DE102008013468A1 - Adjustable filtering circuit for use in radio frequency identification reader device, has band-stop filter arranged in downstream side of one of the mixing stages, for suppressing carrier signal in intermediate frequency signal - Google Patents

Abstract

The adjustable filtering circuit (300) has an adjustable oscillator (314) that generates an oscillator signal, and input mixing stages (318,320) that mix input signal with the oscillator signal, to generate an intermediate frequency signal. A band-stop filter (322) is arranged in the downstream side of one of the mixing stages, for suppressing carrier signal in the intermediate frequency signal. An independent claim is included for radio frequency identification (RFID) reader device.

Description

RFID systems are used in many different areas of automatic identification. Particularly in the field of industrial automation, the use of RFID systems is advantageous. The growing demands of quality management and quality assurance in automated production can be easily realized with RFID systems. Production and measurement data can easily be stored and carried along in the production on the object itself. They are thus available and documented at any time on any object. This also makes the manufacturing more flexible. Production parameters can be stored on the object and directly read out in the respective street. Objects can be taken out of the series at any time and added later without confusion. A failure of the central computer also poses no problem in terms of the unique identification and localization of the objects by the decentralized storage of the data on RFID tags ( Klaus Finkenzeller: RFID Handbook, 3rd edition, Carl Hanser Verlag, Munich ).

in principle In RFID systems, a distinction is made between coupled and microwave systems. at the coupled systems, the data exchange between the Reader unit and tag through capacitive or inductive coupling. These systems typically operate at the frequencies of 135 kHz or 13.56 MHz. In the microwave systems, a non-reactive, electromagnetic coupling via the far field of an antenna. Here, the frequency ranges 860-930 MHz, 2.45 GHz and 5.8 GHz.

One essential part of the available RFID systems work according to the so-called backscatter (or backscatter) method. In these systems, the reader unit sends a constant, sinusoidal Carrier signal (English continuous wave, CW), the day with Energy supplied. Furthermore, a part of this carrier signal reflected on the day. About changing the antenna backscatter properties the tag modulates the data to be transferred to this Carrier signal, which acts as sidebands around this signal are recognizable in the frequency range.

• 1. Funkfelddämpfung α dB / r in Dezibel in Abhängigkeit der Entfernung r und der Wellenlänge λ (λ = c/f):
  
• 2. Empfangsleistung nach der Tagantenne P dBm / te in dBm in Abhängigkeit der Sendeleistung der Readereinheit P dBm / rs in dBm, vom Gewinn der Sendeantenne G dB / s in Dezibel, vom Gewinn der Tagantenne G dB / t in Dezibel sowie der Funkfelddämpfung a in Dezibel:
  
• 3. Sendeleistung des Tags Ps dBm / ts in dBm in Abhängigkeit der Empfangsleistung des Tags, der Modulationsverluste am Tag P dBm / mod in Dezibel:
  
• 4. Empfangsleitung an der Readereinheit P dBm / re in dBm in Abhängigkeit der beschriebenen Parameter:
  

Based on the well-known equations for the field propagation, the essential reception and transmission powers of a transmission path can be described as follows:

• 1. Field attenuation α dB / r in decibels as a function of the distance r and the wavelength λ (λ = c / f):
  
• 2. Reception power after the tag antenna P dBm / te in dBm as a function of the transmission power of the reader unit P dBm / rs in dBm, from the gain of the transmitting antenna G dB / s in decibels, from the gain of the tag antenna G dB / t in decibels and the radio field attenuation a in decibels:
  
• 3. Transmission power of the tag Ps dBm / ts in dBm as a function of the received power of the tag, the modulation losses on the day P dBm / mod in decibels:
  
• 4. Receive line at the reader unit P dBm / re in dBm depending on the described parameters:
  

Furthermore, a system dynamic D required for the distance can be specified in decibels over the difference between the transmission power P dBm / rs of the received power P dBm / re of the reader unit:

In the standards ISO-IEC 18000-6C such as ETSI EN 302 208-1 The UHF RFID systems are regulated in the frequency range 860 MHz-930 MHz. In this case, for example, a maximum transmission power of 2 W ERP (equivalent radiated power) as well as the modulation methods to be used are specified.

