JP2008278025A - Radio tag reader - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a reliable radio tag configured to perform carrier sense free from an error even when a strong radio wave is inputted from another radio tag reader communicating by a frequency in another channel, and to secure stable carrier sense sensitivity in the channel band of the communication. <P>SOLUTION: The radio tag reader comprises: a modulator 22 which modulates a high-frequency signal sent by an oscillator oscillating the high-frequency signal; a frequency converter 10 which converts a reception signal into a baseband signal based upon the high-frequency signal modulated by the modulator 22; a carrier sense unit 34 which executes the carrier sense where the baseband signal is received and passed through band-stop filters 32 and 34, whether there is the same frequency component with a transmission frequency of transmission is detected before the transmission from baseband signals having passed through the band-stop filters 32 and 34, and when the component is not detected, transmission of the radio tag reader is allowed; and a modulated signal control unit 50 which alternates and inputs f1 and f2 to the modulator at intervals of predetermined time. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a wireless tag reader that performs communication with a wireless tag.

  There is a backscatter method as a communication method of a wireless tag such as an RFID (radio frequency identification) tag and a wireless tag reader. In this backscatter method, the wireless tag reader transmits an unmodulated radio wave to the radio tag, the radio tag receives this radio wave and converts it to an operating DC power source, and the radio tag is the impedance of the antenna of the radio tag. The received radio wave is reflected and absorbed by changing. The wireless tag reader performs data communication with the wireless tag by receiving a reflected wave from the wireless tag. That is, the wireless tag reader receives radio waves from the radio tag while transmitting radio waves for operating power. This operation is specific to communication between the wireless tag and the wireless tag reader.

  Such a wireless tag reader is equipped with a carrier sense function in order to avoid interference with other wireless tag readers. The carrier sense function is a function that scans a received signal that is the same as the frequency (channel) to be transmitted for a certain period of time before the RFID tag reader transmits, and checks whether other RFID tag readers are communicating. If it can be confirmed that no other wireless tag reader is communicating, the local station can start communication. The carrier sense execution time, sensitivity, and the like are determined by the Radio Law. For example, in the case of Japan, a carrier sense sensitivity of -74 dBm is required for a wireless tag reader capable of outputting up to 1 W.

Further, a direct conversion method is often used as a receiving method of the wireless tag reader. The direct conversion method is well known as generating a baseband signal without using an intermediate frequency by making the frequency of the local oscillator substantially the same as the reception frequency. This direct conversion type wireless tag reader also performs carrier sense based on the amplitude of the baseband signal (for example, Patent Document 1).
JP 2007-28583 A

  Usually, the carrier sense sensitivity of the wireless tag reader is higher than the reception sensitivity during communication with the wireless tag. For this reason, a high frequency amplifier for amplifying the received signal is provided, and the gain is increased at the time of carrier sensing, or the high frequency amplifier is used only at the time of carrier sensing without using the high frequency amplifier at the time of normal reception. It is common to increase the gain at the time of sensing.

  However, in this case, when a large level signal is received at the time of carrier sense, the received signal is easily saturated in the baseband. For example, even if the transmission frequency of another RFID tag reader is different from that of the own station, if a strong radio wave emitted from the RFID tag reader is input to the own station, the received signal is saturated and distorted in the baseband part. As a result, a frequency component corresponding to the communication speed of the other station appears in the carrier sense band of the own station, causing a problem of unnecessary carrier sensing.

  Therefore, it is conceivable to provide a filter for attenuating a frequency component corresponding to the communication speed of the local station, for example, a band rejection filter, and perform carrier sense by passing the baseband signal converted from the received signal through the band rejection filter.

  However, in this case, the level of the baseband signal that has passed through the band rejection filter becomes unstable in the channel band to be carrier sensed. For this reason, it becomes difficult to sense the carrier for a specific frequency, or on the contrary, the sensitivity is too good for a specific frequency, and the carrier sense is detected even for a minute input level or noise. There is a possibility.

