IN-PHASE AND QUADRATURE DEMODULATOR FOR RFID SYSTEM WITHOUT DIRECTIONAL COUPLER
FIELD OF THE INVENTION
The present invention relates to an object management system wherein information bearing electronically coded radio frequency identification (RFID) tags are attached to objects which are to be identified, sorted, controlled and/or audited. In particular the present invention relates to a demodulator for deriving or decoding information included in reply signals that backscatter from the tags.
BACKGROUND OF THE INVENTION
The object management system of the present invention may include information passing between an interrogator which creates an electromagnetic interrogation field, and electronically coded tags, which respond by issuing a reply signal that is detected by the interrogator, decoded and consequently supplied to other apparatus in the sorting, controlling or auditing process. The objects may be animate or inanimate. In some variants of the system the interrogation medium may be other than electromagnetic, such as optical and/or acoustic.
Under normal operation the tags may be passive having no internal energy source and may obtain energy for their reply from the interrogation field, or they may be active and may contain an internal energy source, for example a battery. Such tags respond only when they are within or have recently passed through the interrogation field. The interrogation field may include a function of signalling to an active tag when to commence a reply or series of replies, or may in the case of passive tags provide energy, a portion of which may be used in constructing the reply.
An illustration of a simple electronic tag reading system is provided in FIG. 1A. The present invention considers tags that backscatter a portion of the incident RF energy. Such backscattered signals may be achieved by periodically loading and/or detuning the antenna of the tag, in accordance with an identity code representing data stored in the tag. The periodic loading may be resistive and/or capacitive and may result in modulation represented by a sub-carrier, or sub-carriers related to a carrier frequency. There may be plural mechanisms to produce the sub-carrier(s). For example, the sub-carrier may be produced by employing an on chip oscillator, or by dividing down an incident RF carrier. The modulation waveform used to switch the load may include the sub-carrier(s), suitably modulated, for example by Amplitude Shift Keying (ASK), Phase Shift Keying (PSK) or Frequency Shift Keying (FSK), or derivatives of such schemes, and is commonly referred to as a base-band signal. The base-band signal should be clearly reproduced in the interrogator's receiver in order to extract and process data from tag.
A desirable feature of the system is that circuits employed within the interrogator are able to process the relatively weak signal backscattered or received from the tag. The received signal is likely to be in the range of pico (10"12) to femto (10~15) Watts. Typically the received signal may be of the order of 10μV and may be impressed upon a transmitted signal of the order of 60V peak to peak. Because the interrogator contains within it both the transmitter and receiver (refer FIG 1 ), the received signal is typically mixed with a local oscillator to extract a copy of the original base-band signal used by the tag to modulate the backscattered signal. The local oscillator has a phase relationship with the transmitted signal, and consequently a phase relationship with the received backscattered signal. If a single channel receiver is used to demodulate the signal by mixing the derived received signal with the derived local oscillator, the physical position of the tag with respect to the antenna and the length of the antenna cable, may be such that it results in the derived received signal being in phase with the derived local oscillator. In this situation the output of the mixer will be nil, and the tag is commonly referred to as being in a null. To avoid the above situation, and to cater for variations in cable lengths used to connect the antenna, two channels designated I and Q respectively may be used. One channel (I) signal backscattered from the tag, may be in phase with the local oscillator and the other channel (Q) signal, may be in quadrature with the local oscillator to detect the above described modulations. In this arrangement, if one channel results in the tag being in a null, the other channel will always be at a maximum.
SUMMARY OF THE INVENTION
The demodulator of the present invention may make use of different mechanisms to derive the in-phase (I) and quadrature (Q) base-band signals respectively. The I and Q base-band signals may be derived by considering the entire forward and return signal paths as forming a distributed transmission line. The forward signal path includes the path from the transmitter power amplifier, through the transmitter's output and/or band-pass and low pass filters through the interrogator antenna and via the transmission medium (air) to the tags antenna. The return signal path includes the path from the tag's antenna, via the transmission medium, through the interrogator antenna and into the receiver's and/or transmitter's filters. Significantly, the demodulator of the present invention requires no directional coupler to isolate the signal transmitted via the antenna, from the return or received signal. Hence no transmit signal needs to be cancelled, for quality performance of circuitry.
The I channel may be derived by means of an amplitude to voltage converter. In one form the I channel may be derived by amplitude detecting the received signal at a well-chosen point in the transmitter's output path such as the low pass filter. Modulation produced by variation of the tag's impedance may influence amplitude and/or phase in the radio frequency signal reflected from the tag. When the reflected signal enters the same path that is used by the transmitted signal, it gives rise to an effect commonly known as a standing wave.
The optimum tapping point on the transmission line for the I channel is the point at which the Voltage Standing Wave Ratio (VSWR) is at a maximum when a tag is present in the interrogation field. The VSWR to be detected may be in the range 1 :1000001. In one form the tapping point may be chosen by selecting and/or adjusting components employed in the transmitter's output path filter. Amplitude detection may exploit a known amplitude detecting circuit such as a diode and capacitor or at least one non-linear amplifier. However, a preferred form of detection for this application may be via the base emitter junction of a low noise bipolar transistor, whereby the base-band signal may be available at its collector output. Noise in the I channel may be minimised by finely adjusting the selected filter components.
