Analog Communications

Overview Communication systems Analog Modulation Digital Modulation AM FM Digital Modulation ASK FSK Modems

Communication systems Digital Analog The block diagram on the top shows the blocks common to all communication systems

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:

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) AM radio FM radio/TV

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 Baseband signal (electrical signal) EM waves (modulated signal) Transmitter Input transducer Transmission Channel Modulator EM waves (modulated signal) Carrier Baseband signal (electrical signal) Receiver Output transducer Demodulator

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

Amplitude Modulation

Amplitude Modulation Uses a higher frequency carrier Most efficient use of frequency Time and Frequency Domain Susceptible to Noise

Sine Wave Peak Amplitude (A) Frequency (f) Phase () General Sine wave 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) = Ac(1 + Ka m(t))cos(2fct) Where Ka 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. fc is the carrier frequency m(t) = Am cos(2fmt) Where fc >> fm.

Varying Sine Waves

Higher Frequency Carrier Signal time Carrier time

Higher Frequency Carrier Power Spectral Density watts Carrier Signal frequency Baseband

Time Domain Signal time Carrier time

Time Domain Continued time

Amplitude Modulation Carrier wave Baseband signal Modulated wave Amplitude varying-frequency constant

Some AM Waveforms

Detection of Signal time time

Frequency Domain Unmodulated Modulated watts Carrier Signal frequency Baseband watts Modulated Carrier Signal frequency Baseband 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)

Upper and Lower Sidebands

Upper and Lower Sidebands 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.

Answer

Percentage Modulation Determination

Example 2 An unmodulated carrier is 300Vp-p. Calculate %m when its maximum p-p value reaches: a. 400 b. 500 c. 600

Answers 33.3% b. 66.7% c. 100%

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 tx’s 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.