  In consideration of the above circumstances, the present invention enables carrier sense without error even when a strong radio wave is input from another RFID tag reader that communicates at a frequency of another channel, and is stable in the channel band of the communication. An object of the present invention is to provide a wireless tag reader that can ensure the carrier sense sensitivity.

  According to a first aspect of the present invention, there is provided a radio tag reader comprising: an oscillator that emits a high-frequency signal; a modulation unit that modulates a high-frequency signal emitted from the transmitter; and a baseband signal based on a high-frequency signal that is modulated by the modulation unit. Before the transmission of the RFID tag reader, and the presence of the same frequency component as the transmission frequency is detected from the baseband signal, and the carrier sense that allows the RFID tag reader to be transmitted if there is no detection Carrier sensing means, and modulation control means for selecting the modulation frequency of the modulation means to any one of a plurality of settings during execution of the carrier sense and inputting the selected modulation signal to the modulation means; Is provided.

  According to the wireless tag reader of the present invention, even when a strong radio wave is input from another wireless tag reader communicating at a frequency of another channel, carrier sense without error is possible and stable carrier sense sensitivity in the channel band of the communication. Can be secured.

  [1] A first embodiment of the present invention will be described below with reference to the drawings. The principal part of the wireless tag reader which employ | adopted the direct conversion system as a communication system is shown in FIG.

  A circulator (directional coupler) 3 is connected to a transmission / reception antenna 1 via a band pass filter (band pass filter; BPF) 2. The band-pass filter 2 passes a frequency component in a predetermined band for transmission / reception among reception signals (high-frequency signals) of the antenna 1. The received signal that has passed through the band-pass filter 2 is guided to the amplification / non-amplification switching circuit (amplifying means) 4 by the circulator 3.

  The amplification / non-amplification switching circuit 4 amplifies or non-amplifies the received signal according to a command from the signal processing unit 20, and switches (SW) 5, 6 and a high frequency amplifier (low noise amplifier; also referred to as LNA). It consists of seven. Among these, the switch 5 supplies the received signal from the band pass filter 2 to one of the switch 6 and the high frequency amplifier 7, and does not pass through the high frequency amplifier 7 during non-carrier sense (normal reception). Directly supplied to the switcher 6 and supplied to the switcher 6 via the high frequency amplifier 7 at the time of carrier sense. The high frequency amplifier 7 is for increasing the reception sensitivity, and amplifies the reception signal supplied from the switch 5 to a level necessary for the carrier sense sensitivity of the carrier sense unit 36 described later. The reception signal amplified by the high frequency amplifier 7 is supplied to the switch 6. The switch 6 selectively outputs one of the reception signal from the switch 5 and the reception signal from the high frequency amplifier 7, and outputs the reception signal supplied from the switch 5 when non-carrier sense is performed. During the carrier sense, the reception signal supplied from the high frequency amplifier 7 is output. In addition, switching means for controlling the switches 5 and 6 to amplify the received signal based on an instruction from the signal processing unit 20 at the time of performing carrier sense is provided.

  The received signal that has passed through the amplification / non-amplification switching circuit 4 is supplied to a frequency conversion unit (conversion means) 10. The frequency converter 10 converts the received signal supplied from the amplification / non-amplification switching circuit 4 into a baseband signal based on a high-frequency signal supplied from a modulator 22 described later. Degree phase shifter 19 and the like. Further, the received signal that has passed through the frequency conversion unit 10 is supplied to the baseband unit 24. The baseband unit 24 includes low-pass filters (LPF) 12 and 16, low-frequency amplifiers 13 and 17, analog / digital (A / D) converters 14 and 18, and the like.

  The mixer 11 generates a baseband signal IA having an in-phase component (I component) by mixing a reception signal from the amplification / non-amplification switching circuit 4 and an output of a modulator 22 described later. The baseband signal IA passes through the low-pass filter 12, is amplified by the low-frequency amplifier 13, is digitally converted by the analog / digital converter 14, and is supplied to the signal processing unit 20.