The Q channel may be derived by means of a phase to voltage converter. In one form the Q channel may be derived by multiplying an appropriately phased version of the received signal obtained from a different phase point in the transmitter output path, or the receiver path filter, in a double gate FET or a logic AND gate or a Schmitt Trigger circuit or circuits, with a reference signal derived from the transmitter local oscillator.
The optimum tapping point on the transmission line for the Q channel is at an inflection point of the VSWR. Both I and Q tapping points may be defined by signals present on the forward signal path from the transmitter's power amplifier, through filters, antenna, and ending on the tag, and the return signal path from the tag, antenna, through filters, and back to the transmitter's power amplifier.
According to one aspect of the present invention there is provided a demodulator for deriving, without use of a directional coupler, a base band signal received on a transmission line that connects a transmitter to an antenna including means for obtaining a signal from at least one tapping point on said transmission line; and means for mixing said obtained signal with a reference signal associated with a local oscillator of said transmitter.
According to a further aspect of the present invention there is provided a method of demodulating, without use of a directional coupler, a base band signal received on a transmission line that connects a transmitter to an antenna including obtaining a signal from at least one tapping point on said transmission line and mixing said obtained signal with a reference signal associated with a local oscillator of said transmitter. DESCRIPTION OF A PREFERRED EMBODIMENT
A preferred embodiment of the present invention will now be described with reference to the accompanying drawings wherein:
FIGS. 1A and 1 B show common electronic tag reading systems to which the present invention may be applied;
FIG. 2 shows a block diagram of a typical interrogator;
FIG. 3 shows one embodiment of a practical implementation of the interrogator;
FIG. 4 shows a waveform associated with the standing wave on the transmission line; and
FIG. 5 shows an alternative embodiment of a practical implementation of the interrogator.
FIG.1A shows a conventional electronic tag reading system employing separate transmitting and receiving antennas. FIG. 1 B shows an alternative arrangement in which the two antennas in FIG. 1A are replaced with a common transmitting/receiving antenna.
Referring to FIG. 2, the interrogator includes a logic device drive (LDD) 20 connected to power amplifier 21. Power amplifier 21 is connected to transmitting (or combined transmitting/receiving) antenna 22 via output transformer 23, impedance matching circuitry 24, low pass filter 25, notch filter 26 and band pass filter 27. As noted above the signal path from power amplifier 21 to the tag antenna and back again may be considered and analysed as a distributed transmission line. That being so, the points marked 1-5 including intermediate points along the transmission line may be potential tapping points for processing in respective I and Q channel detectors. Fine adjustment of phase at the tapping points may be obtained by employing existing components in elements 21-27 along the transmission line or by introducing additional components into the transmission line as required to accurately adjust the phase at the tapping points. It will be clear to the skilled addressee that the location and nature of such components depends largely on the detailed solution that is required.
FIG. 3 shows one form of LDD 20 including clock generator 30 and divider 31 driving a plurality of AND gates configured to generate in phase and out of phase " T" pulses for driving transmitter power amplifier 21 arranged in a push- pull configuration. The output of power amplifier 21 is connected to an antenna (not shown) via output transformer 23, notch filter 26 and matching circuit 24. Matching circuit 24 comprising three inductors and two capacitors also provides a delay line function to the l-channel detector.
The l-channel is derived by means of amplitude to voltage converter 35. Amplitude to voltage converter 35 utilizes the base emitter junction of low noise bipolar transistor 36. The base or input of transistor 36 is connected to a tapping point on the transmission line at which the VSWR is selected by judicious choice of component values to be at a maximum (refer FIG.4). The output of the l-channel detector is available at collector 37 of transistor 36 and represents a base-band signal.
The Q-channel is derived by means of phase to voltage converter 38. Phase to voltage converter 38 utilizes double gate FET 39. One gate of FET 39 is connected via delay line 40 to a tapping point on the transmission line at which the VSWR is at a point of inflection (refer FIG. 4). The second gate of FET 39 is connected to a frequency and timing reference derived from LDD 20 as shown in FIG. 3. The timing reference is associated with the local oscillator signal generated in LDD 20 of the transmitter. To minimize phase noise or jitter it is desirable that the timing reference be obtained or tapped from a signal stage of the transmitter that is relatively close to a driving input to power amplifier 21. The output of the Q-channel detector is available at drain 41 of FET 39 and represents a base band signal. FIG. 5 shows an alternative embodiment of a practical implementation of the interrogator of FIG. 3 in which amplitude to voltage converter 35 comprises an inverter and phase to voltage converter 38 comprises a dual input logic (AND) gate. Phase to voltage converter 38 may alternatively be implemented with two Schmitt Trigger circuits.
Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention.