Patent Application
20070030609
This invention relates to Radio Frequency Identification (RFID) readers.
The invention is better understood by reading the following detailed description with reference to the accompanying drawings in which:
RFID readers are rather unique among communication systems in that they typically transmit and receive simultaneously, on the same frequency, usually using a homodyne or superheterodyne receiver topology. In a monostatic RFID reader system (single antenna, combined transmit and receive), the fact that the RFID reader's transmitter is active while the receiver is connected to the same antenna introduces a potential failure mode. This is because a damaged cable connection between the reader and the antenna may result in reflection of most of the transmitter's output power back into the receiver's input port.
Since a typical RFID reader transmitter has an output power of about 1 Watt (+30 dBm) while the mixer burnout threshold for a typical RFID reader's front-end mixer is about 100 mW (+20 dBm), terminating a monostatic RFID reader's antenna port in a return loss of less than 10 dB can lead to mixer burnout within an extremely short time (nanoseconds to microseconds). An RF short circuit or an RF open circuit correspond to a return loss of 0 dB, meaning all of the transmitted power is reflected back to the receiver. A (typically safe) return loss of 10 dB would result in a reflection of only about 10% of the transmitted power back to the receiver. An unconnected or broken cable connection between the reader and the antenna can thus lead to catastrophic failure of the RFID reader's receiver because of excess reflected power.
A similar problem of receiver damage may occur in a bistatic RFID reader (separate transmit/receive antenna) if the reader's antenna is damaged in a way that results in unusually low transmit-receive isolation, or if an installer accidentally connects the reader's transmitter output to its receiver input.
Attempting to transmit into an un-terminated antenna port is not an infrequent event during installation or maintenance, when the reader's installer is busy connecting and disconnecting antennas. It can also occur if a cable or antenna is broken during use, for example by a forklift accident or by a human accidentally bumping into the antenna cables.
Accordingly, in one aspect, this invention provides methods and devices for preventing RFID receiver burnout by detecting a mismatched antenna port condition and reacting to sufficiently quickly (generally as quickly as possible) in order to prevent damage to the reader.
In another aspect, this invention provides an antenna integrity monitoring signal to the RFID reader's processor, so that a disconnected antenna will cause a fault notification from the RFID reader system to a human, or to a higher-level software system that manages RFID readers.
In the case of a bistatic (separate transmit/receive antennas) RFID reader, e.g., reader 18 as shown in
Solutions to this problem are shown in
A different scheme is employed in the system shown in
The methods shown in
In Mode 1, labeled “Hardware protection” in
It should be appreciated that any or all of these methods may be used singly or in combination to achieve the required response time to prevent receiver damage. The choice of which mode or modes are used is one of receiver design.
Certain elements in
In Mode 2, the mixer signal due to the reader's transmitter is either thresholded into a binary (single-bit) value, or digitized into a multi-bit digital representation of an analog voltage. This signal is then used as an input to the reader's microprocessor (or DSP) (not shown), for example as an input to a software power servo loop. In this case, during its operation, the software power servo loop checks the value of the mixer signal against a software threshold and does not permit the transmitter power to exceed the safe region of operation. Alternatively, the reader's software might check the mixer signal either periodically, or at any time the software changes the operating parameters of the reader hardware. Furthermore, this digitized mixer signal could also be used as an interrupt input to the microprocessor or DSP to signal a fault asynchronously.
These two modes (Mode 1 and Mode 2) have properties that make it desirable to employ them in concert. Clearly, if it is desirable for the reader to make a notification to the user that an antenna circuit fault has occurred, at least Mode 2 should be employed, to give the reader's microprocessor or DSP a signal that the mixer(s) are (or may be) on the verge of destruction. However, because of the finite processing speed required for the microprocessor to act on this notification and shut down the transmitter, it may be too late to prevent near-instantaneous destruction of the receiver system. This may be especially true if the reader's processor is busy with other processing tasks at the time the fault occurs.
The approaches of Mode 1 take the reader's processor out of the shutdown loop by connecting the mixer monitoring circuit to the transmitter directly, and by providing high-speed methods of inhibiting the transmitter, such as by disabling the power amplifier (Mode 1b) or by switching off various signal paths (Mode 1a or 1c). These high speed switching tasks can be performed on a time scale (nanoseconds to microseconds, depending on the particulars of the design) which can act in sufficient time to save the receiver from destruction. However the Mode 1 approaches, if not used in combination with Mode 2, do not include notification of the reader's processor that a fault has occurred. Thus, a combination of the two modes should preferentially be employed.
It should be appreciated that there are many ways of detecting excess transmitter power impinging upon the mixer(s), including measuring mixer current or voltages using a high impedance opamp (operational amplifier) or comparator circuit, employing a transimpedance amplifier or resistor to convert mixer current to voltage, or employing a peak detector circuit to capture the peak value of such a current or voltage.
Furthermore, the mixer signal may be either sliced to a binary indication (mixer signal exceeds safe values, or not) or it may be digitized by an analog-to-digital converter (ADC) and its derivative or absolute value examined by either a hardware or software means.
Still further, there are many ways of disabling the transmitter including those mentioned (switching the signal paths, or the power amplifier bias), or by employing any means known to switch either DC or RF signals, including FETs (field-effect transistors), bipolar transistors, PIN (Positive-Intrinsic-Negative) diodes, switching diodes, relays, etc.
Additionally, should a low noise amplifier (LNA) or other fragile active or passive device be employed in the circuit ahead of the mixer(s), the threshold of damage to those components can be used instead of the mixer threshold of damage, should they be more vulnerable than the mixer(s) in any given design.
This approach is equally useful either in the monostatic case shown in
The argument about mixer current monitoring also applies to the use of the LNA or other fragile device such as an integrated circuit, diode, or transistor, (or, in fact, any device having a non-linear transfer function), as a mixer for the purpose of extracting the burnout monitoring signals.
It should also be appreciated that combinations of these approaches, as implemented either in the analog or digital domain, or in hardware or software, have been explicitly recognized and contemplated herein. The integration of these functions into an integrated circuit is also contemplated.
It should also be appreciated that the approaches described with reference to
The present invention thus provides a relatively simple and straightforward method of adding a significant level of burnout protection to an RFID reader front end. It is also an inexpensive method, since it makes use of the existing mixer elements (or the parasitic mixers formed by the LNA or other nonlinear devices) to provide the signals needed to determine whether their own burnout is imminent. Furthermore, the reaction time of this circuit can be extremely fast, especially if a direct feedback from the monitoring circuit to the transmitter circuit is employed, so that the front end is exposed to an excessive-power condition for the minimum possible time and thus maximizing their chance of survival.
Thus is provided description of the invention, and of the manner and process of making and using it. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
