TABLE OF CONTENTS All radios must perform certain generic functions. Small signals must be made larger ( amplification), high-frequency signals must be generated ( oscillation), signals at different frequencies must be combined to create new frequencies ( mixing), and signals at wanted frequencies must be accepted while other frequencies are rejected ( filtering). The components that perform these functions are unsurprisingly known as amplifiers, oscillators, mixers, and filters. In modern radios, signals must always be converted to and from digital form, though in RFID radios it can a bit grandiose to assign the terminology analog-to-digital conversion to the simple comparators and switches that are often sufficient for these purposes.
In Figure 4.4 we introduce common symbols for these components, and depict how each component modifies the signals it encounters. Depending on the behavior of the reader antenna, the signal entering the reader may be a mixture of a wide variety of frequencies due to many sources of RF energy at differing frequencies and power levels (1). The band select filter ideally removes most of the signals outside of the band of interest (e.g. 902 928 MHz for RFID in the United States), leaving signals from one or more transmitters at RF frequencies in the wanted band (2). These signals are optionally amplified (3) and then presented to a mixer, where they are mixed with a CW signal from the local oscillator at a constant frequency and amplitude (4). The result, after low-pass filtering (which removes frequencies...
