Analog and Digital Communication Notes<\/p>\n
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Modulation is the process of varying a carrier signal in order to use that signal to convey information. The three key parameters of a sinusoid are its amplitude, its phase and its frequency, all of which can be modified in accordance with an information signal to obtain the modulated signal. A device that performs modulation is known as a modulator and a device that performs the inverse operation of demodulation is known as a demodulator. A device that can do both operations is a modem (a contraction of the two terms).<\/p>\n
In digital modulation, the changes in the signal are chosen from a fixed list (the modulation alphabet) each entry of which conveys a different possible piece of information (a symbol). The alphabet is often conveniently represented on a constellation diagram.<\/p>\n
In analog modulation, the change is applied continuously in response to the data signal. The modulation may be applied to various aspects of the signal as the lists below indicate.<\/p>\n
Analog modulation techniques<\/p>\n
Angular modulation<\/p>\n
Phase modulation (PM)<\/p>\n
Frequency modulation (FM)<\/p>\n
Amplitude modulation (AM)<\/p>\n
Single-sideband modulation (SSB, or SSB-AM), very similar to single-sideband suppressed carrier modulation (SSB-SC)<\/p>\n
Vestigial-sideband modulation (VSB, or VSB-AM)<\/p>\n
Sigma-delta modulation (\u2211\u0394)<\/p>\n
Digital modulation techniques<\/p>\n
Any form of digital modulation necessarily uses a finite number of distinct signals to represent digital data.<\/p>\n
In the case of PSK, a finite number of phases are used.<\/p>\n
In the case of FSK, a finite number of frequencies are used.<\/p>\n
In the case of ASK, a finite number of amplitudes are used. This is very similar to pulse code modulation<\/p>\n
Pulse modulation<\/p>\n
These are hybrid digital and analogue techniques.<\/p>\n
Pulse-code modulation (PCM)<\/p>\n
Pulse-width modulation (PWM)<\/p>\n
Pulse-amplitude modulation (PAM)<\/p>\n
Pulse-position modulation (PPM)<\/p>\n
Pulse-density modulation (PDM)<\/p>\n
Miscellaneous techniques<\/p>\n
The use of on-off keying to transmit Morse code at radio frequencies is known as continuous wave (CW) operation.<\/p>\n
Adaptive modulation<\/p>\n
Trellis coded modulation (TCM) also known as trellis modulation<\/p>\n
Generation and Detection of Am<\/b><\/p>\n
Introduction to Modulation<\/b><\/p>\n
Basic concept is to vary a carrier signal c(t) = Ac<\/sub>\u00a0cos (2\u03c0fc<\/sub>t relative to an analog (information) wave form m(t) orbits {bi<\/sub>}.<\/p>\n Analog modulation varies the amplitude (AM), frequency (FM), or phase (PM) of the carrier c(t).<\/p>\n Digital modulation varies the amplitude (M-AM), phase (PSK), pulse characteristics (PAM), or amplitude and phase (MQAM) of the carrier.<\/p>\n Amplitude Modulation<\/b><\/p>\n Amplitude modulation varies the carrier amplitude according to an analog information signal m(t)<\/p>\n \u2022In standard AM modulation, a constant term is added to the information signal to yield the transmitted signals(t)=Ac[1+ka<\/sub>m(t)] cos(2\u03c0fc<\/sub>t).<\/p>\n \u2022The constant term greatly simplifies demodulation but is wasteful of power.<\/p>\n \u2022The envelope of the transmitted signal is a(t)=Ac|1+ka<\/sub>m(t)|.<\/p>\n \u2022If |kam(t)|<=1t then a(t) is always nonnegative, which simplifies demodulation (can demodulate envelope only), but hurts SNR.<\/p>\n \u2022The spectrum of modulated signal is S(f) = .5 ka\u00a0<\/sub>Ac\u00a0<\/sub>[M (f\u2212fc<\/sub>) + M(f+fc<\/sub>)] + .5 Ac\u00a0<\/sub>[ (f\u2212fc<\/sub>) + \u03b4(f+fc<\/sub>)]<\/p>\n \u2022The percentage modulation of the signal is defined as max t[100|ka<\/sub>m(t)|].