The 90-degree phase shifter 19 outputs a signal obtained by changing the phase of a high-frequency signal supplied from a modulator 22 described later by 90 degrees.
The mixer 15 mixes the received signal from the amplification / non-amplification switching circuit 4 and the high-frequency signal supplied from the 90-degree phase shifter 19 to generate a baseband signal QA having a quadrature component (Q component). . The baseband signal QA passes through the low-pass filter 16 and is amplified by the low-frequency amplifier 17, then digitally converted by the analog / digital converter 18 and supplied to the signal processing unit 20.

  The local oscillator 21 operates in response to a command from the signal processing unit 20 and emits a high-frequency signal (for example, 953 MHz) having substantially the same frequency as the frequency of the reception signal from the wireless tag. This high frequency signal is supplied to the modulator 22. The modulator 22 modulates a high-frequency signal emitted from the local oscillator 21 in accordance with a modulation signal supplied from a digital / analog (D / A) converter 25 described later (ASK modulation), thereby transmitting to the wireless tag. A high-frequency signal that functions as a power source for operating the wireless tag while outputting data is output, or a high-frequency signal that functions only as a power source for operating the wireless tag is output. A high-frequency signal output from the modulator 22 is supplied to the mixer 11, the 90-degree phase shifter 19 and the power amplifier 23. The power amplifier 23 amplifies the power of the input high frequency signal. The amplified high frequency signal is guided to the band pass filter 2 side by the circulator 3, passes through the band pass filter 2, and is transmitted from the antenna 1.

  As shown in FIG. 2, the signal processing unit 20 has a low-pass filter (LPF; second filter) 31 to which the baseband signal IB A / D converted by the baseband unit 24 is supplied, as shown in FIG. A band rejection filter (BEF; first filter) 32 to which a baseband signal IC that has passed through the low-pass filter 31 is supplied, and a low-pass signal to which a baseband signal QB A / D converted by the baseband unit 24 is supplied. A filter (LPF; second filter) 33, a band rejection filter (BEF; first filter) 34 to which a baseband signal QC that has passed through the low-pass filter 33 is supplied, and a baseband that has passed through the low-pass filters 31 and 33. A data demodulator (demodulator) 35 to which signals IC and QC are supplied, and baseband signals ID and Q that have passed through the band rejection filters 32 and 34, respectively. Is supplied to the carrier sense unit (carrier sense means) 36, the control unit 40 to which the demodulation result of the data demodulation unit 35 and the carrier sense result of the carrier sense unit 36 are supplied, and the modulation control unit connected to the control unit 40 (Modulation control means) 50 and the like are provided. The band rejection filters 32 and 34 are dedicated filters when carrier sense is executed.

  The low-pass filters 31 and 33 pass frequency components (for example, 40 kHz) corresponding to the communication speed (for example, 40 kbps) of the wireless tag reader among the baseband signals IB and QB generated by the baseband unit 24, respectively. .

  The band rejection filters 32 and 34 block the frequency components corresponding to the communication speed of the wireless tag reader from the baseband signals IC and QC that have passed through the low-pass filters 31 and 33, respectively.

The data demodulator 35 demodulates the baseband signals IC and QC that have passed through the low-pass filters 31 and 33.
The carrier sense unit 36 detects the same frequency component as the transmission frequency (for example, 953 MHz) from the baseband signal that has passed through the band rejection filters 32 and 34 before the transmission of the wireless tag reader. Carry out carrier sense that allows transmission.

  The modulation control unit 50 generates modulation signals of a plurality of frequencies for carrier sense, and sequentially selects and outputs each modulation signal during execution of carrier sense by the carrier sense unit 36, and transmits data to the wireless tag. In this case, a modulation signal corresponding to the transmission data is generated and output. As shown in FIG. 3, a carrier sense modulation signal M1 generation unit 51a, a carrier sense modulation signal M2 generation unit 51b, and a transmission modulation signal are provided. An Md generation unit 52, a selection unit 53, and the like are included.

