PROCESS OF AMPLITUDE MODULATION, IN PARTICULAR FOR READER WITHOUT CONTACT, ETDISPOSITIF OF IMPLEMENTATION

The present invention relates to processes for amplitude modulating a carrier frequency.

It is particularly useful in the contactless readers, such as those that are used for reading out information stored in contactless chip cards or labels said "TAG" for identifying all kinds of products.

It is known that chip cards in contactless reading, and that the labels, said "TAG", are increasingly commonly used and in many cases require the use of a portable reader of a small size which has its own source of integrated energy, a battery, that it is desired to recharge often does that the least possible.

The reader must be of the size and weight as low as possible, and consume minimal power. Power consumption in such a reader is particularly important in the transmission stage of the carrier for interrogating the reading circuit. Since the level of transmission is fixed by the operational requirements, it is important to increase the yield of the stage by reducing the losses to decrease the overall consumption.

The modulation used is of known amplitude modulation in general, by the abbreviation "ASK" (Amplitude Shift Keying), represented schematically in Figure 1.

On this figure, the reader 101 connected to an antenna 102 transmits to the "TAG" 104 103 a wireless signal amplitude modulated. As shown schematically in Figure, the amplitude modulation of the signal 103 is to alter the level of the transmitted wave between a value Vmax, corresponding for example to a bit 1, and a value Vmin, corresponds in this case to a bit 0. It is necessary to keep such level of Vmin minimum transmit to keep the synchronization of the receiver and for distinctness between the reception of an information element and a transmission interruption.

The method most known for amplitude-modulating carrier includes, as shown in Figure 2, to supply a variable voltage in the output stage to amplify the carrier frequency. Thus the output transistor 201 receives on its gate the carrier frequency. It is fed from a power source + by a power supply circuit 202 consisting essentially of a modulation transistor 203 which receives on its gate the modulation signal. A inductance surge 204 is serialized between the circuit 202 and the transistor 201, so as to isolate the power at high frequency RF.

Impedance matching circuitry and filtering 205 eliminates the harmonics and matching the output impedance of the transistor 201 to that of the antenna used, to deliver thereto a valid signal.

The arrangement is particularly suited to the amplifiers Class C or Class D, in which the transistor 201-switched from a saturated state to an off state at the rate of the carrier frequency. In these conditions the signal substantially square thus obtained at the output of this transistor has a level that varies as a function of the supply voltage delivered by the modulation transistor 202. May readily compute the sum to obtain a variation of the supply voltage between the values Vmax and Vmin abovementioned.

The circuit 205 consequently getting a substantially sinusoidal signal from the square wave signal delivered by the transistor 101.

The method has various disadvantages, in particular that of requiring a modulating transistor 202 supporting a power at least equal to that of the output transistor 201, since these two components are to transmit the same power. The thereby dissipates wasteful in the modulation transistor large power, which is to be charged to the battery without contributing to the emission. The dissipation corresponds at the same time to a voltage drop in the modulation transistor, of the order of 200 to 500 millivolts according to the transistor used. This prevents operate the device under a low supply voltage, thus requiring a battery voltage greater than that which is strictly necessary, which increases the stresses at the battery.

Finally the cost of this solution is high since it a larger number of components, some of which, in particular the modulation transistor, are themselves relatively expensive.

Other known methods of modulating, in particular purely linear in which the carrier frequency is modulated at small level, which is relatively easy, to be then amplified to the power level required to output. The amplification is then performed using a linear amplifier which has difficulties of construction and being bulky, expensive and power consumption.

In the, the invention provides a method for amplitude modulation, in particular for contactless reader, wherein is applied to a power transistor a signal at the frequency f of the carrier to modulate, essentially characterized in that the control signal is a rectangular signal formed successive pulse at frequency f and width modulated.

According to another feature, the transistor is supplied by means of an energy storage circuit and isolating the continuous supply of the high frequency directly from a power source, and the width of the pulses is provided to lock and unlock the transistor during times allowing said energy storage circuit and isolation of discharging at least a portion of its energy into the output circuit for obtaining the desired amplitude for the modulation.

