Radio Frequency (Continued....)

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The transceiver handles the process of converting the low-frequency baseband signal into a higher-frequency RF signal though a process called upconversion. Within the transceiver, an RF carrier signal is generated at the frequency that will be used for the final RF signal. This RF carrier is then combined with the baseband signal to upconvert the low-frequency baseband to the higher RF carrier frequency. Figure 2-2A shows both the low-frequency baseband signal and the RF carrier signal that are used in the upconversion process. Figure 2-2B shows the result of an ideal upconversion where the modulated RF signal is identical to the original baseband signal; except, it is now centered at a much higher frequency. In addition to upconverting outgoing signals, the transceiver also downconverts incoming RF signals to low-frequency baseband signals. The downconversion process is the inverse of the upconversion process.

The process of upconverting a modulated baseband signal into a modulated RF signal




Although the RF signal exiting the transceiver is now at a suitable frequency for wireless transmission, it is still too weak to travel over any appreciable distance. Likewise, any signal being received by the antenna is too weak to be sent directly to the transceiver. The RF front-end serves two functions: It amplifies signals coming from the transceiver to a power level suitable for transmission, and it amplifies weak signals coming from the antenna to a level that can be detected by the transceiver. The quality and performance of the RF front-end is what determines a radio's overall RF performance. Specifications such as output power and receiver sensitivity are directly determined by the front-end components.

The last component in the radio is the antenna itself, whose purpose is to interface the electric currents flowing in the radio's circuitry to electromagnetic waves in free space. Depending on the radio's intended application, the type of antenna used can vary widely. The quality and performance of the antenna used in a radio has as much impact on its total performance as that of the RF front-end. For this reason, it is important to understand how to properly utilize the type of antenna used in any particular application.


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