28-05-2013, 12:59 PM
AM modulation
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Carrier Acquisition
In the different AM modulation methods where a carrier is not transmitted (DSBSC, SSB, … etc), we assumed that the carrier that was used in the transmitter is also available at the receiver. This is a big assumption since it may be easy to generate a sine wave, however, it is difficult to get a sine wave with the SAME FREQUENCY and PHASE as the sine wave that was used in the transmitter. Therefore, the concept of carrier acquisition is important for these types of modulation.
Carrier Acquisition in DSBSC
Since at the input of a DSBSC demodulator system, only the transmitted signal (and random noise) is available, the demodulator must have some method to extract the carrier from the received signal. It is known that the carrier frequency is located at the middle of the DSBSC signal as shown below.
In theory, a filter with an extremely narrowband (few Hz) that is centered at the carrier frequency c would be able to extract a signal that represents the carrier frequency. However, this assumes that the message signal has a DC component that when modulated this DC component moves to the carrier frequency. Having a DC component in the message signal that will be modulated using DSBSC is not necessarily the case. In fact, it is generally not the case. Take for example an audio signal. Audio signals do not contain DC since and audio signal travels through the air molecules in the form of variations in the air pressure around the atmospheric pressure. A microphone receives these variations in the air pressure and generates an electric signal with no DC. Also, digital data in the form of binary numbers is generally represented by 1’s and 0’s that have voltages of +5 V and –5 V, respectively. Such a signal generally has no DC. So, in practice, the message signals that are modulated using DSBSC (or other modulation techniques) do not have a DC component as shown in the following figure.