• – Sendeleistung 2 W ERP,
• – Betriebsfrequenz 866,5 MHz,
• – Antennengewinne der Readerantenne und der Tagantenne von jeweils 6 dBi und 1 dBi,
• – Modulationsverluste von 6 dB
• – sowie Reflexionsverluste von 0,5 dB

ergibt sich eine Eingangsleitung von ca. –19,7 dBm mit einer korrespondierenden notwendigen Systemdynamik von 55,9 dB als Differenz zwischen der Sendeleistung und der Empfangsleistung für diese Entfernung. Jede Verdopplung der Entfernung führt zu einer Reduzierung der Eingangsleistung um 12 dB, korrespondierend zu einer 12 dB höhere Systemdynamik.For a distance of one meter, this results in a radio field attenuation of about 31.2 dB, the increases with each doubling of the transmission distance by a factor of 6 dB. Under realistic conditions:

• - transmission power 2 W ERP,
• Operating frequency 866.5 MHz,
• Antenna gains of the reader antenna and the tag antenna of 6 dBi and 1 dBi respectively,
• - Modulation losses of 6 dB
• - and reflection losses of 0.5 dB

results in an input line of about -19.7 dBm with a corresponding necessary system dynamics of 55.9 dB as the difference between the transmission power and the received power for this distance. Each doubling of the distance results in a 12 dB reduction of the input power, corresponding to a 12 dB higher system dynamic.

The backscatter method simultaneously sends the carrier signal and receives the tag response (duplex mode). This can be realized by using a circulator ( 1 ). The existing isolation or crosstalk between the transmit port and the receive path is an important system feature and contributes greatly to system dynamics. For commercially available circulators, the crosstalk is in the range of 20 to 30 dB, so that without the use of further measures to increase this value, the weaker day responses are "overshadowed" by the crosstalk and therefore no longer error free to demodulate.

The The invention described in this document allows a Suppression of the crosstalk from the carrier signal in the input path combined with a simultaneous frequency-selective Filter the operating channel, especially in a multi-reader environment necessary is. This will cause the system dynamics to be around Suppression of the carrier signal increases, resulting in an increase in range or improvement the bit error rate at the same distance results.

One Another important aspect in the dimensioning of a transmitting / receiving system is the calculation of the input noise power N, which is directly proportional to the system bandwidth .DELTA.f. For commercially available Systems are usually acoustic surface filters (SAW filter) used to pass the operating band. By the use of the invention described in this document, the effective filter bandwidth from the bandwidth of the operating channel be reduced to the effective channel bandwidth. This can the input noise by a factor between 10 and 100 for each RFID systems at 868 MHz or 2.45 GHz are reduced.

State of the art

• 1. Der Einsatz von zwei getrennten Antennen zum Senden und Empfangen anstatt einem Zirkulator und einer kombinierten Sende-/Empfangsantenne. Hierbei kann durch eine optimale Ausrichtung der einzelnen Antennen sowie der Einsatz von Dämpfungselementen an geeigneten Stellen der jeweiligen Antennengehäuse der Übersprecher zwischen dem Sende- und dem Empfangspfad weiter verbessert werden. Praktische Werte liegen hierbei um –40 dB. Nachteilig bei dieser Vorgehensweise ist die Empfindlichkeit des Übersprechers bezüglich Umwelteinflüssen in der Nähe des Systems oder des Tags, wie größeren reflektierenden Flächen, die zu einem großen Übertragungsfaktor zwischen der Sende- und der Empfangsantenne führen, der als Störsignal die schwache Tag-Antwort überschatten kann.
• 2. Erweiterung der Schaltung durch ein festes, parallel geschaltetes Bauelement, das zusammen mit dem Ersatzschaltparameter des Übersprechers einen Resonanzkreis mit einer Resonanzfrequenz bei der Betriebsfrequenz bildet, wodurch der Übersprecher für die Betriebsfrequenz in Abhängigkeit der Güte des eingesetzten Elements um einige Dekaden zu kompensieren ist. Dieser Ansatz kann mit dem Einsatz von wenigen SMD-Komponenten realisiert werden und ist kostengünstig. Der Nachteil dieses Vorgehens besteht – in der invarianten Frequenzlage der Kompensation, wodurch der Einsatz auf eine einzige Frequenz beschränkt wird, – im geringen Maß der Kompensation, der durch der diskreten Werten der verfügbaren Komponenten hervorgerufen wird, – in der Inflexibilität der Kompensation im Bezug auf Änderungen an der Sende- und/oder der Empfangsantenne – sowie in der schmalbandigen Kompensation, wodurch eine starke Abhängigkeit der absoluten Frequenzlage der Kompensation von den Fertigungstoleranzen entsteht, die dazu führt, dass die tatsächlich bei der Betriebsfrequenz erreichbare Verbesserung der Isolation stark variieren kann.

