Today, technology is centered on communication. Signals are used to transmit information between a transmitter and a receiver. Through modulation, these signals convey the information. One of the types of modulation methods used in signal transmission is pulse amplitude modulation. The simplest type of modulation is pulse amplitude modulation. The message data is encoded in the amplitude of a series of signal pulses using an analog to digital conversion technique. In this article, a general overview of pulse amplitude modulation, or PAM, is covered.

What is Pulse Amplitude Modulation?

The most fundamental type of pulse modulation is pulse amplitude modulation. Each sample in this modulation is proportional to the amplitude of the modulating signal, and the signal is sampled at regular intervals. Prior to our in-depth study, PAM explains the modulation concepts.

What is Modulation

Modulation is the process of altering a carrier signal’s properties such as amplitude, frequency, and width. It involves incorporating data into the carrier signal. A steady waveform with constant amplitude and frequency is a carrier signal.

Electromagnetic signals like radio, laser, and optical signals frequently undergo modulation. For transmission over telecommunication, audio, video, images, and text data are added to the carrier signal.

Types of Modulation

Modulation is categorized into two types depending on the type of signal.

• Continuous-wave Modulation
• Pulse Modulation

The categories for pulse and continuous-wave modulation are shown below.

Continuous-wave Modulation

A carrier signal is used in continuous wave modulation to modulate the message signal. Modulation can be accomplished by adjusting three parameters: frequency, amplitude, and phase. There are thus three different types of modulations.

1. Amplitude Modulation
2. Frequency Modulation
3. Phase Modulation

Pulse Modulation

By using pulses to transmit information along with the signal, a technique known as pulse modulation is used. There are two types: digital pulse modulation and analog pulse modulation.

Analog pulse modulation is classified as

• Pulse Amplitude Modulation (PAM)
• Pulse Width Modulation (PWM)
• Pulse Position Modulation (PPM)

Digital modulation is classified as

• Pulse Code Modulation
• Delta Modulation

Pulse Amplitude Modulation

With pulse amplitude modulation, the instantaneous amplitude of the modulation signal regulates the amplitude of each pulse. It is a modulation system where each sample is proportional to the signal’s amplitude at the time of sampling and the signal is sampled at regular intervals. By encoding the data in a series of signal pulses’ amplitudes, this method transmits the data.

There are two types of sampling techniques for transmitting a signal using PAM. They are:

1. Flat Top PAM
2. Natural PAM

Flat Top PAM

Each pulse’s amplitude is inversely proportional to the modulating signal’s amplitude at the time of the pulse. Regarding the analog signal that will be sampled, the signal’s amplitude cannot be changed. The amplitude’s tops are still flat.

Natural PAM

Each pulse’s amplitude is inversely proportional to the modulating signal’s amplitude at the time of the pulse. The remainder of the half-cycle is then spent with the pulse’s amplitude.

The unmodulated carrier signal in pulse modulation is a periodic train of signals. Consequently, the pulse train can be explained as follows.

Where ‘A’ is the unmodulated pulse amplitude

‘τ’ is pulse width

The pulse trains periodic time can be denoted as ‘Ts’

The modulating signal in PAM can alter the signal amplitudes. By multiplying the carrier signal and the modulating signal in this instance, the modulating signal, such as m(t), PAM, can be produced. The o/p is a series of pulses that allow the modulating signal to change the signal amplitudes. As the pulses mimic the contour of the modulating signal, the particular type of PAM is known as normal PAM. The pulse train behaves toward the modulator like a periodic switching signal. Once the switch is turned ON, the modulating signal samples can start to be supplied toward the output. The sampling period is the periodic time of the pulse train.

Fs = 1/Ts

The natural pulse amplitude modulation equation can be described as the following.

The modulated pulse train can be described like

E(t) = m(t) +Up(t)

= a0 m(t) + a1 m(t) cos2πnt/Ts + a2 m(t) cos4πnt/Ts+….

The modulated signal in the equation above consists of a modulating signal that is multiplied by the dc term, such as ‘a0’a series of DSBSC-based components that are produced as a result of the harmonics in the pulse signal. The division ∆ between these shouldn’t be lower than zero to prevent the low-edge of the DSBSC range from overlapping through the lower frequency range. So

W + ∆ = fs – W, with ∆ ≥ 0

fs ≥ 2W

The sampling frequency must, according to this mandatory statement, be at least twice as high as the maximum frequency of the modulating signal. The spectra can no longer be divided by filtering if the sampling state is not met, allowing for the emergence of such overlap. Aliasing is the term used to describe this effect because the highest frequency components found in the DSBSC range appear in the lower frequency portion of the spectrum. The modulation signal can first be passed through an anti-aliasing filter to cut off the signal spectrum at W value in order to avoid aliasing.

The ‘fs’ (sampling frequency) = 2W which is called the Nyquist Frequency due to its wideband nature, pulse amplitude modulation includes an extremely limited range of applications for direct signal transmission. It is used in instrumentation systems & in ADC for computer interfacing.

How PAM Signal is generated?

The following pulse amplitude modulation block diagram can be used to generate PAM. PAM, or pulse amplitude modulation, is the most fundamental form of pulse modulation. With PAM, each sample of the signal can be made in relation to the amplitude of the modulating signal at the sampling moment.

