Pulse Amplitude Modulation
Viraj Deshpande
Software Engineer at State Street | ETC Graduate | C++, Python, Java, R, JavaScript | Software Development and Creative Problem Solving Enthusiast
I. Pulse Amplitude Modulation
Pulse amplitude modulation is a technique in which the amplitude of each pulse is controlled by the instantaneous amplitude of the modulation signal. It is a modulation system in which the signal is sampled at regular intervals and each sample is made proportional to the amplitude of the signal at the instant of sampling. This technique transmits the data by encoding in the amplitude of a series of signal pulses.
There are two types of sampling techniques for transmitting a signal using PAM. They are:
● Flat Top PAM
● Natural PAM
Flat Top PAM: The amplitude of each pulse is directly proportional to modulating signal amplitude at the time of pulse occurrence. The amplitude of the signal cannot be changed with respect to the analog signal to be sampled. The tops of the amplitude remain flat.
Natural PAM: The amplitude of each pulse is directly proportional to modulating signal amplitude at the time of pulse occurrence. Then follows the amplitude of the pulse for the rest of the half-cycle.
Natural PAM
II. Components
1. IC 555 timer(1)
2. NPN transistor(1)
3. battery 5V(1)
4. capacitors(2)
5. resistors(5)
6. function generator(1)
7. oscilloscope(1)
Ⅲ Circuit Diagram
Ⅳ Image of Output:
Ⅴ Working of Circuit:
The circuit can be broadly divided into 3 parts:
1. 5V AC Signal - AC signal of 5V and 1kHz frequency is used to generate the modulating sinusoidal signal.
2. Pulse Train generator - IC 555 is used for generating pulse train which will act as carrier signal in this circuit. The output of the oscillator would be passed to the base of the transistor.
3. Pulse amplitude modulator - Finally, the transistor would take inputs from AC signal and pulse oscillator and generate the pulse modulated wave at its output which would be displayed on the oscillator.
● Working of IC 555 - When the output at pin 3 is HIGH, the capacitor charges up through the resistor. When the voltage across the capacitor reaches 2/3Vcc, pin 6 causes the output at pin 3 to change state and goes LOW. The capacitor now discharges back through the same resistor until pin 2 reaches 1/3Vcc causing the output to change state once again. The capacitor continually charges and discharges between 2/3Vcc and 1/3Vcc back and forth through the same resistor creating a HIGH and LOW state at the output, pin 3.
● Working of transistor - Whenever the timer gives high output, it is passed to the base of the transistor that basically connects the collector emitter terminals. Due to this, the terminal across R4 is short and no current passes through it thus generating no output.
On the other hand, when there is no supply from timer, the transistor which is acting as switch would open thus all the current would pass through R4 thus giving high output. This process is continued causing the natural sampling of the carrier pulse with respect to modulating signal.
Ⅵ Observations
1. Threshold pin voltage = 3.33V
2. Trigger pin voltage = 1.66V
3. DC power supply = 5V
4. AC power supply = 5V
5. Frequency of modulating wave = 1kHz
Ⅶ Calculations
● Calculation of control voltage and negative terminal of trigger voltage:
As there is voltage divider within the IC 555,
Let Vc = Control voltage
V-tr = voltage across negative terminal of comparator having its positive terminal connected at trigger pin.
Vcc = 5V
Vcc - IR = Vc
(Vcc - Vc)/I = R --1
Similarly,
(Vc - V-tr)/I = R --2
V-tr/I = R --3
Substituting 3 in 2 and 1 we get;
● V-tr = Vcc/3 = 5/3 = 1.66V
● Vth = 2Vcc/3 = 3.33V
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Ⅷ Applications of PAM
● It is used in Ethernet communication.
● It is used in many micro-controllers for generating control signals.
● It is used in Photo-biology.
● It is used as an electronic driver for LED lighting.
● PAM is used in the Ethernet network which is used to connect two systems & used to transfer data among these systems. So PAM is used in Ethernet communications.
● The control signals can be generated in various microcontrollers by using PAM
● This modulation technique is mostly used in digital data transmission & applications changed by PCM & PPM. Mostly all phone modems which are faster above 300 bit/s utilize QAM (quadrature amplitude modulation).