  The carrier sense modulation signal M1 generation unit 51a generates and outputs a rectangular wave modulation signal M1 having a frequency f1. The carrier sense modulation signal M2 generation unit 51b generates and outputs a rectangular wave modulation signal M2 having a frequency f2 (> f1). These frequencies f1 and f2 are frequencies that are located above and below the stop frequency of the band stop filters 32 and 34. f1 is lower than the stop frequency and f2 is higher than the stop frequency. The transmission modulation signal generator 52 generates and outputs a modulation signal Md corresponding to the transmission data supplied from the controller 40. The selection unit 53 selects one of the modulation signals M1, M2, and Md according to the selection signal supplied from the control unit 40, and outputs it as the modulation signal TXD. In other words, the modulation control means performs control to select a modulation frequency of the modulation means from any of a plurality of settings and to input the selected modulation signal to the modulation means during carrier sense. .

  The modulation signal TXD output from the modulation control unit 50 is input to the modulator 22 via the digital / analog converter 25. With this input, especially during carrier sense, the modulation frequency of the modulator 22 is switched to a plurality of values in order.

The control unit 40 includes the following means (1) to (4) as main functions.
(1) Means for instructing the carrier sense unit 36 to execute carrier sense for a predetermined time before data transmission to the wireless tag.

  (2) A means for permitting or prohibiting data transmission to the wireless tag according to the carrier sense result of the carrier sense unit 36.

  (3) Means for supplying transmission data to the modulation controller 50 when data transmission is permitted.

(4) Means for recognizing the demodulated data of the data demodulator 35 when receiving from the wireless tag (during non-carrier sensing).
Next, the operation will be described.
A high frequency signal received by the antenna 1 is input to the amplification / non-amplification switching circuit 4 via the band pass filter 2 and the circulator 3. At this time, if carrier sense is not executed (during normal reception), the received signal input to the amplification / non-amplification switching circuit 4 is supplied to the frequency conversion unit 10 without being amplified. When the carrier sense is executed, the received signal input to the amplification / non-amplification switching circuit 4 is amplified by the high frequency amplifier 7 and supplied to the frequency conversion unit 10 in order to obtain the required carrier sense sensitivity.

  The received signal supplied to the frequency converter 10 is converted into baseband signals IA and QA based on the high frequency signal from the modulator 22. The baseband signals IB and QB converted into digital signals by the baseband unit 24 are input to the data demodulating unit 35 through the low-pass filters 31 and 33, and the low-pass filters 31 and 33 and the band rejection. The signal is input to the carrier sense unit 36 through the filters 32 and 34.

  The carrier sense unit 36 performs carrier sense before transmission by the wireless tag reader. For example, in the case of trying to start transmission at a frequency (953 MHz) of 5 channels in the frequency band for the wireless tag allocated for Japan, the same frequency component as the transmission frequency is generated by the low-pass filters 31, 33 and It is detected from the baseband signals ID and QD that have passed through the band rejection filters 32 and 34. If the amplitude values (RSSI values) of the baseband signals ID and QD are less than the set value, it is determined that there is no frequency component (carrier) that is the same as the transmission frequency. In this case, transmission is allowed. However, if the amplitude values of the baseband signals ID and QD are greater than or equal to the set values, it is determined that there is the same frequency component (carrier) as the transmission frequency. In this case, transmission is prohibited.

  At the time of this carrier sense, the modulation control unit 50 alternately outputs the modulation signal M1 of the frequency f1 and the modulation signal M2 of the frequency f2 by predetermined time (for example, 250 μsec) as shown in FIG. Along with the switching output of the modulation signals M1 and M2, as shown in FIG. 5, the baseband signals I and B of the line between the low frequency amplifiers 13 and 17 and the analog / digital converters 14 and 18 in the baseband unit 24 are obtained. The frequency of Q is alternately switched to f1 and f2 for a predetermined time.

