In amplitude modulation (AM), the carrier frequency is kept constant for several reasons, including practical considerations and the need for efficient transmission and reception of the modulated signal. Here are the main reasons for maintaining a constant carrier frequency in AM:
Separation of signal components: The carrier frequency acts as a reference for demodulation at the receiver. By keeping it constant, it becomes easier to separate the modulating signal (the information-bearing signal) from the carrier signal during demodulation. This allows the receiver to extract the original signal accurately.
Compatibility and standardization: A constant carrier frequency allows for compatibility and standardization across different transmission and reception equipment. AM broadcasting, for example, operates on specific carrier frequencies, such as 540 kHz to 1600 kHz on the AM band. This ensures that receivers can tune in to the desired station by selecting the appropriate carrier frequency.
Efficient power usage: By maintaining a constant carrier frequency, it becomes possible to optimize the transmitter's power output to match the requirements of the transmission system. This ensures efficient use of power resources, as the transmitter does not need to vary its power output based on the modulating signal.
Transmission infrastructure: The infrastructure for transmitting and receiving AM signals, such as antennas and tuning circuits, is designed to handle a specific carrier frequency. Changing the carrier frequency would require significant modifications to the infrastructure, which is not practical or cost-effective.
Receiver design simplicity: Keeping the carrier frequency constant simplifies the design of AM receivers. The receiver can focus on extracting the modulating signal by demodulating the amplitude variations of the carrier, rather than dealing with a varying carrier frequency.
It's important to note that in certain modulation schemes, such as frequency modulation (FM), the carrier frequency does vary to represent the modulating signal. However, in the case of AM, a constant carrier frequency is preferred for the reasons mentioned above.