Ⅸ Component Information
● 555 Timer Block Diagram
• Pin 1. – Ground, The ground pin connects the 555 timer to the negative (0v) supply rail.
• Pin 2. – Trigger, The negative input to comparator No 1. A negative pulse on this pin “sets” the internal Flip-flop when the voltage drops below 1/3Vcc causing the output to switch from a “LOW” to a “HIGH” state.
• Pin 3. – Output, The output pin can drive any TTL circuit and is capable of sourcing or sinking up to 200mA of current at an output voltage equal to approximately Vcc – 1.5V so small speakers, LEDs or motors can be connected directly to the output.
• Pin 4. – Reset, This pin is used to “reset” the internal Flip-flop controlling the state of the output, pin 3. This is an active-low input and is generally connected to a logic “1” level when not used to prevent any unwanted resetting of the output.
• Pin 5. – Control Voltage, This pin controls the timing of the 555 by overriding the 2/3Vcc level of the voltage divider network. By applying a voltage to this pin the width of the output signal can be varied independently of the RC timing network. When not used it is connected to ground via a 10nF capacitor to eliminate any noise.
• Pin 6. – Threshold, The positive input to comparator No 2. This pin is used to reset the Flip-flop when the voltage applied to it exceeds 2/3Vcc causing the output to switch from “HIGH” to “LOW” state. This pin connects directly to the RC timing circuit.
• Pin 7. – Discharge, The discharge pin is connected directly to the Collector of an internal NPN transistor which is used to “discharge” the timing capacitor to ground when the output at pin 3 switches “LOW”.
• Pin 8. – Supply +Vcc, This is the power supply pin and for general purpose TTL 555 timers is between 4.5V and 15V.
● Transistor as a switch
Cut-off Region:
Here the operating conditions of the transistor are zero input base current ( IB ), zero output collector current ( IC ) and maximum collector voltage ( VCE ) which results in a large depletion layer and no current flowing through the device. Therefore the transistor is switched “Fully-OFF”.
Cut-off Characteristics
transistor switch in cut-off
• The input and Base are grounded ( 0v )
• Base-Emitter voltage VBE < 0.7v
• Base-Emitter junction is reverse biased
• Base-Collector junction is reverse biased
• Transistor is “fully-OFF” ( Cut-off region )
• No Collector current flows ( IC = 0 )
• VOUT = VCE = VCC = ”1″
• Transistor operates as an “open switch”
Then we can define the “cut-off region” or “OFF mode” when using a bipolar transistor as a switch as being, both junctions reverse biased, VB < 0.7v and IC = 0. For a PNP transistor, the Emitter potential must be negative with respect to the Base.
Saturation Region:
Here the transistor will be biased so that the maximum amount of base current is applied, resulting in maximum collector current resulting in the minimum collector emitter voltage drop which results in the depletion layer being as small as possible and maximum current flowing through the transistor. Therefore the transistor is switched “Fully-ON”.
Saturation Characteristics
transistor switch in saturation
• The input and Base are connected to VCC
• Base-Emitter voltage VBE > 0.7v
• Base-Emitter junction is forward biased
• Base-Collector junction is forward biased
• Transistor is “fully-ON” ( saturation region )
• Max Collector current flows ( IC = Vcc/RL )
• VCE = 0 ( ideal saturation )
• VOUT = VCE = ”0″
• Transistor operates as a “closed switch”
Ⅹ Conclusion
The circuit successfully modulates the carrier pulse train generated using IC 555 using modulating signal of 5V and these two signals are controlled using an npn transistor that eventually generates the output that is pulse amplitude modulated signal. Thus, the circuit made would ensure that the carrier signal would get naturally sampled and its amplitude would vary proportional to the modulating signal. We also calculated various values to generate the suitable output. In future, this modulated signal can be further demodulated easily .
Here, the working of transistor as a switch and the IC 555 as pulse train generator was successfully studied and implemented. Moreover, the charging and discharging properties of the capacitor were also used for continuous generation of pulses.
Ⅺ References