  Here, the operation of the band rejection filters 32 and 34 will be described. For example, when starting transmission at a frequency of 5 channels (953 MHz) in the frequency band for UHF band wireless tags allocated for Japan, carrier sense is executed for the same 5 channels before starting transmission. If no carrier is detected as a result of this carrier sense, transmission is started.

  When a strong radio wave from a wireless tag reader that is transmitting on a channel other than 5 channels is input to the own station, the received signal is saturated and distorted in the baseband unit 24. A spectrum appears at 40 kHz equal to the frequency component (modulation frequency) corresponding to the speed. Since this spectrum is included in a baseband band (for example, 0 to 100 kHz) for performing carrier sense, it becomes easy to erroneously perform carrier sense.

  Therefore, low-pass filters 31 and 33 that pass a frequency component (40 kHz) corresponding to the communication speed (40 kbps) of the wireless tag reader among the baseband signals I and Q converted by the baseband unit 24 are provided. The baseband signals IC and QC that have passed through the band-pass filters 31 and 33 are demodulated, and the frequency corresponding to the communication speed (40 kbps) of the wireless tag reader among the baseband signals IC and QC that have passed through the low-pass filters 31 and 33. Band stop filters 32 and 34 for blocking components (40 kHz) are provided, and carrier sense is executed for baseband signals ID and QD that have passed through the band stop filters 32 and 34. Thereby, even when a strong radio wave is input from another RFID tag reader that communicates at a frequency of another channel and amplified by the amplification / non-amplification switching circuit 4, erroneous carrier sense can be prevented.

  However, the Japanese radio law stipulates that carrier sense sensitivity (for example, −74 dBm) is satisfied in a channel band for communication during carrier sense. Since the channel band assigned for the UHF band wireless tag is 200 kHz, it is necessary to maintain carrier sense sensitivity in the range of 953 MHz ± 100 kHz in the case of 5 channels.

  As a specific example, the communication speed between the wireless tag reader and the wireless tag is 40 kbps, the cutoff frequency of the low-pass filters 31 and 33 in the signal processing unit 20 is about 100 kHz, and the blocking frequencies of the band-stop filters 32 and 34 are set. When the carrier sense modulation frequency for the high frequency signal of the local oscillator 21 is 40 kHz and the signal of −74 dBm, which is the standard value of carrier sense, is input to the antenna 1, the amplitude values (RSSI) of the baseband signals ID and QD The relationship between (value) and frequency is as shown in FIG. That is, the minimum value of the amplitude value is “57” at 953 MHz, the maximum value of the amplitude value is “102” at 953.02 MHz, and the amplitude value varies between the minimum value and the maximum value. It can be seen (variation width = 102−57 = 45).

In particular, since the stop frequency of the band stop filters 32 and 34 is 40 kHz, the amplitude value at 953 MHz is small.
That is, when the modulation frequency input to the modulator 22 is 40 kHz, the 953 MHz high-frequency signal emitted from the local oscillator 21 is modulated at 40 kHz. When the antenna 1 receives a 953 MHz high-frequency signal, the received signal and the 953 MHz high-frequency signal modulated at 40 kHz are mixed by the mixers 11 and 15, thereby obtaining 40 kHz baseband signals I and Q. However, since the 40 kHz baseband signals I and Q are attenuated by the band rejection filters 32 and 34, the amplitude values of the baseband signals ID and QD supplied to the carrier sense unit 36 are very small values. For this reason, the amplitude value at 953 MHz is small.

  Ideally, the amplitude values of the baseband signals ID and QD are preferably constant within the channel band. This is because if the amplitude value is constant within the channel band, the carrier sense sensitivity is constant for any frequency within the channel band. When the amplitude value varies within the channel band, the carrier sense sensitivity also varies. In such a case, there is a possibility that carrier sense is unnecessarily performed with respect to a low level received signal or noise.

  In order to prevent such unnecessary carrier sense, in the present embodiment, at the time of carrier sense, the modulation control unit 50 alternately outputs the modulation signal M1 and the modulation signal M2 every predetermined time (for example, 250 μsec), and thereby the antenna When the received signal input from 1 is 953 MHz, baseband signals I and Q whose wave numbers are alternately switched between f1 and f2 every predetermined time are obtained.

  The carrier sense unit 36 averages the amplitude values of the baseband signals ID and QD over an even multiple of a predetermined time for switching (for example, 250 μsec), and performs carrier sense on the averaged amplitude value. As a result, an amplitude value with little fluctuation in time can be obtained, and highly reliable carrier sense that is not affected by instantaneous noise or the like can be achieved. For example, when the predetermined switching time is 250 μsec, it is preferable to set 500 μsec, which is twice as long, or 1 msec, which is four times as long as the averaging time.

  Baseband signal ID, QD amplitude value when the modulation frequency is single f1 (= 11.7 kHz), baseband signal ID, QD amplitude value when f2 (= 90.9 kHz) single, f1 and f2 FIG. 7 shows the experimentally confirmed amplitude values of the baseband signals ID and QD when they are alternately switched at predetermined time intervals.

  The fluctuation range of the amplitude value when the modulation frequency is single f1 and the fluctuation range of the amplitude value when f2 is single are larger than the fluctuation range of the amplitude value when 40 kHz is single as shown in FIG. When f1 and f2 are alternately switched at predetermined time intervals, the fluctuation range of the amplitude value is “23” from the minimum value “67” at 953.09 MHz to the maximum value “90” at 953.04 MHz, 40 kHz It is improved to about half of the single case.

  In this way, the band rejection filters 32 and 34 are provided for carrier sense, and further, the modulation frequency at the time of carrier sense is alternately switched between f1 and f2 for a predetermined time, thereby allowing other radios communicating at the frequency of another channel. Even when a strong radio wave is input from the tag reader, stable carrier sense without error is possible, and stable carrier sense sensitivity can be secured within the communication channel band of the wireless tag reader.

[2] A second embodiment of the present invention will be described.
As shown in FIG. 8, the modulation control unit 50 includes a carrier sense modulation signal M1 generation unit 51a, a carrier sense modulation signal M2 generation unit 51b, a carrier sense modulation signal M3 generation unit 51c, and a transmission modulation signal Md generation unit. 52 and a selection unit 53.

  The carrier signal modulation signal generators 51a and 51b generate and output rectangular-wave modulation signals M1 and M2 having frequencies f1 and f2, as in the first embodiment. The carrier sense modulation signal M3 generation unit 51c generates and outputs a rectangular wave modulation signal M3 having a frequency f3.

  The relationship between the frequencies f1, f2, and f3 is “f1 <f3 <f2”. For example, 11.7 kHz, which is lower than the stop frequency 40 kHz of the band rejection filters 32 and 34, is selected as f1, for example, 90.9 kHz higher than the stop frequency 40kHz of the band rejection filters 32, 34 is selected as f2, and the band rejection is as f3. The same value as the blocking frequency 40 kHz of the filters 32 and 34 is selected.

  The selection unit 53 selects any one of the modulation signals M1, M2, M3, and Md according to the selection signal supplied from the control unit 40, and outputs it as the modulation signal TXD.

The operation will be described.
At the time of carrier sense, the modulation control unit 50 switches and outputs the modulation signal M1 of the frequency f1, the modulation signal M2 of the frequency f2, and the modulation signal M3 of the frequency f3 in order for a predetermined time (for example, 250 μsec). Along with the switching output of the modulation signals M1, M2, and M3, when the reception signal input from the antenna 1 is 953 MHz, the frequencies of the baseband signals I and Q are sequentially switched to f1, f2, and f3 by a predetermined time.

