PAMTDMGenRec -- Overview
PAM Modulation - Time Division Multiplexing
OBJECTIVES:
At the end of performing this experiment, learners would be able to:
• Describe the concept of PAM - TDM
• Obtain the PAM -TDM from given input
• Understand the working of LF398 IC (sample-and-hold circuit)
EQUIPMENT:
• IC LF398
• Signal generator
• Resistors – 47 kΩ, 1.5 kΩ
• Capacitor – 0.1 µF
• +/- 15V DC Power Supply
• Digital Storage Oscilloscope & probes
• Connecting wires & Bread Board
DESIGN:
(Recovery stage) for Low Pass Filter
f = 500Hz
R=?
let C= 0.1µF
f=
R= 3.18kΩ
THEORY:
• LF398 is a monolithic sample-and-hold circuit utilizing BI-FET
technology for accurate fast acquisition of input signal.
• A sample and hold circuit is an analog device that samples
(captures) the voltage of a continuously varying analog signal and
holds (locks) its value at a constant level for a specified minimum
period of time (hold time). They are typically used in analog-to-
digital converters to eliminate variations in input signal that can
corrupt the conversion process.
•PAM is one basic type of analog pulse modulation . In PAM, the
amplitude of regurlarly spaced rectanglar pulses vary with the
instantaneous sample values of an analog signal in a one to one
fashion.
• TDM is a technique used for transmitting several analog message
signals over a communication channel by dividing the time frame
into slots, one slot for each message signal.
Reference reading:
B Kanmani, “Some applications of the combination: LM-741 and LF -
398”, WASET CESSE 2009: International conference on Computer,
Electrical and Systems science and Engineering, Rome, 28th-
30thApril, Italy, 2009.Volume 52, April 2009, ISSN: 2070-3724,
pages 335-340.
Acknowledgement
Mr.Shreenivas B for converting laboratory experiment to Tektronix
courseware format
PAMTDMGenRec -- Procedures
Step 1
Circuit setup:
Build the following circuit with given component values
Step 2
• Use a signal generator to generate analog inputs message - I
(sine wave of 500 Hz for example), message-II (triangular wave of 1
KHz for example) and sampling (square wave signal). The analog
input will be set to 1 kHz Sine wave(or triangular wave) and
sampling signal will be 15-20 kHz Square-wave of 50% duty cycle.
• Turn on the supply of the circuit and enable signal generator that
is feeding signal to the circuit.
Step 3
• Connect the DSO probe – CH1 at analog input (pin # 3 of LF398
IC), CH2 at analog input (pin # 6 of LF398 IC), CH3 at sampling
signal input (pin # 8 of LF398 IC) and CH4 at output (pin # 5 of the
LF398 IC).
• Perform Autoset on DSO and capture the output signal.
Step 4
• Configure PEAK-to-PEAK measurement on the input and output
signal.
• Observe – input and output on DSO and record the signal
Step 5
Following figure shows the two message signals to be time division
multiplexed
Step 6
Following figure shows the Carrier signals and the corresponding
PAM-TDM waveform
Step 7
Circuit setup to recover message - I :
• Build the following circuit with given component values
• The output of PAM-TDM circuit is given as an input to the recovery
circuit as shown in following figure, sampling signal input (pin # 8 of
LF398 IC)The same square wave used for generation of the PAM-
TDM is cascaded with recovery circuit
Step 8
Step 9
• Cut-off frequency of LPF: Highest frequency component in
message signal - I
• Output of IC LF 398 is given to LPF as an input, to recover the
message - I
• Record the measurement and Observe – input, output on DSO
Step 10
Circuit setup to recover message - II :
• Build the following circuit with given component values
• The output of PAM-TDM circuit is given as an input to the recovery
circuit as shown in following figure,sampling signal input (pin # 8 of
LF398 IC)is the same square wave used for generation of the PAM-
TDM is cascaded with recovery circuit
Step 11
Step 12
•Cut-off frequency of LPF: Highest frequency component in
message signal - II
•Output of IC LF 398 is given to LPF as an input, to recover the
message - II
•Record the measurement and Observe – input, output on DSO
Step 13
Observation:
1) Simultaneously two signals are transmitted on the same channel
2) Multiplexing of two signals can be verified.
Open-ended Question / Can you answer this?
What will be the effect on output waveform if:
1) Order of the LPF is increased
