RF Simulation Demo: Pulse Amplitude Modulation

The objective of this experiment is to understand the underlying principles of the pulse amplitude modulation and demodulation. This demo is designed to illustrate the theory behind PAM, and introduce practical aspects of its implementation.

Theory

Pulse modulation may be used to transmit analog information, such as continuous speech or data. It is a system in which continuous waveforms are sampled at regular intervals. Information regarding the signal is transmitted only at the sampling times, together with any synchronizing pulses that may be required. At the receiving end, the original waveforms may be reconstituted from the information regarding the samples, if these are taken frequently enough. Despite the fact that information about the signal is not supplied continuously as in AM and FM, the resulting receiver output can have negligible distortion.

Pulse modulation may be subdivided broadly into two categories, analog and digital. In the former, the indication of sample amplitude may be infinitely variable, while in the latter a code which indicates the sample amplitude to the nearest predetermined level is sent. Pulse-amplitude and pulse-time modulation are both analog, while the pulse code and delta modulation systems are both digital. All the modulation systems to be discussed have sampling in common, but they differ from each other in the manner of indicating the sample amplitude.

The two types of analog pulse modulation, pulse–amplitude and pulse-time modulation correspond roughly to amplitude and frequency modulation. Pulse Amplitude Modulation, the simplest form of pulse modulation. It forms an excellent introduction to pulse modulation in general. PAM is a pulse 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. The pulses are then sent by either wire or cable, or else are used to modulate a
carrier.

The two types are double-polarity PAM which is self-explanatory and single-polarity PAM, in which a fixed dc level is added to the signal, to ensure that the pulses are always positive. The ability to use constant-amplitude pulses is a major advantage of pulse modulation, and since PAM does not utilize constant-amplitude pulses, it is infrequently used. When it is used, the pulses frequency-modulate the carrier.

It is very easy to generate and demodulate PAM. In a generator, the signal to be converted to PAM is fed to one input of an AND gate. Pulses at the sampling frequency are applied to the other input of the AND gate to open it during the wanted time intervals. The output of the gate then consists of pulses at the sampling rate, equal in amplitude to the signal voltage at each instant. The pulses are then passed through a pulse-shaping network, which gives them flat tops. As mentioned above, frequency modulation is then employed, so that the system becomes PAM-FM. In the receiver the pulses are first recovered with a standard FM demodulator. They are then fed to an ordinary diode detector, which is followed by a low-pass filter. If the cutoff frequency of this filter is high enough to pass the highest signal frequency, but low enough to remove the sampling frequency ripple, an undistorted replica of the original signal is reproduced.

Demonstration

1) Place the Simulate Signal function VI in the block diagram and configure it as signal type of sine wave form with the frequency of 100Hz.

2) Place the another Simulate Signal function VI in the block diagram and configure it as signal type of square wave form with the frequency of 10Hz.

3) Place the Multiply VI in the Block diagram in order to multiply the sine signal with the square wave form signal.

4) Place the waveform graphs in the front panel and wire the blocks to the each output function in the block diagram.

5) Place the sub diagrams in side the loop in order to repeat the operation until the stop button is enabled.

On the front panel (below) we can observe the input message signal, the carrier signal, the pulse amplitude modulated signal, as well as the demodulated output. The effects of changing the frequencies of the message and carrier signals can be easily seen.

Requirements

Filename: pam.vi

Software Requirements

Application Software: LabVIEW Full Development System 7.1

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