  Baseband signal ID, QD amplitude value when modulation frequency is single f1 (= 11.7 kHz), baseband signal ID, QD amplitude value when f2 (= 90.9 kHz) single, f3 (= 40 kHz) The amplitude values of baseband signals ID and QD in the case of a single case, and the amplitude values of baseband signals ID and QD when the modulation frequency is sequentially switched to f1, f2, and f3 are confirmed by experiments. Nine.

  The fluctuation range of the amplitude value in the case of a single 40 kHz is large, and the fluctuation range of the amplitude value in the case of a single f1 and a single f2 is further larger than that of the single 40 kHz, but f1, f2, f3 Are sequentially switched from the minimum value “74” at 953.08 MHz to the maximum value “90” at 953.04 MHz, about 1/40 of a single 40 kHz. This is improved to 70% of the case of switching between two frequencies of 3, f1, and f2.

Other configurations, operations, and effects are the same as those in the first embodiment. Therefore, the description is omitted.
[3] A third embodiment of the present invention will be described.
As shown in FIG. 10, the modulation control unit 50 includes a carrier sense modulation signal M1 generation unit 51a, a carrier sense modulation signal M2 generation unit 51b, a transmission modulation signal Md generation unit 52, a selection unit 53, and a superposition unit 54. have.

  The carrier signal modulation signal generators 51a and 51b generate and output rectangular-wave modulation signals M1 and M2 having frequencies f1 and f2, as in the first embodiment. For example, 11.7 kHz, which is lower than the stop frequency 40 kHz of the band rejection filters 32, 34, is selected as f1, and for example, 90.9 kHz, which is higher than the stop frequency 40 kHz of the band rejection filters 32, 34, is selected as f2.

  The superimposing unit 54 superimposes the modulation signals M1 and M2 and outputs a modulation signal whose level changes at two frequencies f1 and f2, as shown in FIG. This modulated signal is supplied to the selector 53.

  The selection unit 53 selects one of the modulation signal supplied from the superimposition unit 54 and the modulation signal Md supplied from the transmission modulation signal Md generation unit 52 according to the selection signal supplied from the control unit 40, Is output as a modulation signal TXD.

The operation will be described.
At the time of carrier sense, a modulation signal whose level changes at two frequencies f1 and f2 is supplied to the modulator 22 and AM-modulated. By this supply, the fluctuation range of the amplitude values of the baseband signals ID and QD in the channel band can be suppressed small. Thereby, stable carrier sense sensitivity can be ensured in the channel band.

Other configurations, operations, and effects are the same as those in the first embodiment. Therefore, the description is omitted.
[4] Modification
In each of the above embodiments, ASK modulation or AM modulation has been described as an example of the modulation scheme of the modulator 22, but not limited to ASK modulation and AM modulation, for example, BPSK modulation may be employed. The same effect can be obtained by using a low-pass filter having a characteristic of blocking the frequency component (40 kHz) in place of the band rejection filters 32 and 34. Further, a variable gain is used as the high-frequency amplifier 7 in the amplification / non-amplification switching circuit 4 so that the high-frequency amplifier 7 is operated at a low gain when carrier sense is not executed, and the high-frequency amplifier 7 is operated at a high gain when carrier sense is executed. You may make it operate | move by. In this case, the switches 5 and 6 of the amplification / non-amplification switching circuit 4 can be dispensed with.

  The signals generated by the modulation signal generation units 51a, 51b, and 51c at the time of carrier sensing have been described as rectangular waves. However, the signals are not limited to rectangular waves, but may be waveforms that are band-limited by a low-pass filter or the like.

  In addition, the present invention is not limited to the above embodiment, and various modifications can be made without departing from the scope of the invention.