The invention also relates to a device for implementing said method, mainly characterized in that it comprises a first transistor powered by an energy storage circuit and isolation of the high frequency, a reference oscillator at a frequency equal to or multiple of f, a duty cycle generator to generate from the reference oscillator a rectangular signal width modulated by a signal indicative of the level to be sent and for applying the rectangular signal to the control electrode of the transistor, and a matching circuit filter and powered by the first transistor for delivering the amplitude modulated signal.

The energy storage circuit and insulation may be performed straightforwardly by an inductance surge.

According to another feature, the duty cycle generator comprises a resistor and a diode connected in parallel and supplied by the reference oscillator, a first capacitor charged by the diode with the resistor to determine a first time constant, a second capacitor charged by said diode under the control of a second transistor to determine with the first capacitor and the resistor a second time constant, and a comparator operating in Schmidt trigger, whose input is connected to the common point to the capacitors and to the resistor and whose output is connected to the control electrode of the first transistor.

According to another feature, the direction of the diode is reversed, the resistor is replaced by a capacitor and the capacitors are replaced by resistors.

According to another feature, 1' reference oscillator operates at a frequency which is a multiple of the frequency f and the duty cycle generator is formed of digital circuits using the signal of the reference oscillator to obtain by divisions and pattern a signal at the frequency f with the duty cycle desired.

According to another feature, the matching circuit and of filtering further comprises means for switching reactive elements to accommodate circuit duty cycles used.

Other features and advantages of the invention will appear clearly in the following description, presented of non-limiting example facing the appended Figures which represent:

-figure 1, a schematic representation of a transmission between a reader and a "TAG" using an amplitude modulated signal;

-figure 2, a diagram of the amplitude modulator operating by varying the supply voltage, according to a known technique;

-figure 3, the schematic diagram of a modulation system according to the invention;

As shown in Figure 3, according to the invention the output transistor 301 is directly supplied from a power source + constant voltage via a shock impedance 304.

It has taken in this example an inductance of shock, of course any circuit capable of storing energy and isolate the continuous supply of the high frequency may be suitable. Could also take a tuned LC circuit to the carrier frequency.

The transistor 301 is a switching transistor of any type, but having the properties desired frequency and voltage, herein a MOS transistor, which receives on its gate a signal input for the switching operating effectively, i.e. between a saturated state and an off state.

In this way it operates in a class D and the matching circuit 305 and filtering eliminates unwanted harmonics corresponding to the signal obtained by square output of the transistor, to apply the antenna with a substantially sinusoidal signal with a known impedance.

In these conditions, when the transistor 301 is saturated, the impedance 304 is charged when assuming the locked mode, it discharges in the antenna through the circuit 305. In the level of maximum voltage obtained in the per part of discharge depends on the duration thereof, i.e. the time during which the transistor is off, and another portion, of the time during which the inductance may have recharge, to the extent that this duration is not sufficient, taking into account the value of the inductance, to obtain a full charge.

Relationships between these and the value of the inductance can be obtained with ease from the elementary laws of electricity, according to a known method.

The invention therefore provides the power the gate of the transistor 301 with a rectangular signal T=l/f period corresponding to the frequency f carrier to obtain, at the output of the modulation circuit, and in amplitude modulation of the output signal by varying the duty cycle of the rectangular signal to obtain the voltage variations output corresponding to the desired modulation.

The biases that the duty cycle is defined as the ratio between the conductive duration t of the transistor and the period T of the carrier frequency, itself corresponding to the ratio between the duration of the high level of the carrier frequency and its period t.

Thus to obtain an output signal corresponding to the amplitude Vmin of Figure 1, is applied to the gate of the transistor 301 a signal 306 tl low duration relative to the period t. In this example, the transistor 301 is on for the high levels and applied to its gate blocked by the low levels. Therefore, it is passing for the duration tl, allowing the inductance 304 to charge for a time sufficient for the voltage has the value Vmin. Transistor 301 When the locks, the inductance 304 discharges into the matching network 305.

By against When is desired at the output of the circuit a signal 305 to the voltage Vmax, is increased the duration of the high level applied to the gate of the transistor 301 t2 to a value corresponding to the pulse 307. The inductance 304 discharges thus longer, resulting in an output voltage above the maximum voltage obtained with the pulse duration 306 tl. The duration t2 is arranged to obtain voltage Vmax corresponding to the other state of the modulation with respect to the voltage Vmin.

In practice, so that the assembly works well and efficiency is maximum, use a maximum value of the duty cycle which is substantially equal to 50%.

