Signal Modulation: Why Does ASK/PSK (QAM) Need Bandwidth?

[LostInSpace]

How come a signal modulated using Amplitud Shift Keying and/or Phase Shift Keying (such as QAM) require a bandwidth? I mean, since the frequence is constant; only the amplitude and phase is changed.

Thanks in advance!

 

[NoTime]

I mean, since the frequence is constant

It is not constant! The slope dV/dT changes from that of the unmodulated sine wave. This means that for a small periods of time it is different frequencies.

 

[ravenprp]

Signal modulation is a technique used to transmit information over a communication channel by varying certain properties of the signal, such as amplitude, frequency, or phase. In the case of Amplitude Shift Keying (ASK) and Phase Shift Keying (PSK), the amplitude and phase of the signal are modulated to represent the transmitted information.

One of the main reasons why ASK/PSK (QAM) signals require bandwidth is because of the nature of the modulation process. When the amplitude or phase of a signal is changed, it results in the signal containing a range of frequencies. This is known as spectral broadening and it occurs because the original signal is being spread out over a wider frequency range.

In order to accurately transmit the modulated signal, the channel must have enough bandwidth to accommodate this spread out signal. If the channel does not have enough bandwidth, the signal will experience distortion and information may be lost. This is why ASK/PSK (QAM) signals need bandwidth - to ensure that the original signal is accurately transmitted without any loss of information.

Additionally, the amount of bandwidth required for ASK/PSK (QAM) signals also depends on the modulation scheme used. For example, QAM (Quadrature Amplitude Modulation) uses both amplitude and phase modulation, resulting in a more complex signal with a wider bandwidth requirement compared to ASK or PSK alone.

In summary, the need for bandwidth in ASK/PSK (QAM) signals is a result of the modulation process and the requirement to accurately transmit the modulated signal without any distortion or loss of information.