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Unformatted text preview: 52 www.rfdesign.com February 2008 Design Tip Spectral-mask shaping for a Class 1 Gen 2 PR-ASK RFID I/Q modulator By Doug Stuetzle A n integrated I/Q modulator such as the LT5568-2 can generate the 900 MHz transmit signal for a Class 1 Gen 2 UHF RFID reader. While there are less-expensive alternatives for example directly modulating a VCOan I/Q modulator offers more flexibility to gener- ate multiprotocol signals to meet RFID standards. For example, a Class 1 Gen 2 reader can send information to one or more tags by several different modulation formats. The RF carrier may be modulated by double-sideband amplitude shift keying (DSB-ASK), single-sideband amplitude shift keying (SSB-ASK), or phase-reversed amplitude shift keying (PR-ASK) using a pulse-interval encoding format. To implement SSB-ASK modulation, the I and Q inputs of a bal- anced modulator are driven with in-phase and quadrature signals; the quadrature signal is the Hilbert transform of the in-phase signal. The resulting modulator output is a LO-suppressed, single-sideband RF sig- nal. Alternatively, to generate a double-sideband RFID signal, the circuit required to drive such a modulator is simple. Only the I (or Q) port of the modulator need be driven with the baseband signal. The unused port is left unconnected or not driven. Thus, a single digital-to-analog port is left unconnected or not driven....
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