AT501055A4 - METHOD OF DEMODULATING AND READING DEVICE FOR EVALUATING LOAD MODULATED SIGNALS - Google Patents

Description

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The invention relates to a method for the demodulation of load-modulated signals in a system in which a reading device delivers a high-frequency carrier signal to a transmitting antenna, passive transponders in the area of influence of the antenna receive the signal and load-modulate the received signal and in the reading device the retroactively modulated RF Signal is demodulated to evaluate the modulation signal.

Likewise, the invention relates to a reading device for evaluating load-modulated signals, with an RF generator for supplying an antenna with a carrier signal, a demodulation circuit and an evaluation circuit.

Transponders and readers, in English usage and mostly "RFID tags". and "Reader" called, are known and used in many fields, eg. For example, in transportation, finance, access control, etc. Systems and application examples are described in, for example, " RFID Handbook " Klaus Finkenzeller, John Wiley & Sons, Ltd., Second Edition 2003, ISBN 0-470-84402-7.

A prior art system for reading tags is shown in FIG. A reading device LES (Reader) with transmitting antenna supplied via a magnetic alternating field, eg. B. in 13.56 MHz band, on the one hand transponder (tags) TRA, which also have an antenna, with energy (passive tags) and on the other hand, the reader LES can communicate with the transponder TRA (downlink). The RF signal taken from a transponder TRA is rectified and with the rectified voltage can z. B. an energy storage to supply the transponder electronics are charged. If a transponder has absorbed enough energy, it can receive signals from a reader. However, it can also send signals to the reader using a load modulation retroactively applied to the carrier (uplink). In the aforementioned RFID Handbook " RFID Handbook " is such a load modulation described in connection with Figure 3.16 on page 45.

A load modulation causes the uplink in the reader a signal that can occur in different modes, either as amplitude modulation or as a phase modulation or as a hybrid of the two types of modulation. If a signal with amplitude modulation is present, the demodulation is possible with little technical effort with the aid of a peak detector (envelope detector, diode detector). However, this method fails for signals that are predominantly phase-modulated. FOLLOWED! he

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Figs. 2a and 2b show the case of pure amplitude modulation. At the time shown in FIG. 2a, the carrier signal st and the modulation signal sm are in phase and overlap each other constructively to form a resulting signal sr. At the time shown in Fig. 2b, carrier st and modulation sm destructively interfere with each other to produce a signal sr. As a result, the signal sm occurs as an amplitude modulation in the signal sr.

Figs. 3a and 3b show the case of pure phase modulation. At the time given in FIG. 3a, the modulation sm is lagging 90 degrees with respect to the carrier st and a resulting signal sr results. At the point in time relevant to FIG. 3b, the modulation sm is leading by 90 degrees relative to the carrier st and leading it the result is a signal sr. However, the lengths of the resulting pointers in FIGS. 3a and 3b do not differ, so that the modulation is not recognizable in amplitude either.

The occurrence of both types of modulation is in particular when using multiple readers (M readers LES in Fig. 1), which are either also (mutually phase-shifted, but frequency-synchronous) are driven or used only as a receiver, inevitable. Such systems are used, for example, to generate spatially varying field vectors in order to avoid orientation-related (reader-antenna to tag-antenna orientation) detection gaps. When using multiple readers and the maximum distance at which a communication between the reader and tags is possible, in short, the range can be increased.

However, the occurrence of both types of modulation can also be a consequence of the interaction between multiple transponders and readers.

The prior art knows two methods for the mode-independent demodulation of load modulated signals. The first method uses synchronous sampling, placing high demands on the electronics. The second method uses a cable of a certain length to achieve a mode conversion (from phase modulation to amplitude modulation), thereby enabling cost effective demodulation with the aid of diodes. A mode conversion can also be realized by means of filters. For readers whose size must be kept low, the use of additional cables proves naturally disadvantageous. On the other hand, filters have to show a very pronounced frequency behavior in order to bring about the desired mode conversion, which alone is associated with considerable technical effort because of the temperature dependence of the electronic components.

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The stated problems that the load modulation can occur as both amplitude and phase modulation and in particular the different phase position of the uplink signal in the use of multiple out of phase antennas is inevitable, are known and methods have been proposed to solve these problems.

