Pulse amplitude modulation (PAM) is a modulation technique used in digital signal processing. PAM involves the use of two separate signals: a carrier signal and a modulating signal. The carrier signal is the steady, unvarying element of the pair that remains constant at all times. The modulating signal acts as the dynamic element of the pair, changing its amplitude with each new input.
Pulse amplitude modulation is a very basic type of pulse modulation. This is because here, each signal is sampled at regular intervals. Also, the amplitude of each sample is proportional to the modulating signal.
For example, imagine you want to send a message to another person by way of Morse code. You would begin by speaking clearly and loudly so that your friend could hear what you were saying clearly.
In this case, your voice represents the “carrier” signal; it doesn’t change over time but remains constant throughout your speech—unlike the rapid up-and-down fluctuations of your speech that make up the “modulating” part of this example.
- 1 Understanding the modulation
- 2 How is the Pulse Amplitude Modulation signal generated?
- 3 Types of Pulse Amplitude Modulation
- 4 The circuit design of Pulse Amplitude Modulation
- 5 Demodulation of PAM
- 6 Applications of PAM
- 7 Advantages of PAM
- 8 Disadvantages of PAM
- 9 Conclusion
Understanding the modulation
Before we get deeper into pulse amplitude modulation, let’s first understand the basics of modulation. This is a process of changing the characteristics of a carrier signal. It can be amplitude frequency and height among many others.
We can define a carrier signal as a steady waveform that has a well-defined frequency and amplitude. When the modulation is introduced to this signal, you will witness a change in the behavior and features of the waveform.
Modulation is further divided into two main classes. These are continuous-wave modulation and pulse modulation. A continuous wave is one where the carrier signal remains constant. It is also known as AM modulation. On the other hand, pulse-amplitude modulation is a type of modulation where the carrier signal changes over time.
How is the Pulse Amplitude Modulation signal generated?
Pulse amplitude modulation is the process by which a waveform is produced by changing the amplitude of a carrier wave. This process is referred to as Pulse-Amplitude Modulation (PAM).
The PAM signal can be generated using any digital signal processing technique. The most common digital signal processing technique used in PAM signals is Discrete Cosine Transform (DCT). In DCT, the input data stream is divided into frames, each containing 8 or 16 samples.
The DCT then calculates the discrete cosine transform of each frame and generates two new frames—one with zero mean and one with maximum mean.
The two new frames are then combined together to form a single new frame. This process repeats itself multiple times until the entire input data stream has been transformed into a single pulse amplitude modulated signal with constant amplitude.
Types of Pulse Amplitude Modulation
There are two major classes of PAM. These are;
-Flat top PAM
What is the difference between the two?
Flat Top PAM
In this type of PAM, the amplitude of the carrier wave is kept constant throughout the duration of the pulse. This is made possible by using a constant amplitude carrier wave. It also implies that the amplitude of the signal remains the same or is not affected by the analog signal. The resultant effect is the top of the amplitude; will remain flat hence the name.
In this type of PAM, the amplitude of the carrier wave will vary between zero and maximal amplitude. This is made possible by using a variable carrier waveform.
Flat Top PAM has more advantages than Natural PAM as it can be used over a wider range of frequencies and in higher data rates. Flat top modulations have been used to transmit data since the 1960s, mainly for television transmission systems, where they are known as FM or frequency modulation (FM).
They are also useful for wireless microphones, such as those found in video surveillance systems that record audio signals onto an audio track on audiotape or other media.
While flat top modulations offer excellent performance at low bit rates (e.g., for telecommunication applications), they require complex signal processing circuitry to generate their own carrier waves at high data rates (e.g., for cable television applications).
The complexity required to generate flat-top modulated signals makes them impractical for many communication applications that require low-cost transmitter circuitry and thus have restricted bandwidths (e.g., wireless microphones).
In terms of the operation modes, pulse amplitude modulation is further classified into two categories. These are Single Polarity PAM and Double Polarity PAM.
Single polarity PAM is also known as unipolar or unidirectional modulation. In this case, an ideal DC bias is introduced to the signal to ensure that all the pulses remain positive despite the behavior of the waveform.
On the other hand, double polarity PAM is where the pulses have both positive and negative attributes.
So, which one is better between the two?
Single polarity PAM is more efficient in terms of bandwidth utilization. The reason is, the bandwidth of the modulation signal is less than that of the carrier wave.
Double polarity PAM is useful in cases where the user needs to modulate a carrier wave at a higher frequency than that of the data signal. This can be achieved by using a lower value for f in comparison to the data rate.
A drawback, however, is that it takes more time for the pulse-shaping circuitry to generate pulses at a high speed and thus requires more time to modulate a higher frequency carrier wave on top of a lower data rate signal.
The circuit design of Pulse Amplitude Modulation
A Pulse Amplitude Modulation is a result of a combination of both the pure sine wave modulating signal and a square wave generator which is designed to generate the carrier pulse for the modulation purpose.
