Communications: electrical – Selective – Interrogation response
Reexamination Certificate
1998-08-18
2003-03-18
Zimmerman, Brian (Department: 2635)
Communications: electrical
Selective
Interrogation response
C340S005800, C340S572500, C340S505000, C235S380000
Reexamination Certificate
active
06535108
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of modulating a carrier frequency with a modulating signal. More specifically, the invention relates to the modulation of a carrier frequency of a resonant circuit used in radio frequency tagging.
BACKGROUND OF THE INVENTION
A resonant circuit is one in which the values of circuit resistance, R, capacitance, C, and inductance, L, are chosen such that the reactance of the resonant circuit is a minimum at a resonant frequency.
In amplitude modulated radio reception, the antenna circuit is tuned to resonate with the carrier frequency of the particular radio frequency circuit. This tuning to resonance or near resonance of the RF antenna circuit provides a means for discriminating against the many other carrier frequencies used by other broadcasting stations, thereby allowing the listener to choose one station without interference from the others broadcasting simultaneously.
Information content is impressed on the carrier via a mixing scheme known as modulation in which a nonlinear clement combines the carrier with the frequencies containing the information to yield a sum and difference frequency signal. This mixing process takes place at the broadcast station. The information is subsequently retrieved by an amplitude modulated radio receiver which separates the carrier via rectification and filtering leaving only the information signal, a method generally referred to as demodulation. Other types of modulating and demodulating circuits are well known, e.g., circuits using frequency modulation, and pulse modulation.
In prior Radio Frequency (RF) tagging art, a resonant circuit is disposed on a thin insulating dielectric substrate to form a tag for use in electronic article detection (EAS) schemes. Generally, the coil of the resonant circuit consists of a closed loop of a conducting element which has a certain value of resistance and inductance. A capacitive element which forms part of this closed loop consists of two separate areas of thin metal conducting film disposed on opposite sides of the dielectric. The tag is attached to articles to be protected from theft. An RF signal at or near the resonant frequency of the resonant circuit is emitted from a base station. When the tag is in the RF field, the tag's absorption can lead to a change in the tank circuit current of the base station and a power dip in a receiving coil. Either one of these two effects can be used to sense the presence of the tag and hence the item to which it is attached. Thus, an alarm can be made to sound when either of these effects are sensed by a pickup coil or by an amplifier, indicating improper removal of an item. To deactivate the tag, a relatively high RF power pulse can be applied at the counter at which the point-of-sale of the item takes place. This high power acts to short the capacitor or burn out a weak portion of the coil. In either case, the circuit is no longer resonant and will not respond to the RF interrogation from the base station. In that case, the customer who has made a legitimate purchase at the point-of-sale counter can pass through the interrogation-sensing gate without setting off an alarm. It is clear from this description that these tags, once deactivated, arc not reusable. In addition, in the configuration just described, the tags are capable of only conveying one bit of information. Thus, they cannot give any information regarding the item's identification and are useful only for anti-theft applications. This kind of tag is normally classified as a single bit tag.
Some RF tags consist of a resonant coil or a double sided coil containing two thin film capacitors with the plate of each capacitor on opposite sides of the dielectric. Such tags can be used for source tagging and have an initial frequency that is different from the frequency used at the retail establishment for theft protection. For example, in U.S. Pat. No. 5,081,445 (assigned to Checkpoint), the tag is designated as being in a deactivated state until the first capacitor is shorted by means of a high power RF pulse at the then resonant frequency. Disabling the capacitor shifts the resonant frequency of the RF circuit to the store interrogation frequency. A second deactivation pulse is used to disable the second capacitor at the point-of-sale when payment is received for the item to which the tag is attached. At this stage, the tag is no longer usable and has been permanently destroyed.
Some additional art discloses two or more frequencies that can be obtained on a RF coil tag by altering the capacitance of the circuit. In one case, a strong DC electric field is applied to change the effective dielectric constant of the capacitor. Thus, the circuit has two resonant frequencies depending on the value of the applied electric field. Due to the ferroelectric hysteresis, the tag can be deactivated by the application of a DC field. However, it can also be reactivated and hence re-used by applying a DC field of opposite polarity (U.S. Pat. No. 5,257,009, assigned to Sensormatic). In an earlier embodiment, a set of capacitors connected in parallel attached to an inductance have been described in U.S. Pat. No. 5,111,186 assigned to Sensormatic in which each dielectric of the set of capacitors varies in thickness. In this manner, a series of resonant frequencies can be obtained by applying different voltages (electric fields). Each of the capacitors then changes capacitance at a different electric field (voltage) levels depending on the thickness of the dielectric. There can be some concern regarding the high voltages required for creating the change in the dielectric. Also, using this apparatus, the remanent state of ferroelectric tends not to be very stable for long periods of time. Additional concern relates to the dielectrics, which are also piezoelectric materials which have properties quite sensitive to stress.
U.S. Pat. No. 5,218,189, assigned to Checkpoint, provides an array of series capacitors connected in parallel with an inductor. Here, the resonance can be altered by selectively shorting one or more of the capacitors, thereby changing the resonant frequency of the resulting circuit. A frequency code can thereby be established by disabling or burning out selective capacitors at the time of interrogation, those capacitors becoming disabled which at the time of manufacture of the tag were “dimpled”. The disadvantage is that the item or person is subject to high r.f. fields during interrogation. Also, the range of frequencies that needs to be scanned is necessarily large. This makes detection difficult since the requirement to scan a large band of frequencies puts a strong demand on the flat response of the detector circuit.
U.S. Pat. No. 4,745,401 describes an embodiment for a reusable tag. It is comprised of two ferromagnetic elements, one soft (low coercivity) and one hard (high coercivity) both physically covering a portion of an R.F. coil. The ferromagnetic element with high coercivity can be magnetized to apply a bias field to the soft material to put the latter into saturation. In that state, the R.F. field generates very small hysteresis losses leading to a relatively high Q of the tag circuit. On the other hand, when the hard magnet is demagnetized, the RF Field results in hysteresis losses in the soft material which lowers the Q of the circuit. This change in Q can be used to determine whether a tag is active or has been deactivated. While this constitutes a reusable tag, the change in Q tends to be small and thereby some what more difficult to distinguish from other effects that attenuate the absorption.
In U.S. Pat. No. 3,500,373 an apparatus is described for interrogating and sensing the presence of a RF resonant tag. Here the interrogating frequency is swept around a center frequency. In general, there is very little radiation emitted except when the tag is present in the field of tile emitter. Thus, when there is no tag in the antenna field, very little energy is lost from the antenna circuit. When the swept frequency coincides with th
Gambino Richard Joseph
Schrott Alejandro Gabriel
Von Gutfeld Robert Jacob
Dalencourt Yves
Intermec IP Corp.
Zimmerman Brian
LandOfFree
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