Radio 102: Pulse Modulation

(Author: Tymon Hytem | Article Views: 23 | Word Count: 465 | Published: Sep 23rd 2007)

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In pulse modulation continuous waveform is sampled at regular intervals and the information collected at the sampling times together with synchronizing pulses if any is transmitted. At the receiving end the original waveform may be reconstituted from the received signal. Pulse modulation differs from amplitude modulation and frequency modulation in the fact that the signal is not supplied continuously. But in spite of this the signal at the receiver output may have negligible distortion.

Pulse modulation can be classified into two broad categories that is analog pulse modulation and digital pulse modulation. Analog pulse modulation can be further divided into pulse amplitude modulation and pulse time modulation. Digital pulse modulation can be pulse code modulation and delta modulation. All these pulse modulation have sampling in common but they differ in the manner of indicating the sampled amplitude. Pulse amplitude modulation is the simplest form of pulse modulation. The signal is sampled at regular intervals and each sample is made proportional to the magnitude of the signal at the instant of sampling. These sampled pulses may then be sent either directly by wire or cable to the receiving end or may be made to modulate a carrier before transmission.

Pulse amplitude modulation is of two types. Double polarity pulse amplitude modulation and single polarity pulse amplitude modulation. Pulse amplitude modulation is in which a suitable fixed direct current level is added to the signal to ensure that all the pulses are positive going. This type of modulation is not popularly used since the pulses are not of constant amplitude. When used the pulses frequency modulate the carrier to generate pulse amplitude modulated voltage. We feed the signal to be converted into pulse amplitude modulated to one input of an AND gate while the pulses at sampling frequency are fed to the other input of the AND gate to open the gate during the desired time intervals. The output of the AND gate then is in the form of pulses at the sampling rate with amplitude proportional to the magnitude of signal at each instant. These pulses are then fed to a pulse shaping network at the output of which we get flat topped pulses. These pulse amplitude modulated pulses then frequency modulate a carrier to result in pulse amplitude modulated-frequency modulated. At the receiving end pulse amplitude modulated-frequency modulated are fed to a frequency modulator detector whose output is fed to a diode detector. The output of diode detector is fed to a low pass filter. The cut off frequency of this low pass filter is kept high enough to pass the highest signal frequency but low enough to remove the sampling frequency ripple. Then at the output of this filter we get back the original signal in undistorted form.

About the Author

Tymon Hytem has worked in the electronics feild for the past 15 years. He enjoys helping people decide on electronic gadgets from finding the right phone for your business and can help you choose the perfect Background Music for your business needs.

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