Known approaches for reducing the carrier signal level in the receive path are limited primarily to the suppression of the crosstalk between the transmit and the receive path. Which includes:

• 1. The use of two separate transmit and receive antennas instead of one circulator and one combined transmit / receive antenna. This can be further improved by an optimal alignment of the individual antennas and the use of damping elements at suitable locations of the respective antenna housing, the crosstalk between the transmission and the reception path. Practical values are -40 dB. A disadvantage of this approach is the sensitivity of the crosstalk to environmental influences in the vicinity of the system or the tag, such as larger reflective surfaces, which lead to a large transmission factor between the transmitting and the receiving antenna, which can overshadow the weak tag response as an interference signal.
• 2. Extension of the circuit by a fixed, parallel-connected component, which forms a resonant circuit with a resonant frequency at the operating frequency together with the equivalent switching parameters of the crosstalk, whereby the crosstalk for the operating frequency depending on the quality of the element used to compensate for several decades. This approach can be realized with the use of a few SMD components and is inexpensive. The disadvantage of this approach is - in the invariant frequency position of the compensation, which limits the use to a single frequency, - in the small amount of compensation caused by the discrete values of the available components, - in the inflexibility of the compensation with respect to Changes to the transmitting and / or the receiving antenna - As well as in the narrow-band compensation, whereby a strong dependence of the absolute frequency position of the compensation of the manufacturing tolerances arises, which means that the actually achievable at the operating frequency improvement of the insulation can vary widely.

By the use of components with variable values, such as Varactor diodes, may have some flexibility regarding frequency position and compensation. Practical values are -35 dB.

• 1 Durch den Einsatz einer digitalen Signalverarbeitung durch beispielsweise DSPs (Digital Signal Processors) ist eine schmalbandige Unterdrückung von bis zu –80 dB möglich. Weiterhin kann bei diesem Ansatz frequenzselektive Filter mit unterschiedlichen Bandbreiten realisiert werden. Nachteilig bei einer digitalen Signalverarbeitung ist einerseits der höhere In vestitionsbedarf, sowohl für die Hardwareanschaffung als auch für die bei der Software-Erstellung benötigte Entwicklungszeit und zugehörigen Kosten. Andererseits beanspruchen die Hardware relativ viel Platz und ist, in abhängig von den zu realisierenden Anforderungen, wesentlich langsamer als eine analoge Schaltung.
• 2 Eine weitere Möglichkeit zur Realisierung einer verbesserten Unterdrückung des Trägersignals im Empfangspfad wird in „System combining radio frequency transmitter and receiver using circulator and method for cancelling transmission signal thereof” ( US Patent 6,567,648 B1 vom 20.05.2006) beschrieben. Hierbei wird eine Unterdrückung des Übersprechers dadurch realisiert, dass ein Teil des Empfangssignals („Rx + Tx” in den Bildern 3 und 4) ausgekoppelt und in der Phase und Amplitude derart geändert wird, dass die Anteile des Trägersignals im ursprünglichen Empfangssignal und in diesem ausgekoppelten Signalpfad der gleichen Amplitude, jedoch um 180° in der Phase verschoben, zusammen geführt werden können. Hierbei wird eine Filterung des Rx-Signals notwendig (Komponente 24 in den Bildern 3 und 4), die aufgrund des geringen Frequenzversatzes zwischen dem Trägersignal und der Tag-Antwort beim Backscatter-Verfahren in RFID-Systemen nur durch eine digitale Signalverarbeitung realisierbar wäre.

