FR2644308A1 - Device for modulating an electromagnetic wave - Google Patents

Abstract

The invention relates to a device for modulating an incident wave of frequency F to generate a reflected wave modulated in amplitude and in phase. This device comprises an antenna 2 capable of picking up and retransmitting on the frequency F, an oscillating circuit 3 coupled as a reflector on the antenna and provided with a variable reactance 8, 9 which can be controlled in voltage so as to be able to give it a frequency d 'tuning as a function of its impedance and surrounding the frequency F of the incident wave, and a control voltage generator 1 coupled to the variable reactance 8, 9 to generate a time-varying control voltage Sc, having a spectral range at more equal to the width of the passband of the oscillating circuit 3. The invention provides an autonomous modulation device, of small size, which can be applied to portable badges, labels, miniaturized beacons, etc.

Description

DEVICE FOR MODULATING AN ELECTROMACNETIC WAVE
The invention relates to a device for modulating an incident electromagnetic wave of determined frequency in order to generate a reflected wave modulated in amplitude and in phase. The device can be adapted to modulate waves in a wide frequency range, from low frequencies to millimeter waves.

 The electromagnetic wave modulators are of very diverse types and allow information to be inserted into an incident wave for the purpose of transmitting it, this information being of any kind, depending on the application presence detection, identification, transmission of coded information ...

 The invention relates to a modulation device used to modulate the incident wave with a view to inserting therein modifiable information which can be controlled locally from the device. It proposes to provide a modulation device leading to very low energy consumption, so as to give it both good autonomy and a very small footprint.

 Generally, the devices of the type mentioned above in which the modulation is controllable, are produced by means of electronic circuits with transistors or diodes requiring currents or voltages too high to allow their application on autonomous systems of small bulk. In particular, these devices are not compatible with power supply using miniaturized batteries or a small number of solar cells.

 An objective of the invention is to provide a new modulation device applicable to autonomous systems of small size, in particular portable badges, labels on object, miniaturized beacons ...

capable of modulating, on demand, an incident wave and transmitting it without material contact. This modulation can be a signature specific to the modulator, or be representative of information to be transmitted (measurement in the case of a sensor, recognition information allowing presence detection and, where appropriate, location).

To this end, the device according to the invention making it possible to modulate an incident wave of frequency F determined in order to generate a reflected wave modulated in amplitude and in phase, comprises in combination
- a real impedance antenna Za, capable of picking up and retransmitting on the aforementioned frequency F,
- an oscillating circuit coupled as a reflector on the antenna and having a bandwidth of predetermined width,
a variable voltage-controllable reactance forming part of said oscillating circuit so as to give it a tuning frequency F var as a function of its impedance, said oscillating circuit having characteristics adapted to allow a variation of the tuning frequency Fvar on a range surrounding the frequency F of incident wave so that, for Fvar = F, said oscillating circuit has a minimum reflection coefficient,
- a control voltage generator, coupled to the variable reactance in view. of its control and adapted to generate a control voltage variable over time, having a spectral range less than or equal to the width of the passband of the oscillating circuit.

 Thus, in the device of the invention, the modulation is carried out by variation of the reflection coefficient of an oscillating circuit which is obtained by variation of its tuning frequency: this variation is generated by the variable reactance which is controlled at this effect by the control voltage from the generator. Among the variable reactances currently available, there are some which consume extremely low powers (of the order of nanowatt or less) and which require low control voltages (of the order of a few volts) under extremely low currents (of nanoampere order or less).

 Thus, the modulation device according to the invention is compatible with miniaturized power supplies of very low power, since the oscillating circuit and its reactance consume only a very low power, and the control voltage generator can itself be even provided for very low power (low output voltage, extremely low output current), so that it is possible to use, for this generator, conventional integrated circuits (low voltage MOS type) whose internal consumption is very low (around microwatt).

 In particular, the variable reactance is preferably carried out by means of a reverse biased diode, associated with a resistor, said diode having an equivalent capacity Cvar variable as a function of the control voltage, capable of taking a value C allowing equality var = F. This choice of reactance leads to an optimized modulation device in terms of energy consumption.

