Communication systems Analog Modulation – AM – FM Digital Modulation – ASK – FSK Modems Overview
The block diagram on the top shows the blocks common to all communication systems Communication systems Digital Analog
Remember the components of a communications system: Input transducer: The device that converts a physical signal from source to an electrical, mechanical or electromagnetic signal more suitable for communicating Transmitter: The device that sends the transduced signal Transmission channel: The physical medium on which the signal is carried Receiver: The device that recovers the transmitted signal from the channel Output transducer: The device that converts the received signal back into a useful quantity
Analog Modulation The purpose of a communication system is to transmit information signals (baseband signals) through a communication channel The term baseband is used to designate the band of frequencies representing the original signal as delivered by the input transducer – For example, the voice signal from a microphone is a baseband signal, and contains frequencies in the range of 0-3000 Hz – The hello wave is a baseband signal:
AM radioFM radio/TV Since this baseband signal must be transmitted through a communication channel such as air using electromagnetic waves, an appropriate procedure is needed to shift the range of baseband frequencies to other frequency ranges suitable for transmission, and a corresponding shift back to the original frequency range after reception. This is called the process of modulation and demodulation Remember the radio spectrum: For example, an AM radio system transmits electromagnetic waves with frequencies of around a few hundred kHz (MF band) The FM radio system must operate with frequencies in the range of 88-108 MHz (VHF band)
Since the baseband signal contains frequencies in the audio frequency range (3 kHz), some form of frequency-band shifting must be employed for the radio system to operate satisfactorily This process is accomplished by a device called a modulator The transmitter block in any communications system contains the modulator device The receiver block in any communications system contains the demodulator device The modulator modulates a carrier wave (the electromagnetic wave) which has a frequency that is selected from an appropriate band in the radio spectrum – For example, the frequency of a carrier wave for FM can be chosen from the VHF band of the radio spectrum – For AM, the frequency of the carrier wave may be chosen to be around a few hundred kHz (from the MF band of the radio spectrum) The demodulator extracts the original baseband signal from the received modulated signal To Summarize: Modulation is the process of impressing a low-frequency information signal (baseband signal )onto a higher frequency carrier signal Modulation is done to bring information signals up to the Radio Frequency (or higher) signal
Basic analog communications system Modulator Demodulator Transmission Channel Input transducer Transmitter Receiver Output transducer Carrier EM waves (modulated signal) Baseband signal (electrical signal)
Types of Analog Modulation Amplitude Modulation (AM) Amplitude modulation is the process of varying the amplitude of a carrier wave in proportion to the amplitude of a baseband signal. The frequency of the carrier remains constant Frequency Modulation (FM) Frequency modulation is the process of varying the frequency of a carrier wave in proportion to the amplitude of a baseband signal. The amplitude of the carrier remains constant Phase Modulation (PM) Another form of analog modulation technique which we will not discuss
Uses a higher frequency carrier Most efficient use of frequency Time and Frequency Domain Susceptible to Noise
Sine Wave Peak Amplitude (A) – maximum strength of signal – volts Frequency (f) – Rate of change of signal – Hertz (Hz) or cycles per second – Period = time for one repetition (T) – T = 1/f Phase ( ) – Relative position in time, from 0-2*pi General Sine wave
S(t) = A c (1 + K a m(t))cos(2f c t) Where K a is a constant. m(t) is the baseband message. For an audio signal, the spectrum of m(t) would typically be in the range of 300 Hz - 3 kHz. f c is the carrier frequency m(t) = A m cos(2f m t) Where f c >> f m.
Frequency Domain Unmodulated frequency Signal Carrier Baseband watts Modulated frequency Signal Carrier Baseband watts Baseband
The AM Waveform The three components that form the AM waveform are listed below: 1. The lower-side frequency (fc - fi) 2. The carrier frequency (fc) 3. The upper-side frequency (fc + fi) If a radio signal of frequency with1000KHz is mixed with an audio signal of 1 KHz the output will be A signal at 1000 KHz (Carrier wave) A signal at 1001 KHz (upper sideband) A signal at 999 KHz (lower sideband)
In most systems the intelligence signal is a complex waveform containing components from roughly 200Hz to 3KHz. If this complex waveform is used to modulate the carrier there would be a whole band of side frequencies. The band of frequencies above the carrier is term the upper sideband. The band of frequencies below the carrier is called the lower sideband.
Example A 1.4MHz carrier is modulated by a music signal that has frequency components from 20Hz to 10kHz. Determine the range of frequencies generated for the upper and lower sidebands.
Points to be Remembered Amplitude modulation is the process of varying the amplitude of a carrier wave in proportion to the amplitude of a baseband signal. The frequency of the carrier remains constant The function of the carrier in AM is simply to provide a signal to heterodyne (mix) with the modulated audio, to convert all the AF components to a higher frequency. The bandwidth of an AM signal is equal to twice the highest freq. [ In commercial AM broadcast txs the freq of the modulating audio is permitted to be as high as 4.5 KHz, and since a double side system is used, commercial stations therefore have a bandwidth of 9 KHz] The bandwidth does not depend on the power of the modulating signal.