1. Field of the Invention
This invention of a Radio Frequency Identification Data-detector circuit for RFID labels applies in the commerce and service areas where there is a need for labeling goods and service orders. Commerce of goods and services has relied increasingly on labeling and electronic data reading of goods and services being marketed. An example of this are the labels containing bar codes that allow access to information about the product in a database, giving access to other information such as price, weight, quantity available in stock, etc.
2. Description of the Related Art
Bar codes are being replaced by RFID including labels, readers, etc. One advantage of using RFIDs is that information can be stored and rewritten on the label itself. Another advantage is that the label does not need to be correctly positioned in front of an optical reader, as with barcode labels.
Many of the RFID label circuits are passive. This invention can be implemented using semiconductor devices, in particular the passive RFID technology. The RFID technology has a large and growing number of applications that include vehicle identification, smart card systems, animal identification, and general applications in the field of logistics such as production and inventory control, in retail to replace the barcode labels, in airports to identify airline luggage, in healthcare to trace medicines.
In an RFID system, goods and services are labeled with RFID labels. In this system, it is desirable that the labels are easily read and without failures. Reading errors can occur if labels are far from the readers. These errors occur mainly in the case of passive RFID labels, which absorb energy from electromagnetic waves emitted by the reader, because the wave energy is attenuated with the square of the distance to the player. For this reason, it is important that passive RFID labels are able to operate with low power consumption. Thus, the labels are able to work even away from the reader, which increases label sensitivity and system reliability as a whole. An example of the signal being received and demodulated is shown in FIG. 1. The signal is a high frequency signal (2) that oscillates within a range of values limited by an envelope (1). FIG. 2 shows more details on the concept of modulation by amplitude. Within the ellipse (3) shown in FIG. 2 the oscillation of the high frequency signal is shown (4), which occurs within the envelope limits (1). The envelope will vary over time, and can reach a maximum high value (5) and a minimum high value (6). To demodulate the signal shown in FIGS. 1 and 2, it is necessary to detect the envelope (1). Typically the top of the envelope (1) is used to detect data. The detected envelope (7) must be compared to a reference as constant as possible. It is desirable that this constant reference is between the maximum high value (8) and the minimum high value (9) of the detected envelope (7). The data are detected by detecting intersections between the detected envelope (7) and the reference.
With help of FIG. 2, it is possible to understand that the low quality of the signal transmitted by the reader and received by the label is not connected only to the signal power, but also to the difference between the signals modulating the information, i.e., a high power signal but with a small relative difference between the maximum high value (5) and the minimum high value (6) of the package is difficult to be demodulated.
Circuits to detect data on the RFID labels already exist. Among the existing approaches, we can mention three types of circuit. A first class circuit uses voltage (level) displacer. The voltage level displacer operates displacing and/or amplifying the received signal package, as to facilitate signal demodulation by increasing the distance between the high part and the lower part of the package. A second class of circuits uses a mobile average to compare with the package. A third class of circuits uses a fixed reference voltage.
Circuits that use level shifters present problems in terms of distortion and consumption. The distortion problem is due to the maximum output voltage. Thus, if the input package is already high, it is not possible to increase the maximum high value (5) of the envelope (1) and the minimum high value (6) of the envelope (1) and the information can be lost, since the difference between the maximum high value (5) of the envelope (1) and the minimum high value (6) of the envelope (1) would decrease. The consumption problem occurs because the increase (voltage shifting) requires power, increasing circuit consumption.
Circuits using mobile average can present problems due to average variation. As the mobile average is not constant and varies with time. Thus, the intersection points between the package and the reference signal given by the mobile average can be either anticipated or delayed in relation and a constant ideal reference. This feature can lead to data loss due to loss of synchronism. Also, if the envelope stays too long in its maximum high value (5) or at its minimum high value (6) the average value used as reference can approach the value of the detected envelope itself (7), leading to false intersections between the average (used as reference) and the detected envelope (7). This problem is often treated with additional circuitry hysteresis, limiting the sensitivity, because in order to detect the difference between the maximum high value (8) of the detected envelope (7) and the minimum high value (9) of the detected envelope (7) must be higher than the hysteresis voltage value. Circuits that use fixed reference voltage can have problems with the envelope signal depth. If the fixed reference is greater than the maximum high value (8) of the detected envelope (7) or lower than the minimum high value (9) of the detected envelope (7), there is no intersection between the fixed reference and the detected envelope (7) and therefore the data is lost.
Furthermore, all three mentioned approaches have a linear or less than linear behavior regarding the detected envelope (7). It will be seen that the invention presents more than a linear gain by using a switching circuit that forces envelope detection upon approaching zero when the minimum high value (6) of the envelope (1) is detected.