The diode is preferably chosen so that its equivalent capacity C for Fv ar = F satisfies the relation:

This choice of characteristic of the diode limits the parasitic losses consumed in the oscillating circuit, because the impedances of diode and antenna are of the same order, which allows a transformation of impedance at minimum loss. The preferential values corresponding to the relation

où a var est la variation de l'impédance # ver correspondant à la plage de variation de la fréquence d'accord F ver
Un tel agencement optimise la valeur maximum du coefficient de réflexion de sorte que la variation d'amplitude de l'onde réfléchie modulée est maximale entre ltétst de réflexion minimum et l'état de réflexion maximum.De plus, la condition

combinée avec la condition précédente

permet d'assurer la transformation d'impédance à perte minimum Les valeurs préférentielles correspondent à la relation

According to a first embodiment, the variable reactance is carried out by means of a variable internal impedance diode # ver and a series resistance of value R satisfying the relation

where a var is the variation of the impedance # ver corresponding to the range of variation of the tuning frequency F ver
Such an arrangement optimizes the maximum value of the reflection coefficient so that the amplitude variation of the modulated reflected wave is maximum between the minimum reflection state and the maximum reflection state.

combined with the previous condition

ensures the transformation of impedance at minimum loss The preferred values correspond to the relationship

According to another embodiment, the variable reactance is carried out by means of a variable internal admittance diode Yvar and a parallel resistance of conductance G satisfying the relation

The same comments as above can be made.

 In addition, in order to make the amplitude modulation maximum for a given control voltage, the oscillating circuit (including the variable reactance) is provided to present characteristics such that the tuning frequency F var varies within a range [ F, F fa 7 where ss F is the maximum algebraic amplitude of variation of FVars the oscillating circuit then having a minimum coefficient of reflection for one of the limits of the range (rvar = F).

 The amplitude of the reflected wave thus varies between a very low value and a maximum value, which optimizes the modulation.

 The device according to the invention can be applied to modulate an incident wave in a frequency range F less than 3 GHz. In this case, the oscillating circuit comprises a combination of inductors, capacitors and localized resistors, coupled to the variable reactance produced by means of a Schottky diode, or of a diode with PN junction, reverse biased.

In practice, the Schottky diode will be chosen for lower voltages of the voltage generator (for example of peak-to-peak amplitude less than 3 volts), the PN junction diode being chosen for higher voltages (for example of amplitude peak to peak greater than 2 volts). The reverse-biased Schottky diode indeed exhibits variations in capacitance which are more sensitive to variations in the control voltage than the junction diode.
PN. On the other hand, the latter is more accessible on the market and less expensive so that, in certain applications, it may be advantageous to choose this junction at the price of an increase in the control voltage.

 In the case where the device according to the invention is applied in a frequency range F of between 1 and 30 GHz, the oscillating circuit can comprise an association of sections of printed lines, capacitors and resistors, coupled to the variable reactance produced on either by a Schottky diode (preferably associated with a control voltage less than 3 volts), or by a PN junction diode (preferably associated with a control voltage greater than 2 volts).

 The device according to the invention can finally be applied to modulate an incident wave of frequency F greater than 10 GHz. In this case, the oscillating circuit can comprise a combination of waveguides and obstacles coupled to the variable reactance (the latter being as previously carried out by a Schottky diode or a diode with PN junction reverse biased, according to the voltage of generator control>.

 With regard to the recovery values of the abovementioned frequency ranges, the technologies concerned can be chosen interchangeably.

The description which follows with reference to the appended drawings presents, by way of nonlimiting examples, three versions of the device according to the invention and illustrates their operation; on these drawings
FIG. 1 is an electronic diagram of a first version (device with localized constants),
FIGS. 2a, 2b, 2c, 2d show the appearance of waves and of electrical signals generated in this device,
FIG. 3 is a diagram of a second version (device with distributed constants drawn on a scale equal to approximately 5),
- Figures 4 and 5 are diagrams in section through perpendicular planes AA and BB, of a third version in waveguide technology tå scale equal to approximately 5).

 The modulation device shown in FIG. 1 is intended to modulate an incident wave of frequency F included in the VHF range around 100 MHz. This device produced in local constant technology consists of a printed circuit integrated into a label, for example for the recognition of an object.

 To this end, an incident wave 0zinc of frequency F is sent to the device (FIG. 2a) and the recognition is carried out by analyzing the reflected wave ref modulated in amplitude (FIG. 2d) which the device of the invention re-emits.

