Lecture 9

COMMUNICATION SYSTEMS Lecture # 9 28th Feb 2007 Instructor

WASEEM KHAN

Centre for Advanced Studies in Engineering

Superheterodyne Receiver A superheterodyne receiver converts the received RF signal to an intermediate frequency (IF) first.

fIF = fRF -fLO Tunable

In the second stage, after necessary filtering, IF signal is demodulated to get the baseband signal. It is used for demodulating amplitude as well as angle modulated signals. Practically all radio and TV receivers are of superheterodyne type. Tunable oscillator is used to tune a particular channel; as fLO varies, received fRF changes accordingly (fIF remains constant).

Problem A message signal is given by

m(t) = 2cos(10 t) + 5cos(5 t) The message signal modulates a carrier signal given by

c(t) = cos(500 t) Sketch the Fourier transform of the message signal and the modulated signal.

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Digital Communications Most of the modern communication systems are employing digital techniques. If the original information is not digital, it is digitized before transmission. The information is represented by a sequence of ones and zeros. Sequence of bits is converted to signal that can propagate through channel. The receiver extracts the same sequence of bits from the received signal and converts back to analog (if the information was originally analog).

Digital Communications What is digital? The information is digital, not the transmitted signal

Why digital? -

Easy to regenerate the distorted signal

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Regenerative repeaters along the transmission path can detect a digital signal and retransmit a new, clean (noise free) signal These repeaters prevent accumulation of noise along the path

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This is not possible with analog communication systems

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Digital Communications Why Digital? Good processing techniques are available for digital signals, such as -

Data compression (or source coding) Error Correction (or channel coding) Equalization

Easy to mix signals and data using digital techniques -

Voice, video, text, etc. can be combined

Flexible and low-cost hardware Encryption techniques are easier to implement

Modulation in Digital Systems Modulation is a process by which one or more characteristic properties of a sinusoidal waveform are varied according to the message signal. The variable characteristics of a sinusoidal waveform are Amplitude Phase Frequency

These characteristics when changed, declare three major categories of modulation Amplitude Shift Keying (ASK) Phase Shift Keying (PSK) Frequency Shift Keying (FSK)

Modulation in Digital Systems Generally speaking modulation is the process of shifting of baseband signal to a high frequency band. This is bandpass modulation. Digital baseband modulation is the process of converting the bit-stream into a sequence of baseband symbols or baseband signal. M-ary signal may assume one of M possible symbols representing log2(M) bits. A binary signal can represent a single bit while an 8-ary signal represents 3 bits. Once the information is converted into a bit-stream, groups of bits are mapped onto available symbol-set.

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Amplitude Shift Keying(ASK) Modulation Process In Amplitude Shift Keying (ASK), the amplitude of the carrier is switched between two (or more) levels according to the digital data For BASK (also called ON-OFF Keying (OOK)), one and zero are represented by two amplitude levels A1 and A0

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