All known approaches broadly use I (in-phase, 0 °) and Q (quadrature, 90 °) signals to successfully demodulate both in amplitude modulation (mostly with I-channel) and in phase modulation (mostly with Q-channel) achieve. It is always possible to select I and Q sample signals so that the maximum in one channel and no signal in the other channel occurs. Therefore, there are also realizations, which use only one channel, in which the demodulator is to be regarded as I or Q demodulator depending on the signal.

According to WO 04/107595 A1, I and Q signals are determined by means of a zero-cross detector module, a phase shifter and choppers and filter components and the demodulation is carried out with the aid of a mixer, wherein the mixture is carried out with two scanning signals shifted by 90 °.

US Pat. No. 6,684,830 also describes a system in which I / Q demodulation is used, the demodulation likewise taking place with the aid of a mixer and subsequent filtering

In contrast, no mixer is needed in the present invention. The demodulation of I and Q channel can be realized with a simple envelope demodulator (eg diode with filter capacitor). Even zero-cross detectors and fast switches (choppers) are not needed.

According to WO 03/049275 A1, I and Q are sampled by tapping at specific locations within the signal path. With the aid of these signals, a sampling (sampling) takes place, there is thus a synchronous sampling with two scanning signals shifted by 90 °. Such a sampling also applies to WO 02/23465 A2, according to which a suitable sampling time is determined in order to successfully demodulate both in phase and amplitude modulation. Another example of synchronous sampling is provided by US 6028503, according to which only a synchronous demodulator is used whose sampling times are shifted depending on the phase position of the signal to be detected.

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Furthermore, WO 96/18969 describes a method for the demodulation of signals with possible occurrence of both types of modulation, the demodulation taking place with the aid of a phase demodulator, also in conjunction with an envelope demodulator

JP 63052082 discloses a method in which I and Q signals are derived from the modulated signal with the aid of two mixers, the signals obtained subsequently being fed to an envelope demodulator and the stronger of the two output signals being used for further processing.

An object of the invention is to demodulate load-modulated signals with little effort, both in terms of cost and physical dimensions.

This object is achieved with a method of the type mentioned, in which according to the invention a frequency synchronous, a phase angle shifted subcarrier is added, both the resulting sum signal and the carrier signal are subjected to amplitude demodulation and the demodulated signals are further processed for evaluation ,

A suitable further processing provides that of the two demodulated signals selected that with the higher signal quality and further processed for evaluation.

In most cases, the flexibility of processing is increased when at least one of the demodulated signals undergoes analog-to-digital conversion.

It is advantageous in view of a sufficiently good sum signal, when the phase angle between the carrier signal and subcarrier is selected in a range of 120 ° to 150 °.

In practice, it has proved to be advantageous if the phase angle between the carrier signal and the subcarrier and / or the amplitude of the subcarrier are chosen so that the phase between the carrier signal and the resulting sum signal is substantially 90 °

Likewise, the object underlying the invention is achieved with a reader of the subject type, which is characterized according to the invention by an auxiliary signal generator for generating a frequency synchronous to the carrier signal, displaced by a phase angle subcarrier, a summing circuit for summing the

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Carrier signal with the subcarrier, a demodulator for demodulating the carrier signal applied to the antenna, and a demodulator for demodulating the summation signal generated in the summation circuit.

An advantageous development of a reading device according to the invention is characterized in that a selection circuit is provided, to which the two signals demodulated in the demodulators are fed and whose output signal is fed to the evaluation circuit.

The invention together with further advantages is explained in more detail below by way of example embodiments, which are illustrated in the drawing. In this show

1 schematically shows a system with readable, passive transponders, as it corresponds to the prior art,

2a and 2b at two times in vector diagrams carrier and modulation at pure amplitude modulation,

3a and 3b at two times in vector diagrams carrier and modulation pure phase modulation,

Fig. 4a and 4b in vector diagrams, the method according to the invention and Fig. 5 is a block diagram of a reading device according to the invention.

In the following description of the invention, reference is made to FIGS. 4 a, 4b and 5, wherein first the block diagram of FIG. 5 is prefixed. An RF generator GEN fed with a frequency fo possibly via a driver amplifier VER with a carrier signal st an antenna ANT, in whose magnetic field one (or more) transponder TRA may be.

With the aid of an auxiliary signal generator HSG, which is also fed by the RF generator GEN, a to the carrier signal st frequency synchronous, to a phase angle φ shifted subcarrier sh generated. The auxiliary signal generator HSG contains a controllable phase shifter which adjusts a 90 ° phase between the carrier signal and the resulting sum signal sr (see below FIGS. 4a, b) as a function of signals supplied thereto, namely the carrier signal st and a resulting sum signal sr (see below) ,

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The output signal of the Hüfsignalgebers HSG, the subcarrier sh is supplied together with the carrier signal st a summing SUM.

