Amplitude Modulation Flashcards
What is the need for modulation?
Audio frequencies have a very limited wave, therefor modulation is needed to superimpose an audio frequency onto a radio frequency to greatly increase he range or distance it can be propagated
What is the easiest way of determining an AM signal on the kit?
Big constant carrier frequency down the middle
May hear voice over the headphones
Define modulation
Modulation is the the process whereby information is superimposed upon a Radio Frequency carrier wave
What is the modulating wave?
The waveform of the original intelligence
What is the carrier wave
The waveform that is used to ‘carry’ the modulating wave to gain further distance of transmission
What is the modulated wave?
The waveform created by encoding the modulating wave into the carrier wave.
This is the waveform that is transmitted
Define amplitude modulation
Amplitude modulation is a method in which the amplitude of the modulated wave is varied in accordance with the amplitude of the modulating wave
In relation to the carrier frequency, what are the upper and lower side bands?
The upper side band is the carrier frequency plus the modulating frequency (fc + fm)
And the lower side band is the carrier frequency minus the modulating frequency
(fc - fm)
The power of the modulating wave is divided between the two side ands, each getting half the power.
The upper and lower side bands are mirror images of each other, this is known as a double side band.
What is the transmission bandwidth?
The bandwidth is the difference in hertz between the lowest and highest frequency components of the waveform
Or = 2 x Modulating frequency
Always 6.8Khz for AM
What is the AM envelope?
It is the outline of the amplitude variations in respect to time.
Define double side band
Is where both bands of frequencies produced by AM modulation are transmitted equally either side of the carrier wave
What is the typical frequency range of speech?
300 to 3400Hz
What are the disadvantages of double side band transmissions?
Waste of transmitter power
Excessive bandwidth
Prone to selective fading
Define single side band
Is where only either the upper or lower side band, produced by AM modulation, is transmitted