<\/p>\n \u2022The band width of the modulated signal is twice that of the information signal.<\/p>\n Generation of AM Waves<\/b><\/p>\n \u2022 Multipliers difficult to build in hardware (at least circa 1920)<\/p>\n \u2022 AM waves typically generated using a nonlinear device to obtain the desired multiplication<\/p>\n \u2022 Square law modulator sums carrier c (t) and information m (t) signals, and then squares the m using a nonlinear device. Unwanted terms are filtered out with a band pass filter.<\/p>\n \u2022 Switched modulation sums c (t) and m (t) then passes sum through a switch, which approximately multiplies it by a periodic square wave. This generates the desired signal plus extra terms that are filtered out.<\/p>\n Detection of AM Waves<\/b><\/p>\n \u2022 AM detection typically entails tradeoffs between performance and complexity (cost).<\/p>\n \u2022 Square law detector squares the received signal followed by a low pass filter. This detection is simple but introduces an unwanted distortion term proportional to m2<\/sup>(t).<\/p>\n \u2022 Envelope detector is a simple circuit for AM detection consisting of resistors, a capacitor, and a diode. It only works when |ka<\/sub>m(t)| <= 1t (Can\u2019t detect sign change). The RLC circuit must track envelope but not the carrier (f\u22121c<<<\/p>\n Main Points:<\/b><\/p>\n \u2022 Modulation is the process of encoding a message signal orbits into a carrier signal.<\/p>\n \u2022 AM modulation modulates the amplitude of the carrier wave form with a message signal.<\/p>\n \u2022 A constant term is added to the message signal to simplify demodulation: this is wasteful of power and hurts SNR.<\/p>\n \u2022 AM waves are typically generated using nonlinear devices.<\/p>\n \u2022 AM waves can be demodulated using nonlinear devices with some distortion.<\/p>\n \u2022 Envelope detectors are simple cheap devices to detect AM waves with no distortion, but they have poor SNR.<\/p>\n","protected":false},"excerpt":{"rendered":" Analog and Digital Communication Notes Analog and Digital Communication Modulation is the process of varying a carrier signal in order to use that signal to convey information. The three key parameters of a sinusoid are its amplitude, its phase and its frequency, all of which can be modified in accordance with an information signal to … Read more<\/a><\/p>\n","protected":false},"author":4,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"fifu_image_url":"","fifu_image_alt":""},"categories":[4773],"tags":[2847],"amp_enabled":true,"_links":{"self":[{"href":"https:\/\/www.kopykitab.com\/blog\/wp-json\/wp\/v2\/posts\/29287"}],"collection":[{"href":"https:\/\/www.kopykitab.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.kopykitab.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.kopykitab.com\/blog\/wp-json\/wp\/v2\/users\/4"}],"replies":[{"embeddable":true,"href":"https:\/\/www.kopykitab.com\/blog\/wp-json\/wp\/v2\/comments?post=29287"}],"version-history":[{"count":1,"href":"https:\/\/www.kopykitab.com\/blog\/wp-json\/wp\/v2\/posts\/29287\/revisions"}],"predecessor-version":[{"id":115517,"href":"https:\/\/www.kopykitab.com\/blog\/wp-json\/wp\/v2\/posts\/29287\/revisions\/115517"}],"wp:attachment":[{"href":"https:\/\/www.kopykitab.com\/blog\/wp-json\/wp\/v2\/media?parent=29287"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.kopykitab.com\/blog\/wp-json\/wp\/v2\/categories?post=29287"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.kopykitab.com\/blog\/wp-json\/wp\/v2\/tags?post=29287"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}