock period for reliable sampling of the data.
Now connect the 2-bit digital to analog converter output to the input of the filter and repeat the previous three steps.
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