Comm 04-5: Modulation systems Flashcards
Amplitude modulation
Modulation method where the amplitude of the carrier wave is increased or decreased in accordance with the changes in the audio signal to be conveyed. Amplitude modulation produces a signal with power concentrated at the carrier frequency and in 2 adjacent sidebands. The transmission bandwidth of an AM-signal is more than twice that of the baseband signal
Single Sideband (SSB) modulation
Form of AM where only one sideband is used to convey information, the rest of the signal is suppressed at the transmission.
No intelligence is conveyed by the carrier wave itself
Since USB and LSB are mirror images of one another, it is redundant to send both side bands
Advantage of SSB
Less power required for transmission
½ the bandwidth required
Intercepted signal of better quality
Frequency modulation
Modulation method where the frequency of the carrier wave is modified to reflect the changes in the audio signal to be conveyed
Advantages of FM vs AM
FM signal-to-noise ratio is better than AM
FM has better fidelity at reception
FM allows transmission of signal at constant peak power (because amplitude of the signal does not change)
Disadvantages of FM vs AM
FM needs more complex equipment
FM requires more bandwidth
The main operational disadvantage of FM is that it is restricted to line of sight due to its use of higher frequency (29.999 MHz and above)
Other forms of modulation
Phase modulation
A form of modulation where the phase signal of the carrier wave is varied by the intelligence signal
Normally used to transmit digital signals
Pulse modulation
A form of modulation where the carrier wave is not a continuous alternating wave, but a train of discrete pulses. Like morse code
Frequency shift keying
A form of modulation where a carrier signal of constant amplitude is frequency modulated to correspond to marks and spaces