The block diagram which shows the structure of the principal part of each embodiment. The block diagram which shows the main structures of the signal processing part in FIG. The block diagram which shows the structure of the modulation control part in 1st Embodiment. The figure which shows the waveform of each modulation signal in 1st Embodiment. The figure which shows the waveform of the baseband signal in 1st Embodiment. The figure which shows the relationship between the frequency and amplitude value of a baseband signal when there is only one modulation signal in each embodiment as a reference. The figure which shows the relationship between the frequency and amplitude value of a baseband signal at the time of 2 frequency switching in 1st Embodiment. The block diagram which shows the structure of the modulation control part in 2nd Embodiment. The figure which shows the relationship between the frequency and amplitude value of a baseband signal at the time of 3 frequency switching in 2nd Embodiment. The block diagram which shows the structure of the modulation control part in 3rd Embodiment. The figure which shows the relationship between the frequency and amplitude value of a baseband signal by 2 frequency superimposition of 3rd Embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Antenna, 2 ... Band-pass filter, 3 ... Circulator, 4 ... Amplification / non-amplification switching circuit (amplification means), 5, 6 ... Switch, 7 ... High frequency amplifier, 10 ... Frequency conversion part (conversion means), 11 , 15 ... mixer, 12, 16 ... low-pass filter, 13, 17 ... low-frequency amplifier, 14, 18 ... analog-to-digital converter, 19 ... 90-degree phase shifter, 20 ... signal processing unit, 21 ... local oscillator , 22 ... modulator (modulation means), 23 ... power amplifier, 24 ... baseband section, 25 ... digital-analog converter, 31, 33 ... low-pass filter (second filter), 32, 34 ... band rejection filter (First filter), 35... Data demodulation unit (demodulation unit), 36... Carrier sense unit (carrier sense unit), 40... Control unit, 50 ... modulation control unit (modulation control unit), 51 ... carrier sense time of modulation signal M1 generation unit, 51b ... carrier sense time of modulation signal M2 generation unit, 52 ... transmission time of modulation signal Md generation unit, 53 ... selection unit, 54 ... superimposing unit

Claims (5)

An oscillator that emits a high-frequency signal;
Modulation means for modulating a high-frequency signal emitted from the oscillator;
Conversion means for converting the received signal into a baseband signal based on the high-frequency signal modulated by the modulation means;
Before the transmission of the wireless tag reader, the presence or absence of the same frequency component as the transmission frequency is detected from the baseband signal, and when there is no detection, carrier sense means for performing carrier sense that allows transmission of the wireless tag reader;
Modulation control means for selecting a modulation frequency of the modulation means to any one of a plurality of settings during execution of the carrier sense, and inputting the selected modulation signal to the modulation means;
A wireless tag reader, comprising: An oscillator that emits a high-frequency signal;
Modulation means for modulating a high-frequency signal emitted from the oscillator according to an input modulation signal;
Conversion means for converting the received signal into a baseband signal based on the high-frequency signal modulated by the modulation means;
Before the transmission of the wireless tag reader, the presence or absence of the same frequency component as the transmission frequency is detected from the baseband signal, and when there is no detection, carrier sense means for performing carrier sense that allows transmission of the wireless tag reader;
Modulation control means for generating and superimposing modulation signals of a plurality of frequencies, and inputting the superimposed modulation signal to the modulation means during execution of the carrier sense;
A wireless tag reader, comprising: A first filter for blocking a frequency component corresponding to the communication speed of the wireless tag reader;
3. The wireless tag reader according to claim 1, wherein the carrier sense means detects presence / absence of the same frequency component as a transmission frequency from a baseband signal that has passed through the first filter. 4. The wireless tag reader according to claim 3, wherein the plurality of set frequencies of the modulation control means are two frequencies above and below the blocking frequency of the first filter. 4. The radio according to claim 3, wherein the plurality of set frequencies of the modulation control unit are a total of three frequencies including a stop frequency of the first filter and two frequencies above and below the stop frequency. Tag reader.

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