In fact, taking into account the switching operation of the transistor 301, the output signal thereof is very substantially the signal applied to its gate, amplified voltage and power. This is the matching circuit 305 to achieve the desired sinusoidal signal, whose level variations, i.e. the modulation, correspond to the level changes at the input of said filter circuit. The latter mechanism corresponding to the operation well known Class D.

The implementation of the method corresponding to Figure 3 can be achieved using for example a circuit represented by the block scheme of Figure 4.

In this circuit, a reference oscillator 308 outputs a signal that is at a frequency equal, or optionally multiple, at the desired frequency for the carrier at the output of the matching circuit 305 and filtering. The duty cycle of the signal provided by the oscillator is calibrated to a reference value, for example 50%.

The signal of this oscillator 308 is applied to a duty cycle generator 309 which further receives the signal corresponding to the level to be transmitted and alters the duty cycle of the oscillator signal 308 according to the level which is to be indicated. Therefore, a width modulated signal corresponding to the signal of Figure 3 306/307 This signal is applied to the gate of the switching transistor 301.

The duty ratio generator 309 may be performed any number of ways.

In a first embodiment, represented in Figure 5, an analog device is used in which the signal from the oscillator 308 508, which has a duty cycle equal to 50%, is applied with a diode 519, itself in parallel over a resistor 529, 539 to a capacitor connected to ground. The common electrode to the capacitor and the diode is connected to the input of a comparator 509.

The capacitor 539 is charged by the rising edge of the signal 508 and it discharges from the falling edge of this signal 508, with a time constant determined by the resistor 529. The time constant determined by the capacitor and the resistor determines the duration tl of Figure 3. The circuit 509, realizes into the pulse 307 applied to the gate of the transistor 301. The circuit 509 transforms a signal having characteristics analog into a digital signal (on/off). It may be obtained by a logic circuit to two thresholds or by a Schmidt trigger or by a comparator with single threshold. In the example shown, it is a logic circuit.

For producing the pulse 306, is used as the second capacitor 549, connected in parallel with the capacitor 539 by being connected to ground through a switching transistor 559, whose gate receives a signal corresponding to 1' indication of the level Vmax. When indicating emission Vmax is applied to the gate of this transistor 559, it is saturated and the value of the capacitor 539 is increased the value of the capacitor 54,9. These two capacitors in parallel thus allow, with the resistor 529, a higher time constant, which is adjusted to provide the tl time corresponding to the transmission of the signal Vmin.

Optionally may be other capacitors in parallel for larger number of output levels of the RF carrier.

The duty ratio generator, operating in an analog manner, can be made of other way, for example as in Figure 6 wherein the role is reversed the resistors and capacitors. On this figure, the diode 619 is connected in the reverse direction of the diode 519 and in parallel with a capacitor 629 replacing the resistance 529. The two capacitors and 549 539 are replaced by two resistors and 649 639. Total At is obtained again two time constants corresponding to the combination of the capacitor and of the one or two resistors.

Another is accomplished by performing the duty cycle generator 309 digitally, using an oscillator 308 which delivers a frequency that is a multiple of the frequency of the RF signal output. Can then, using logic circuits operating in a known manner by divisions and logical combinations, dividing the frequency to obtain a control signal of the transistor 301 having the desired RF frequency and the duty cycle required. The ratio may itself be modified at the rate of the modulation by acting on the control logic.

Furthermore, as is known, the reaction of a filter network whereby the circuit 305, to a signal having a variable duty cycle, to provide an output signal substantially sinusoidal, is to some extent dependent on the duty cycle, in particular the performance of the system and the matching of the output impedance.

The invention therefore provides, as improvement, provided at said network

adaptation and one or more a

switching for insertable reagents, such that a further inductance for example, to optimize the performance of the system and the value of the impedance of output based on the duty cycles used.

The control of these switching means will be from the same signal controlling the duty cycle generator 320.

The modulation method and for minimising the number of components and thus reduces the cost of the drive operating according to this method. The reader then has a very good energy efficiency and it can be operated under a supply voltage as low as possible. The structure is particularly well suited for integration in microelectronics ASIC to obtain a dedicated to this function or a subset of an ASIC.

Finally the temperature stability of the assembly is remarkably excellent, due to operation near digital of the assembly.