Demodulators – Amplitude modulation demodulator
Reexamination Certificate
2000-09-05
2001-11-27
Mis, David (Department: 2817)
Demodulators
Amplitude modulation demodulator
C375S320000
Reexamination Certificate
active
06323728
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention:
The invention relates to a data carrier with at least one coil for the contactless reception of amplitude-modulated signals, a rectifier circuit connected downstream of the coil and a circuit configuration for processing and/or storing data.
Such a data carrier is presently on the market mainly in the form of a chip card and is known from German Published, Non-Prosecuted Patent Application DE 196 34 134 A1, corresponding to U.S. application Ser. No. 09/256,283, filed Feb. 23, 1999. In the case of the method described therein for transmitting data between a terminal and a portable data carrier through a wireless electromagnetic transmission link, a 100% ASK modulation of the carrier signal takes place, as is generally customary in the case of such present-day contactless transmissions. Although that switching on/off of the carrier signal is relatively easy to demodulate in the data carrier, it has the disadvantage that no clock signal is available during a blanking interval.
However, in standardizing bodies for a contactless chip card (ISO 14443), the current standpoint is to use not only ON-OFF keying (OOK) but also amplitude-shift keying (ASK) with a degree of modulation of 5 to 15% for the data transmission from a writing/reading device to a card or generally to a data carrier.
However, such modulation is difficult to demodulate since the distance between the writing/reading device and the data carrier can change greatly and, as a result, the amplitude of the received signal is subject to fluctuations which are superposed on the modulation and falsify it. In addition, the circuits in the data carrier have a greatly fluctuating power consumption, which likewise has retroactive effects on the modulation.
The problems referred to are particularly critical in the case of passive data carriers, which have no power supply of their own and obtain their operating energy from the signal being received.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a data carrier for the contactless reception of amplitude-modulated signals, which reduces or even eliminates the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a data carrier, comprising at least one coil for contactless reception of amplitude-modulated signals; a rectifier circuit connected downstream of the at least one coil; a circuit configuration for processing and/or storing data; a supply-voltage control circuit connected in parallel with the circuit configuration; and a current measuring device acting as an amplitude demodulator, the current measuring device connected between the at least one coil and the voltage-supply control circuit.
In the case of the data carrier according to the invention, the demodulator is realized in conjunction with the power supply concept. As a first measure, the circuit configuration has a supply-voltage control circuit connected in parallel therewith. This brings about a decoupling of the currents in the load circuit provided by the circuit configuration and in the supply circuit provided by the coil and the rectifier. Consequently, the current in the supply circuit is only dependent on the power offered by the signal received by the coil. The amplitude modulation is registered with a current measuring device, acting as an amplitude demodulator, for the current in the supply circuit.
In accordance with a first embodiment of the invention, a measuring resistor having terminals connected to a demodulator circuit is disposed in the supply circuit. The demodulator circuit consequently evaluates the voltage drop across the measuring resistor, which is a measure of the amplitude modulation.
In accordance with a second embodiment of the invention, there is provided a current mirror circuit, the output current of which is equal or at least proportional to the current in the supply circuit and consequently to the modulation. The current mirror circuit has the advantage of ensuring that the voltage drop across the diode or the diode transistor in the supply circuit depends on the current in a non-linear relationship and, in the case of great currents, increases only with the root of the current and consequently limits the voltage at the rectifier. It is desirable to reduce the resistance value further in spite of the lower resistance of the current mirror diode in comparison with a measuring resistor. Therefore, in accordance with another feature of the invention, a bias voltage is provided at the gate of the current-mirror diode transistor, whereby the latter becomes more conductive. The bias voltage is advantageously chosen in such a way that the current-mirror diode transistor is still just in saturation.
In accordance with a first embodiment of a bias-voltage generating circuit, there is provided a voltage divider which is connected to the supply voltage, advantageously to the controlled supply voltage.
In accordance with a further feature of the invention, the voltage divider can be constructed as a filter, in order to suppress voltage fluctuations which are caused at the parallel run controller by a changing load.
In accordance with an added feature of the invention, there is provided a further voltage controlling circuit. This circuit may be connected to the supply-voltage controlling circuit in an advantageous way. In other words, it may be supplied with power by the latter, in order to supply a more stable output voltage in this way.
In accordance with a third embodiment of the invention, the current mirror circuit is integrated into the rectifier circuit. As a result, no further voltage drops between the rectifier circuit and the load, since the current mirror circuit shares the use of a diode already contained in the rectifier circuit. In an advantageous way, two current mirror circuits are used, since then the pulse frequency of the input current of the demodulator circuit to be connected downstream of a current mirror circuit is twice as high, so that the filtering in the demodulator circuit is made easier.
In accordance with a further feature of the invention, the voltage drop may also be detected and evaluated at rectifier diodes themselves. For this purpose, at least one further pair of diodes is connected in an advantageous way in parallel with a pair of diodes of the rectifier circuit leading to a supply line. The connecting point of the diodes, disposed with opposite polarity, of this further pair of diodes, is connected to a first input of a demodulator circuit. The supply voltage delivered by the corresponding pair of diodes of the rectifier circuit is fed to a second input of the demodulator circuit, so that the demodulator circuit can evaluate the differential voltage. The diodes represent a non-linear resistance, the characteristic of which must be taken into consideration in the demodulation. This variant of the invention has one advantage which is that no additional voltage drop occurs and another advantage which is that the extension of the rectifier circuit can be carried out very simply in practice.
In accordance with an a concomitant feature of the invention, a combination of the supply voltage control in the load circuit with the current measurement in the supply circuit, is achieved by providing a series control circuit, in which the voltage is controlled at the output of the rectifier circuit. A transistor disposed in the current path between the rectifier circuit and the supply-voltage control circuit is driven in such a way that its resistance changes according to the current in the supply circuit so that the voltage drop across the transistor remains virtually constant. The control signal for the transistor is then a measure of the current in the supply circuit and consequently of the modulation.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and descr
Güngerich Volker
Reiner Robert
Schmitt-Landsiedel Doris
Schraud Gerhard
Greenberg Laurence A.
Infineon - Technologies AG
Lerner Herbert L.
Mis David
Stemer Werner H.
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