There are still few approaches to suppress the carrier signal in the receive path:

• 1 The use of digital signal processing such as DSPs (Digital Signal Processors) enables narrowband suppression of up to -80 dB. Furthermore, in this approach, frequency-selective filters with different bandwidths can be realized. A disadvantage of a digital signal processing on the one hand, the higher investment needs, both for the purchase of hardware as well as for the software development required development time and associated costs. On the other hand, the hardware occupy a relatively large amount of space and, depending on the requirements to be realized, is much slower than an analog circuit.
• A further possibility for realizing an improved suppression of the carrier signal in the receive path is described in "system combining radio frequency transmitter and receiver using method for canceling transmission signal thereof" (US Pat. US Pat. No. 6,567,648 B1 of 20.05.2006). Here, a suppression of the crosstalk is realized in that a part of the received signal ("Rx + Tx" in Figures 3 and 4) is coupled out and changed in phase and amplitude such that the shares of the carrier signal in the original received signal and in this decoupled Signal path of the same amplitude, but shifted by 180 ° in phase, can be performed together. This requires a filtering of the Rx signal (component 24 in Figures 3 and 4), which would be realized only by digital signal processing due to the low frequency offset between the carrier signal and the tag response in the backscatter method in RFID systems.

Alone By using a digital signal processing can be an effective Suppression of the carrier signal according to state technology for individual channels. The requirements for filtering a single operating channel are so high (the relative bandwidth at UHF RFID is for example, 0.23 ‰), which in turn is only a digital one Signal processing a reliable filtering of a single Operating channels.

The in this patent described electrically tuneable channel filter with carrier suppression combines a frequency-selective analog carrier suppression with filtering of the operating channel and may be in addition to those described above Approaches to improved carrier suppression be used in the reception path. The improvement of carrier suppression leads to a proportional improvement of the system dynamics and consequently to an increased range and / or a improved bit error rate. At the same time the input noise is reduces receiver sensitivity and signal-to-noise ratio improved.

Achievable benefits

By the use of the electrical channel filter described in this patent can in addition to the known possibilities to suppress the crosstalk between the Transmit and the receive path a frequency-selective suppression of the carrier signal in the receive path can be achieved. Practical values this is over 40 dB, which is a threefold Increasing the range of an RFID system using the backscatter method can lead.

by virtue of the frequency selectivity of the invention it is possible Systems with a multi-channel operation to operate, or the operating frequency to dynamically vary a system with a single operating channel, to the frequency offset to interferers and / or adjacent Keep systems as large as possible.

The Reduction of the filter bandwidth from the SAW filter for the operating band the frequency-selective channel filter results in an improvement the input noise by a factor between 10 and 100, depending whether the RFID system at 868 MHz or at 2.45 GHz with a each channel bandwidth of up to 500 kHz works.

The technical realization of the invention based on commercially available components is in terms of cost and staff investment much cheaper than a realization by means of digital signal processing. Only with a disproportionate high investment in computing power, for example, in a frequency-selective filtering by a digital signal processing, a reaction time in a comparable order of magnitude possible than that described in this patent electrical channel filter makes it possible.

Description of the invention

2 shows schematically simplified the structure of an embodiment of the electrically tunable channel filter according to the invention. This includes at least one oscillator 114 that has at least one signal splitter 116 at least two mixer stages 118 and 120 with a signal for down-converting the input signal from the input terminal 110 at the down-mixer stage 118 mixed for processing to an intermediate frequency and at the up-converter stage 120 to the operating frequency again mixed up. At the intermediate frequency, the carrier signal is passed through a high frequency band stop filter 122 suppressed and the respective operating channel through a bandpass filter 124 filtered. Optionally, an amplification of the signal by an amplifier stage 126 be performed.