The PAM signal is generated by the sampler, which has two inputs: the sampling/carrier signal and the modulating signal, as shown in the PAM block diagram above. Since the modulating signal is where the data can be carried, the signal amplitude is therefore relative to that signal. The PAM signal is therefore this. The above waveforms, which also include the message and sampling signals, display the PAM signal’s spectrum. The carrier train of signals using the waveform is plotted within the time field.
Pulse modulation is primarily used to transmit analog data, such as speech that would otherwise be continuous.

Circuit Design of Pulse Amplitude Modulation

A square wave generator that creates the carrier pulse and a PAM modulator circuit, along with a pure sine wave modulating signal, are used to create a PAM. The Wien Bridge Oscillator circuit, on which a sine wave generator is based, is used. As a result, the output sine wave may have less distortion. A potentiometer can be used to change the oscillator’s amplitude and frequency thanks to the circuit’s design.

By adjusting the potentiometers R2 and R, the frequency and amplitude of the signal can be changed. The sine wave’s frequency is determined by

F = 1/(2π√R1R2C1C2)

An astable circuit based on op-amps is used to produce the square wave.The square wave is generated more simply by using the op-amp. It is possible to make the pulse’s ON and OFF times identical and to change the frequency without altering either.

The resistance R and capacitance C values determine how long the generated pulses last. The op-amp astable circuit’s period is given by

T = 2.2RC

Types of Pulse Amplitude Modulation

Pulse amplitude modulation is categorized into two types

1. Single Polarity PAM
2. Double Polarity PAM

When using single polarity PAM, the signal is given a suitable fixed DC bias to make sure that all of the pulses are positive. The situation known as double polarity PAM occurs when the pulses are both positive and negative. The amplitude of each pulse in some pulse amplitude modulations may be directly proportional to the instantaneous modulating amplitude once the pulse has occurred. When a pulse is delivered, the amplitude of each signal can be inversely proportional to the instant modulating amplitude in another type of PAM.

In other systems, each pulse’s intensity is primarily determined by the characteristics of the modulating signal, which excludes strength factors like instant phase or frequency. The main advantage of pulse modulation is the ability to use pulses with stable amplitude. PAM is not frequently used because it does not use stable amplitude signals. The frequency of the pulse changes the carrier once it has been used. A pulse amplitude modulation is very easy to create and demodulate. An AND-gate’s single input receives the signal conversion from a generator that can be directed toward PAM.

To open the AND gate during the necessary time intervals, signals at the sampling frequency are sent to the other i/p. The signals are then passed through a network in pulse shape, which gives them a plane top, before they are sampled at the logic gate’s output, which is equivalent in amplitude to the signal voltage every second.

Demodulation of PAM

The PAM signal is fed to the low pass filter in order to demodulate it. The demodulated signal is produced by the low pass filter, which also removes the high-frequency ripples. The inverting amplifier is then used to apply this signal, amplifying it so that the output of the demodulator is almost of equal amplitude to that of the modulating signal.

Pulse Amplitude Modulation Circuit using 555 Timer

The circuit for pulse amplitude modulation using a 555 timer is depicted below. One NPN transistor connected to the output of the 555 IC can be used to generate the pulse amplitude modulation. This IC can be connected in astable mode to generate a pulse train that can be used to sample the audio signal.

This must have a frequency that is at least twice as high as the audio signal. Normally, it is 8 KHz because the audio signal is equal to 3.4 KHz, but this circuit uses 32 KHz for better quality. The base terminal of the NPN transistor can receive the output of the pulse train. A positive clamper connected to the C1 capacitor and D1 diode is used to connect the transistor’s collector terminal to a low-frequency audio signal. With a positive clamper above 0V, the level of the audio signal can be changed. The pulse amplitude modulation signal is present at the transistor’s collector terminal. The IC555-generated signal’s amplitude varies in response to the data signal’s current amplitude.

Advantages

The advantages of pulse amplitude modulation include the following.

• Circuits for transmitters and receivers are straightforward and simple to build.
• Both modulation and demodulation are easy processes.
• The typical copper wires can carry a lot of data quickly, effectively, and efficiently.
• PAM has the ability to transmit the message while also producing other pulse modulation signals.
• It is the simplest type of modulation
• Since there is infinite FM available, frequent PAM development is possible to enable increased data throughput over accessible networks.
• It doesn’t need complicated circuitry for either the transmission or the reception. The transmitter and receiver both have very straightforward circuit designs.
• This modulation can produce other kinds of pulse modulation signals and simultaneously transmit the message.
• It is the fundamental and straightforward method for both modulation and demodulation for all types of digital modulation methods.

Disadvantages

The disadvantages of pulse amplitude modulation include the following.

• Noise will be great.
• Large bandwidth is required for PAM transmission.
• For transmitting PAM signal, BW must be large
• Because the pulse amplitude signal varies, more power is needed for transmission.
• In comparison to other types, noise immunity for this modulation is low. As a result, it is almost the same as AM.
• Because of these fluctuations in the signal’s frequency caused by the message or modulating signal, intrusions will be present.
• When the pulse amplitude signal changes, a lot of power is needed for transmission and even more power is needed to obtain PAM.

Applications of PAM

• It is employed by numerous micro-controllers to produce control signals.
• It is used in Ethernet communication.
• It is used as an electronic driver for LED lighting.
• It is used in Photo-biology.
• PAM can be used to generate the control signals in a variety of microcontrollers.
• So The majority of applications for this modulation technique involve digital data transmission and are PCM- and PPM-changed. QAM (quadrature amplitude modulation) is used by virtually all phone modems that are faster than 300 bit/s.
• PAM is used in Ethernet networks, which link two systems together and transfer data between them. So Ethernet communications make use of PAM.