The design of this sine wave generator is fully based on the Wien Bridge Oscillator circuit. This circuit is a modified version of the Wien Bridge Oscillator which is designed to generate the square wave signal from the sine wave signal.
The circuit also uses a negative feedback loop to produce an amplified sine wave signal. This amplified sine wave signal will be used for modulation purposes.
The amplitude of this sine wave can be controlled by adjusting the duty cycle of the square pulse generator. A potentiometer is used to adjust the duty cycle of the square pulse generator.
The duty cycle of this pulse train will determine how much amplitude will be added to or subtracted from the sine wave input signal. This amplitude control can be adjusted using a potentiometer. When this potentiometer is adjusted in such a way, it will produce two different outputs; one for positive values and another for negative values.
In positive values, this potentiometer has maximum resistance which means that there is zero change in amplitude when you adjust it all the way down (negative value).
As you move this potentiometer towards minimum resistance, it produces maximum amplitude which means that there will be a maximum change in amplitude when you adjust it all the way up (positive value).
Demodulation of PAM
Demodulation refers to the process of removing the modulation from the input signal. In this project, demodulation is achieved by subtracting the square pulse from the sine wave.
Demodulation of PAM is performed with two separate differential amplifiers. The first differential amplifier is used to remove the modulation and its output is used to feed the second differential amplifier which will be used to extract the original sine wave signal.
The first differential amplifier can be constructed using a single op-amp and a pair of resistors, R1 & R2.
Applications of PAM
Pulse amplitude modulation has a wide range of applications in the modern world. Let’s look at some of them:
Versions of ethernet connections such as 10BASE-T and 100BASE-TX use PAM to achieve high data transfer rates. The modulation is achieved by modulating the signal’s amplitude.
Photo biology refers to the field of biology which uses light to control various biological processes. PAM is used in this field to create ultrashort pulses of light. This has led to the development of various technologies in the field such as ultrafast laser, optical tweezers, and optical quantum computing.
Instrumentation amplifiers are used to amplify signals from low-level inputs such as sensors or switches. This is achieved by using a switch (called a pulse generator) that produces a square wave signal with a fixed amplitude and frequency. The exact duration of this square wave signal is controlled by the time constant of an RC circuit which makes it possible to generate pulses with different durations.
Pulse amplitude modulation is used in telecommunication equipment such as cable modems and DSL modems where data transmission rates are limited by the maximum length of the data carrier waveform that can be sent over the communication line without losing significant amounts of information.
Most digital television use PAM for the broadcasting and transmission of data. Here, the data is encoded using a digital signal. The PAM signal is then modulated onto the carrier waveform to produce the television picture.
Advantages of PAM
Here are the top advantages of pulse amplitude modulation:
-Provide an easier option for both modulation and demodulation: This is because the digital signal can be demodulated using a simple passive circuit, which can be easily integrated into the existing analog receiver.
-PAM provides an easy way to mix two or more signals: The PAM signal is used to mix two or more signals. The mixed signal can then be demodulated and processed by an analog receiver.
-Quick transmission of data: The PAM signal can be modulated onto a carrier waveform at a high data rate. This allows the transmitter to send large amounts of data very quickly.
-Pulse width modulation reduces the distortion of the transmitted signal: Pulse width modulation is used in conjunction with amplitude modulation to reduce the distortion of the transmitted signal.
-Does not require a complex circuit: As we have just seen, the PAM signal is very easy to generate and transmit. This means that it does not require a complex circuit to implement.
-Can be used for both digital and analog applications: The PAM signal can be used for both digital and analog applications. This is because the PAM signal can be transmitted using either an analog or a digital carrier waveform.
-PAM has excellent immunity to electromagnetic interference (EMI): The PAM signal has extremely good immunity to EMI. This is because the PAM signal is only transmitted at low power levels and because it uses a very narrow bandwidth.
Disadvantages of PAM
The main demerits of pulse amplitude modulation include;
-Requires a large bandwidth: The PAM signal requires a very large bandwidth to be transmitted. This means that it requires a very large amount of bandwidth to be allocated for the signal.
-Pulse width modulation can introduce distortion: The PAM signal can introduce some distortion when the amplitude is modulated. This is because the maximum amplitude of the pulse is limited by the ratio between the width of the pulses and their period.
-The PAM signal can only transmit information at low power levels: The PAM signal can only transmit information at low power levels. This means that there is a limit on how much information can be transmitted using this technique before interference will corrupt the transmission of information over long distances.
-Low noise immunity: The PAM signal is very susceptible to noise. This means that even if the PAM signal is very powerful, it will still be corrupted by noise if the transmitter and receiver are close together.
At this point, you have adequate information about pulse impulse modulation, how it works, and when it is used. Are you ready to implement it in your circuit or application? Go ahead and use it.
Ensure that all other components in the circuit are of the right quality.
One way of guaranteeing this is by buying from reliable electronic component manufacturers and suppliers. You can find them easily by using the services of reputable sourcing agents.
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