The inserted bandstop filter 122 and the bandpass filter 124 are at a fixed frequency. The task of the mixer stages is to mix the individual operating channels to the intermediate frequency and to mix the filtered signals from the intermediate frequency to the original frequency position.

The order of filtering and amplification is interchangeable. However, it is recommended to use a low-noise amplifier according to the amplification 2 This is preferable because it reduces the resulting chain noise figure and thus improves the signal-to-noise ratio.

Further embodiments of invention

A further embodiment of the invention consists in the second Dispense mixing stage and further evaluation of the tag response at the intermediate frequency.

Farther is it possible to send a second oscillator signal to the second one Mixer lead to the signal at the output of the circuit to mix another, any intermediate frequency to the further evaluation of the tag response on this second intermediate frequency make.

An alternative realization of the electrically tunable channel filter according to the invention is simplified in FIG 3 displayed. In this case, there is a separate filtering of the two sidebands of the tag response between the entrance gate 210 and the exit gate 212 , For this purpose, the input signal via at least one signal divider 230 divided between the two signal paths. The at the downwards mixing stage 218 and the up-mix stage 226 for the mixture necessary oscillator signal in the first signal path in which, for example, the lower sideband can be filtered, is the oscillator 214 over the signal divider 238 made available. The filtering of the first sideband is done by the high quality bandpass filter 222 , where the signal level is optional through the (low noise) amplifier stage 234 can be raised. According to the same principle, filtering of the second side band by the high-quality filter takes place on the second signal path 224 with optional amplification by the (low-noise) amplifier stage 236 , The at the downwards mixing stage 220 and the up-mix stage 228 required oscillator signal is through the oscillator 216 over the signal divider 240 generated. By merging the two filtered sidebands from the two signal paths on the signal splitter 232 A signal is created that essentially consists of the two sidebands, or the tag response.

at In this embodiment, the filtering of the respective Sidebands take place at the same intermediate frequency, whereby the same electrical components for the first and the second path would be usable.

Also In this realization, it is possible to evaluate the day answer through the use of more oscillators for the second mixing stage of the respective sideband an evaluation to perform the tag response at any intermediate frequency.

In this patent, a tunable in the frequency position filter circuit for suppressing a present in the RFID backscatter method carrier signal is presented, the same time filtering of the operating channel through a bandpass filter of the highest quality.

By the suppression of the carrier signal at up to 40 dB, the range of the RFID system can be up to three times increases, or at a constant distance, the bit error rate be improved.

By the use of the highest quality channel filter for Operating band reduces the input noise power of the receiver proportional to the ratio between that according to the State of the art used SAW filter for the operating band and the bandwidth of the operating channel according to this Invention. For RFID systems for the UHF range is the operating band, for example, 3 MHz in Europe and 28 MHz in the USA, while the bandwidth of an operating channel respectively 200 kHz or 500 kHz, so this an improvement the input noise power and thus the system dynamics by one Factor 15 or 56 can be achieved.

by virtue of the frequency selectivity of the filtering can be in this Patent described circuit in RFID systems with a Multi-channel operation can be used. For systems with a single operating channel The frequency position of the carrier signal can be changed dynamically be to make a larger frequency offset to others to allow adjacent system or eventual interferers.

The analog circuit design based on components of the mass electronics ensures fast signal processing and cost-effective implementation. Pictures reference designation 10 transmitting terminal 12 receiving terminal 14 Transmitting / receiving antenna 16 circulator 110 Input port of the circuit 112 Output port of the circuit 114 oscillator 116 signal splitter 118 first mixer stage 120 second mixer stage 122 Band stop at the intermediate frequency 124 Bandpass filter at the intermediate frequency 126 optional amplifier stage at the intermediate frequency 210 Input port of the circuit 212 Output port of the circuit 214 Oscillator for the first sideband 216 Oscillator for the second sideband 218 first mixer stage for the first sideband 220 first mixer stage for the second sideband 222 Band pass filter of the first sideband at the intermediate frequency 224 Band pass filter of the second sideband at the intermediate frequency 226 second mixer stage for the first sideband 228 second mixer stage for the second sideband 230 first signal divider stage 232 second signal divider stage 234 optional amplifier stage of the first sideband 236 optional amplifier stage of the second sideband 238 Signal divider for the first oscillator signal 240 Signal divider for the second oscillator signal

