Physics · Ch 16 — Communication Systems
Mixing up of signals from different transmitters
Mixing up of signals from different transmitters
Mixing Up of Signals from Different Transmitters
A third, more practical, problem arises when many transmitters send out baseband information signals simultaneously: since all these signals occupy roughly the same low-frequency range, they overlap and mix together, and a receiver has no simple way of separating one from another. Transmitting at high frequencies and assigning each message signal its own allotted frequency band avoids this mixing, allowing many signals to travel and be received at once without interference.
Together, the three arguments - antenna size (Section 15.7.1), radiated power (Section 15.7.2), and signal mixing (this section) - establish why the original low-frequency message must be translated onto a high-frequency carrier wave before transmission, in a way that preserves the original information: this process is called modulation.
A carrier wave may be continuous/sinusoidal or in the form of pulses. A sinusoidal carrier is written:
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What this figure shows. Two-part figure stacked vertically, both with a horizontal 'Time' axis (unlabelled scale). (a) A smooth sinusoidal wave with 'Amplitude' marked by a vertical double-headed arrow from the axis to the first peak, and 'Time period T' marked by a horizontal bracket spanning one full cycle; to the right, the relation omega = 2*pi/T is written. (b) A rectangular pulse train (four to five square pulses of alternating high/low levels) with 'Pulse duration' marked as the width of a pulse at its top, 'Pulse rise' and 'Pulse fall' each marked with a short diagonal arrow pointing to the leading/trailing vertical edge of a pulse, and 'Pulse amplitude' marked by a vertical doubl …
What this figure shows. Stack of five aligned time-domain plots (a)-(e), all sharing a common horizontal 'time' axis from 0 to 3 at the bottom, each plot's vertical axis unlabelled with amplitude values shown (mostly -1 to 1, panel (c) -2 to 2). (a) c(t): a fast, constant-amplitude sinusoidal carrier wave. (b) m(t): a slower single sinusoidal modulating wave (about 2 full cycles over the time span). (c) cm(t) for AM: a fast carrier wave whose envelope (peak amplitude) bulges and shrinks following the shape of m(t), the classic AM 'beating' envelope pattern. (d) cm(t) for FM: a fast wave of constant amplitude whose instantaneous frequency (spacing between zero-crossings) visibly compresses and stretches in sync with m(t). (e) cm(t) for PM: visually similar to the FM trace, constant amplitude with the phase/frequency of the oscillations shifting in accordance with m(t). A sidebar link near this figure reads 'Modulation and Demodulation http://iitg.vlab.co.in/?sub=59&brch=163&sim=259&cnt=358'. …