This device comprises a generator 1 of control voltage Sc adapted to generate a voltage Sc variable over time, the shape of which is shown diagrammatically in FIG. 2c. This generator comprises in the example an ALIM power source which can be constituted by several solar cells associated with a reserve capacity; these cells can in particular be constituted by eight 5/5 mm silicon cells capable of generating an output voltage of the order of 3 volts (in a room with average lighting). These cells supply a COD coder of the 8-bit "CMOS standart" type, capable of delivering an eight-bit code in the form of rectangular voltage pulses with an amplitude of 3 volts (FIG. 2b). The rhythm of these pulses is around 10 kilobits per second. This code amplitude modulates an OSC oscillator whose frequency is 100 kilohertz (for example "CMOS standard" quartz oscillator)
The encoder / oscillator assembly is produced in the same integrated circuit. The control signal Sc at the output of the oscillator consists of pulse trains (at the frequency of 100 kilohertz), carrying the code specific to the coder COD. For each object to be distinguished, this code will be different. The spectral range of the control voltage Sc is of the order of 100 kilohertz.

 Furthermore, at its opposite input, the device comprises an antenna 2, in particular a printed antenna, constituted by a conductive spiral of dimensions suitable for picking up and retransmitting on the aforementioned frequency F (100 MHz in the example). This antenna has a real impedance Zs equal in the example to 50 ohms at the frequency F.

 An oscillating circuit 3 is coupled to this antenna 2, a reflector, the tuning of which is controlled in voltage by the control signal Sc through a filter 4.

 The oscillating circuit 3 comprises a coupling capacitor 5, a choke 6, a connecting capacitor 7, a reverse-junction PN 8 diode, of variable internal impedance #var 'and a resistor 9 in series with said diode.

 The diode 8, the resistor 9 and the choke 6 constitute the resonant part of the oscillating circuit, while the capacitor 5 ensures the adaptation between antenna and resonant part and that the capacitor 7 avoids the short-circuit of the control by the self 6 (while having a negligible impedance with respect to high frequency currents).

 The diode 8 can in particular be of the "BA 105 RTC" type manufactured by "PHILIPS"; this diode has a variable equivalent capacity Cvar which, when the diode is biased at zero voltage, is denoted C and is equal to 32 pF.

The characteristics of the circuit are determined to verify the relationship

ohms. La diode présente une résistance interne qui s'ajoute à la résistance 9 donnant une valeur totale de résistance en série R = 5 ohms.The variation in capacitance of the diode which is used in the oscillating circuit is 16 pF corresponding to a variation in impedance

ohms. The diode has an internal resistance which is added to the resistance 9 giving a total value of resistance in series R = 5 ohms.

These values were chosen to satisfy the relationship:

 The conditions indicated above provide a tuning frequency F var varying between the frequency F = 100 MHz and an extreme frequency of the order of r + # r F = 140 MHz, the reflection coefficient of the oscillating circuit being minimum at the lower limit of the variation range (100 MHz) and maximum at its upper limit (140 MHz); the bandwidth of the oscillating circuit has a width of 40 MHz, much greater than the spectral range of the control voltage Sc (100 kiloHz); these conditions make it possible to optimize the modulation efficiency as regards both the insertion loss and the sensitivity to the command.

 The control voltage Sc which generates the variation in capacitance of the diode 8 is delivered to the latter through a filter 4 intended to avoid a transfer of high frequency energy to the generator 1 and to eliminate the DC component of the control voltage. To this end, this filter comprises a shock inductor 10 (for example of the "VK 200 RTC" type), a resistor 11 for suppressing static charge and a connection capacitor 12 eliminating the DC component.

 Thus, as shown in FIG. 2d, the wave reflected by the oscillating circuit 3 and re-emitted by the antenna 2 0rez 'is amplitude modulated as a function of the coded information carried by the control signal Sc, between a negligible value (corresponding to the minimum reflection coefficient) and a #max value (corresponding to the maximum reflection coefficient) equal to the amplitude of the incident wave reduced by insertion losses.

 The modulation device described is very compact and can fit on a thin label, dimensions 5/10 cm; it is autonomous since it is supplied by cells, this supply being authorized by its very low consumption (approximately 30 microwstts).

It should be noted that an equivalent circuit consists in providing a resistance in parallel of the diode, the variation of internal admittance of the diode and the conductance G of the resistance in parallel being linked by the relation equivalent to the previous one

 In addition, the control generator can be provided with a lower number of cells, for example half in this case this generator is produced in "C M OS low voltage" technology and the diode is advantageously replaced by a Schottky diode.

 Figure 3 shows schematically another embodiment of the device of the invention, in printed circuit technology, with distributed constants. This device is in the example intended for the modulation of an incident wave of frequency F = 10 GHz. Its operation is the same as the previous device, only changing the production technology.

 The generator 1 of the control signal Sc has not been shown, nor the resistor 11 and the capacitor 12 of the filter 4 which, being part of the low frequency circuit, remain in technology with local constants as before.

