TABLE OF CONTENTS In this section, we will provide a brief overview of the two basic alternative designs used for radios. In the discussion, we will allude to a number of concepts that will be discussed in more detail later in the chapter; the reader who is at times puzzled is encouraged to charge bravely on in the hopeful expectation that later sections will provide clarification.
As we saw in Chapter 3, data is usually sent over a radio link by imposing a slow modulation on a fast carrier signal. In the case of a typical RFID reader, the transmitted data rate is typically less than 100 kbps, limited largely by the amount of frequency spectrum allocated to the reader. Somewhat faster modulations are often used by the tags: up to 640 kbps for EPCglobal Generation 2 tags, or 3.25 MHz (at much lower data rates) for EPCglobal class 0. All these signals are much lower in frequency than the carrier signal, typically around 900 MHz. These low frequency signals that describe the way the carrier is to be modulated are generally known as baseband signals. One of the key tasks for a radio is to impose this desired baseband modulation on the transmitted signal, and to extract if from the received signal. These operations are collectively referred to as frequency conversion.
There are two basic frequency-conversion architectures: direct conversion, also known as homodyne, and the multiple conversion or heterodyne configuration. Direct conversion schemes proceed directly,...
