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| 16 which is why many high-power radar systems are based on TDM. If the system can achieve different frequencies for transmission and reception, then a frequency-division multiplexing (FDM) method can be used. Some cellular communication systems implement this method, using different transmit and receive frequencies and filter banks that limit adjacent channel leakage. If it is necessary to have both transmitter and receiver activity at the same time and frequency along the same antenna transmission path, then designers can use components such as circulators. A circulator is a three-port device that shifts the signal entering a port to the next port while providing isolation among the ports. In this method, a designer connects a circulator directly to the transit, receive, and antenna paths, where the transmit signal energy will be carried to the antenna path, and the antenna-coupled signal energy will be directed to the receive path. Digital Converters and Synthesizers Many modern RF systems are either fully digital or hybrid analog/digital systems. This means that digital synthesizers generate RF signals—sometimes completely modulated and pre- distorted—and digital converters convert RF signals into digital signals to be processed and even demodulated. Depending on the frequencies and the performance capabilities of the digital electronics involved, designers may need to add only an RF front end (RFFE) between the digital synthesizers and digital converters to create a communication and sensing system. Antennas RF antennas are transducers that convert RF electrical signals to EM radiation and vice versa. They can be made from simple conductive structures, waveguides, or more complicated multi-element arrays that require sophisticated antenna processors to properly control the arrays and process the multitude of incoming and outgoing signals. Make Versus Buy Considerations for RF Modules Though an RF system may contain many components and devices in each signal chain, RF part manufacturers have become extremely proficient at developing complex integrated circuits (ICs) and high-density printed circuit board (PCB) modules that address many challenges of designing an RF signal chain (Figure 3). In the simplest form, an RF module may be a relatively simple filter bank for a multiple-frequency channel wireless communication protocol or as complex as a Figure 3: A generic RF signal chain. (Source: Analog Devices)

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