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• - US 6567648 B1 [0013]

Cited non-patent literature

• - Klaus Finkenzeller: RFID Handbook, 3rd Edition, Carl Hanser Verlag, Munich [0001]
• - ISO-IEC 18000-6C [0006]
• - ETSI EN 302 208-1 [0006]

Claims (4)

Electrically tunable circuit for filtering a transmission channel from an RFID signal with suppression of the carrier signal in the reception path between the input port 110 and the exit gate 112 consisting of a. at least one oscillator 114 for generating a signal required for the mixing operations, b. at least one power divider 116 for dividing the oscillator signal, c. at least two mixer stages 118 and 120 wherein the downmixer stage 118 the input signal mixed to an intermediate frequency and the up-converter stage 120 mixes the filtered signal from the intermediate frequency to the original frequency position, d. a band stop filter 122 with highest quality for suppression of the carrier signal at the intermediate frequency, e. a bandpass filter 124 with highest quality for filtering the operating channel at the intermediate frequency f. and an optional low-noise amplifier stage 126 , which improves the chain noise figure of the circuit and compensates for the conversion losses arising during the mixing processes, the order of the filtering by the band-stop filter 122 and the bandpass filter 124 are interchangeable. Electrically tunable circuit for filtering a transmission channel from an RFID signal with suppression of the carrier signal in the reception path between the input port 210 and the exit gate 212 realized by filtering the two sidebands of the tag response around the carrier signal through two bandpass filters at an intermediate frequency consisting of a. at least one power divider 230 for dividing the input power between the two paths for filtering the individual sidebands, b. at least one oscillator 214 for generating a signal required for the mixing operations at the first (eg upper) sideband, c. at least one power divider 238 for dividing the oscillator signal from the oscillator 214 between the mixer stages for the first (eg upper) sideband, d. at least two mixer stages 218 and 226 for shifting the first signal component to the intermediate frequency and back into the frequency position of the input signal, e. at least one bandpass filter 222 highest quality to filter the first (eg upper) sideband, f. an optional low-noise amplifier stage 234 to reduce the chain noise figure of the circuit as well as to compensate for the conversion losses during the processing of the first (eg upper) sideband, g. at least one oscillator 216 for generating a signal required for the mixing operations in the second (e.g. lower) sideband, h. at least one power divider 240 for dividing the oscillator signal from the oscillator 216 between the mixer stages for the second (eg lower) sideband, i. at least two mixer stages 220 and 228 for shifting the second signal component to the intermediate frequency and back to the frequency position of the input signal, j. at least one bandpass filter 224 highest quality to the second (for example lower) sideband zufiltern, k. an optional low-noise amplifier stage 236 to reduce the chain noise figure of the circuit as well as to compensate for the conversion losses during the processing of the second (eg upper) sideband l. and at least one power divider 232 which merges the two filtered sidebands that are at the frequency position of the input signal. Electrically tunable circuit for filtering a transmission channel from an RFID signal with suppression of the carrier signal in the reception path between the input port 110 and the exit gate 112 according to claim 1, wherein either the supply of the second mixer stage by the oscillator 114 over the signal divider 116 omitted or replaced by a second oscillator. Electrically tunable circuit for filtering a transmission channel from an RFID signal with suppression of the carrier signal in the reception path between the input port 210 and the exit gate 212 realized by filtering the two sidebands of the tag response around the carrier signal by two bandpass filters at an intermediate frequency according to claim 2, either to the supplies of the second mixer stages of the respective signal paths by the oscillators 214 and 216 via the signal divider 238 and 240 omitted or replaced by each additional oscillators.