Interrogator for electronic identification system

Details Interrogator for electronic identification system Interrogator for electronic identification system

1997-10-27

2001-03-20

Tokar, Michael (Department: 2819)

Communications: electrical

Selective

Interrogation response

C375S371000, C327S023000, C455S113000, C455S012100, C340S870030, C342S358000

Reexamination Certificate

active

06204750

ABSTRACT:

RELATED APPLICATIONS
Our co-pending patent application Ser. No. 9505350.0 filed on Mar. 16, 1995 includes a brief reference to an interrogator including a digital filter and capable of detecting small changes in Q factor, and the invention of this application may be applied in such an interrogator.
This invention relates to electronic identification systems, especially systems comprising an interrogator and one or more non-contact transponders.
BACKGROUND OF THE INVENTION
Interrogators and non-contact transponders are known, and one such system is disclosed in European Patent Application Publication No. 0 585 132, CSIR. In that application, a transponder is identified by synchronising an extracted data signal with a clock signal, but such an interrogator cannot detect small frequency changes in a system having a broad, flat Q factor. In EP 0 374 018 Etat Francais, a smart card detector incorporates a Colpitts oscillator of variable frequency, but the frequency variation is substantial and is dependent on two capacitors and the inductance when the detector is coupled to a smart card. Such a detector cannot detect small frequency changes.
SUMMARY OF THE INVENTION
According to the invention, an interrogator for an electronic identification system comprises a first oscillator arranged to vary in frequency in accordance with the frequency of a received signal; a second oscillator arranged to vary in accordance with the frequency of the first oscillator after a delay of a number of cycles of the received signal; and means to detect the phase difference between the first and second oscillators.
Said delay may be provided by the use of a loop filter in phase discriminator means.
The first oscillator may be sensitive to impedance changes and the second oscillator will then be insensitive to impedance changes. The first oscillator may be a Colpitts oscillator arranged to be unstable. The second oscillator may be a voltage controlled oscillator.
Preferably the interrogator further comprises phase-change detection means, such as a digital filter, which further measures the frequency at which the phase changes occur, such frequency measurements indicating the modulation frequency, and therefore the transponder identification signal of a non-contact transponder.
Preferably the input for the digital filter comprises an array of flipflop circuits; a delay line associated with each flipflop circuit and arranged so that the time delay provided by each delay line increases incrementally along the array; and means for providing signals from the Colpitts oscillator and the second oscillator as first and second clock signals, and for providing said clock signals to each flipflop/delay line pair so that when the phase difference between the clock signals corresponds to a time which is less than the time delay provided by a delay line, the associated flipflop circuit provides a first logical output, and when said time is greater than the time delay provided by a delay line, the associated flipflop circuit provides a second logical output.
Preferably there is further provided a second array of flipflop circuits and delay lines connected in opposition to the first array, so that if the first clock signal leads the second clock signal the logical output of the first array changes and if the first clock signal lags the second clock signal the logical output of the second array changes.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described by way of example only with reference to the accompanying drawings in which:
FIG. 1
illustrates in principle an electronic identification circuit;
FIG. 2
illustrates in partial block diagram form an interrogator according to the invention;
FIGS.
3
(
a
) and (
b
) illustrates the changes in current and voltage of the interrogator's antenna;
FIGS.
4
(
a
) and (
b
) illustrates respectively the voltage and frequency output of the Colpitts oscillator, and
FIG.
4
(
c
) illustrates the resultant error signal;
FIG. 5
illustrates the operation of the phase discriminator circuit; and
FIG. 6
illustrates the principle of a novel arrangement to provide the input to a digital filter.


REFERENCES:
patent: 4562437 (1985-12-01), Sasaki et al.
patent: 4725841 (1988-02-01), Nysen et al.
patent: 4751469 (1988-06-01), Nakagawa et al.
patent: 4904964 (1990-02-01), Peng et al.
patent: 4949361 (1990-08-01), Jackson
patent: 5331669 (1994-07-01), Wang et al.
patent: 0 006 691 (1980-01-01), None
patent: 0 253 368 (1988-01-01), None
patent: 0 309 201 (1989-03-01), None
patent: 0 353 807 (1990-02-01), None
patent: 0 374 018 (1990-06-01), None
patent: 0 585 132 (1994-03-01), None
patent: 0 613 253 (1994-08-01), None
patent: 91 13499 (1991-09-01), None

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