4.8: Capsule Summary

4.8 Capsule Summary

Radio transmitters must be accurate, efficient, and transmit within their allowed frequency band. Receivers must be sensitive (but not to criticism), selective, and detect a huge range of signal strength. Both must be flexible. RFID reader radios usually operate in unlicensed bands and thus must support frequency hopping or other interference-mitigation provisions. RFID readers also have the peculiar problem of being both full- and half-duplex; the use of a bistatic antenna configuration may be beneficial. Because they receive a backscattered signal, RFID receivers are generally configured as homodyne rather than heterodyne radios. The leakage from the transmitter creates offsets which must be filtered or blocked.

Radios are constructed of a number of key components. Amplifiers are characterized by gain, power, bandwidth, noise, and distortion properties, which are often reported in terms of second- and third-order intercepts. Mixers are more complex, and in addition to conversion loss, bandwidth, noise, and distortion, one must consider isolation and a large number of possible spurious output frequencies.

Oscillators are generally constructed using positive feedback through a resonant circuit. Oscillator amplitude noise can readily be removed by limiting the output; phase noise is not so easily dealt with. The resonator quality factor plays a critical role in determining the phase noise of the oscillator. Oscillators use a variable component such as a varactor to adjust the frequency of oscillation.

An oscillator is generally embedded in a phase-locked loop to form a synthesizer, which produces an output signal bearing a controlled relationship to...