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Signalling and modulation (Periodic signals (Sinewave - lamda = (c/f)…
Signalling and modulation
Periodic signals
Sinewave - lamda = (c/f) where c is the speed of light, and f is the frequency
Peak Amplitude (A) is the signal strength over time, typically measures in volts
s(t) = A sin (2 .pi. f.t + phi) where phi is the phase, t is time, f is frequency, pi is pi
Frequency - meausred in hertz. the rate at which the signal repeats itself
f = c/lamda
T = 1/f period T is the time for one repeatition
Phase - relative period in time with a single period of signal
Modulation
A baseband message signal, which contains information, is used to control the parameters of a high-frequency carrier signal, so as to impress information onto the carrier
The message or modulating signal, can be either
Analogue m(t)
or Digital d(t)
Modulation is required because aerial dimensions are of the same order as the wavelength.
For baseband speech at 3Khz the wavelength is lamba = (c/f) = 3.10^8 / 3.10^3 = 3.10^5 or 100Km. - thats a bloody big aerial
A carrier signal has 3 parameters than can be modulated (varied) to make it carry information
Amplitude
Frequency
Phase
The baseband signal can be digital or analogue
These leads to 6 kinds of modulation
If the message (baseband) signal m(t) controls Amplitude then we have AM (amplitude modulation)
Analogue (because m(t))
if the message signal m(t) controls frequency then we have FM (frequency modulation)
If the message signal m(t) controls the phase then we have PM (Phase modulation or phi M)
If the message signal d(t) controls amplitude we get ASK (amplitude shift keying)
Digital because d(t)
Special case gives PSK (phase shift keying) called phase reversal keying (PRK)
If the message signal d(t) controls frequency we get FSK frequency shift keying
If the message signal d(t) controls phase we get PSK phase shift keying