There are now two signals available, which can be referred to as I and Q signals. Of these signals, the carrier signal st is a demodulator DEI and the

Subcarrier sh or subcarrier signal supplied to a demodulator DEQ Both demodulators may be simple envelope demodulators based on a diode.

The output signals of the demodulators DIE, DEQ, the demodulated signals si, sq be performed, optionally after an analog / digital conversion in an analog / digital converter ADW to a selection circuit SEL, which z. B. selected on the basis of known signal quality criteria that signal which is best suited for further processing. This selected signal Sn is used in an evaluation circuit ASW in a known manner, for. For identification of tickets, goods etc.

The method used is shown in FIG. 4 a at a first time and in FIG. 4 b at a second time in analogy to FIGS. 2 and 3. While the load-modulated signal sm remains unchanged from the addition, the carrier sr resulting from the carrier signal st and subcarrier sh, which is now amplitude-modulated in the case shown, rotates its position. Both the voice signal and the unchanged carrier are fed to a simple diode demodulator as described above. The described method ensures that at least in one of the two channels I, Q, the load-modulated signal occurs as an amplitude modulation and the demodulation is successful even with only one diode.

With the aid of the auxiliary carrier, however, the resulting sum signal can also be reduced compared with the original signal (FIG. 3), while the load-modulated signal remains unchanged. The ratio of amplitudes of the load modulated signal sm and the resulting sum signal Sr is called a modulation degree or a modulation index. A higher degree of modulation is advantageous in demodulation. For this purpose, further sums can be formed with further auxiliary carriers. For example, the subcarrier may have exactly reversed phase of the carrier and thus also be used in an event as shown in Fig. 4 to increase the modulation index.

The described, here in the block "auxiliary signal generator HSG". contained phase shifter is therefore controllable with advantage, since the load modulation amplitude and phase influenced, which may also be the case that at a certain amount of the subcarrier and

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Vienna, the 7th Λριίΐ 200ΕΓ

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Claims (7)

ß & rs? ···· ··· ······ in which a reading device (LES) delivers a high-frequency carrier signal (ST) to a transmitting antenna (ANT), passive transponders (TRA) in the area of influence of the antenna receive the signal and load-modulate the received signal and in the reading device the retroactive modulated RF signal is demodulated for evaluating the modulation signal, characterized in that a frequency-synchronous subcarrier (sh) shifted by a phase angle (φ) is added to the carrier signal (st), both the resulting sum signal (sr) and the carrier signal (st) of an amplitude demodulation be subjected and the demodulated signals (si, sq) are further processed for evaluation. 2. The method according to claim 1, characterized in that of the two demodulated signals (si, sq) that selected with the higher signal quality and further processed for evaluation. 3. The method according to claim 1 or 2, characterized in that at least one of the demodulated signals (si, sq) is subjected to an analog / digital conversion. 4. The method according to any one of claims 1 to 3, characterized in that the phase angle (φ) between the carrier signal (st) and subcarrier (sh) is selected in a range of 12o ° to 150 ° 5. The method according to any one of claims 1 to 4, characterized in that the phase angle (φ) between the carrier signal (st) and subcarrier (sh) and / or the amplitude of the subcarrier (sh) are selected so that the phase between the carrier signal ( st) and resulting sum signal (sr) is substantially 90 °. 6. reader (LES) for evaluating load-modulated signals, with an RF generator for feeding an antenna (ANT) with a carrier signal (st), a demodulation circuit (DEM) and an evaluation circuit (AWS), -9- # • ♦ · · Characterized by an auxiliary signal generator (HSG) for generating a frequency synchronous to the carrier signal (st), displaced by a phase angle (φ) subcarrier (sh) , a summing circuit (SUM) for summing the carrier signal (st) with the subcarrier (sh), a demodulator (DEI) for demodulating the antenna (ANT) carrier signal (st), and a demodulator (DEQ) for demodulating the Summing circuit (SUM) generated sum signal (sr). 7. Reading device according to claim 6, characterized in that a selection circuit (AWS) is provided, to which the two in the demodulators (DIE, DEQ) demodulated signals (si, sq) are supplied and whose output signal of the evaluation circuit (AWS) is supplied. .Wien, dm 07 april? 0fF