Transmit phase noise is converted to amplitude noise due to the delay-line-discriminator action of the transmitter output, cable, and antenna. Phase noise (from the oscillator) and amplitude noise (from the amplifier chain and the DAC) are important because some of the transmitted signal leaks into the receiver and can limit sensitivity. Power efficiency is usually dominated by the output power amplifier design designers must trade off linearity (and thus transmit bandwidth) against efficiency. Filtering of the transmitted symbols greatly improves the spectral width of the output, but even more improvement can be achieved using phase-reversal amplitude-shift keying, which requires a single mixer for implementation, or single-sideband modulation, which requires a quadrature modulator. Transmitter architectures trade efficiency, cost, and transmit bandwidth. Monostatic RFID readers also employ specialized microwave components – circulators and directional couplers – that are capable of selecting signals based on their direction of travel. There are several architectures for performing this operation, including flash and delta-sigma converters. The received signals must at some point be converted from analog voltages to digital data. The transmitter must be modulated this may be done with a simple switch, or use a digital-to-analog converter, such as a current-steering circuit. Once the signal is converted to baseband, filtering can use discrete components or active filters created by combining an operational amplifier with a frequency-dependent feedback network. Filters built of discrete components are usually limited to quality factors of around 10–20 and thus cannot filter very narrow bands other technologies, such as surface-acoustic-wave devices or dielectric resonators, are used to achieve narrowband high-frequency filtering. Integer-N and fractional-N synthesizers can both be used in RFID applications fractional-N synthesizers are more versatile but more complex to design and can suffer from additional noise.įilters remove unwanted frequencies from a signal. ![]() Oscillators use a variable component such as a varactor to adjust the frequency of oscillation.Īn oscillator is generally embedded in a phase-locked loop to form a synthesizer, which produces an output signal bearing a controlled relationship to a very stable reference frequency. The resonator quality factor plays a critical role in determining the phase noise of the oscillator. Oscillator amplitude noise can readily be removed by limiting the output phase noise is not so easily dealt with. Oscillators are generally constructed using positive feedback through a resonant circuit. 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. Amplifiers are characterized by gain, power, bandwidth, noise, and distortion properties, which are often reported in terms of second- and third-order intercepts. Radios are constructed of a number of key components. ![]() The leakage from the transmitter creates offsets which must be filtered or blocked. Because they receive a backscattered signal, RFID receivers are generally configured as homodyne rather than heterodyne radios. RFID readers also have the peculiar problem of being both full- and half-duplex the use of a bistatic antenna configuration may be beneficial. RFID reader radios usually operate in unlicensed bands and thus must support frequency hopping or other interference-mitigation provisions. Receivers must be sensitive (but not to criticism), selective, and detect a huge range of signal strength. Radio transmitters must be accurate, efficient, and transmit within their allowed frequency band. ![]() Dobkin, in The RF in RFID (Second Edition), 2013 4.8 Capsule Summary
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |