TABLE OF CONTENTS An RFID transmitter has two basic tasks. In the downlink phase, it must provide power to start up passive tags, and modulate its signal so as to send the tags commands and data. In the uplink phase, the transmitter must provide an unmodulated signal that the tag can back-scatter in order to return data to the receiver. The transmitter must be able to operate on any of a number of radio channels with accuracy of a few parts per million in frequency, and switch from one channel to another rapidly, in order to meet the requirements of unlicensed use. In order to obtain good read range, the transmitter should provide the maximum output power allowed by the relevant regulatory bodies. In handheld or portable applications, it should do so while consuming as little DC power as possible.
The transmitter's task is rendered significantly more complex by the presence of other radio devices, and in particular other RFID readers. In normal far-field-coupled operation, the transmitter radiates and can interfere with distant radio receivers. Recall from Chapter 3 that the radiated spectrum of a modulated signal is much wider than that of an unmodulated signal, and that typical RFID modulations, which are optimized for powering the tag, are very inefficient users of spectrum. Regulatory requirements for radiation out of the allowed bands are often very stringent. It is ordinary practice to provide guard bands by refraining from use of channels immediately adjacent to the edge of the allowed...