The device consists of a plate 13 of teflon glass, the rear face (not visible) is fully metallized and has a window symbolized at 14 constitute a slot antenna ("Slot" type). This is coupled to the oscillating circuit constituted by sections of printed lines 15, and by a diode 16 in the example diode
Schottky reverse polarized (for example "THOMSON DP 320").

A postponement of mass 17 with metallized holes establishes the electrical connection with the metallized rear face. Line sections 18 form the equivalent of the impact choke of the filter 4.

A connection block 19 allows the connection with the other elements of the filter and the control generator. The production and dimensioning of line sections are well known to those skilled in the art in the field of microwave frequencies; the diode is chosen as above to present an equivalent capacity and a series resistance, satisfying the relationships already provided.

Figures 4 and 5 show respectively in section AA and BB the oscillating circuit of the device produced in waveguide technology, in order to modulate an incident wave of frequency F = 35 GHz. The antenna (not shown) can in particular be in this case an antenna
Horn.

 The resonant part of the oscillating circuit is constituted by the guide section 20 (of length d) terminated by pole obstacles 21 and 22. The pole 22 ensures the coupling with the antenna guide, while the pole 21 ensures the coupling with the diode 25 of schottky type or with P junction according to the control voltages coming from the generator. The choice of the characteristics of the diode is carried out as previously to satisfy the relationships already defined.

 A section of coaxial line 23 associated with an iron-loaded adhesive joint 24 constitutes the equivalent of the impact self of filter 4.

Claims (10)

 1 / - Device for modulating an incident wave of determined frequency F, intended to generate a reflected wave modulated in amplitude and in phase, characterized in that it comprises in combination  - an antenna (2) of real impedance Za, able to pick up and re-emit on the aforementioned frequency F,  - an oscillating circuit (3) coupled as a reflector on the antenna and having a bandwidth of predetermined width,  - a variable voltage-controllable reactance (8, 9), forming part of said oscillating circuit (3) so as to give it a tuning frequency F var as a function of its impedance, said oscillating circuit having characteristics adapted to allow a variation of the tuning frequency #var on a range surrounding the frequency F of the incident wave so that, for Fvar = F, said oscillating circuit has a minimum reflection coefficient,  - a control voltage generator (1), coupled to the variable reactance (8, 9) for the purpose of its control and adapted to set a time-varying control voltage (Sc), having a spectral range less than or equal to 8 the width of the bandwidth of the oscillating circuit (3).  2 / - Modulation device according to claim 1, characterized in that the variable reactance is carried out by means of a diode (8) polarized in reverse, associated with a resistor (9), said diode having an equivalent capacity C var variable as a function of the control voltage (Sc), capable of taking a value C allowing equality FV8r = F.  3 / - Modulation device according to claim 2, characterized in that the variable reactance is achieved by means of a diode (8) whose equivalent capacity C for # var = F satisfies the relation

 4 / - Modulation device according to one of claims 2 or 3, characterized in that the variable reactance is produced by means of a variable internal impedance diode #var and a series resistance of value R satisfying the relationship

 where # var is the variation of the impedance #ver corresponding to the range of variation of the tuning frequency Fvar  5 / - Modulation device according to one of claims 2 or 3, characterized in that the variable reactance is carried out by means of a variable internal admittance diode #ver and a parallel resistance of conductance G satisfying the relationship

 6 / - Modulation device according to one of claims 1, 2, 3, 4 or 5, characterized in that the oscillating circuit (3) comprising the variable reactance has characteristics such that the tuning frequency F ver varies in a range [F, F + a rj where AF is the maximum algebraic amplitude of variation of FVar, said oscillating circuit thus presenting a minimum coefficient of reflection for one of the limits of the range (Fvar = F).  7 / - Modulation device according to one of claims I to 6, characterized in that the control voltage generator (1) is coupled to the variable reactance (8, 9) by means of a filter (4 ) in order to avoid a transfer of high frequency energy to the generator and to eliminate the DC component of the control voltage.  8 / - Device according to one of claims 1 to 7, for the modulation of an incident wave of frequency F less than 3 GHz, in which the oscillating circuit comprises a combination of inductors, capacitors and localized resistors, coupled to a reactance variable realized by means of a Schottky diode or with PN junction polarized in reverse.  9 / - Device according to one of claims I to 7 for the modulation of an incident wave of frequency F between 1 and 30 GHz, in which the oscillating circuit comprises an association of sections of printed lines, capacitors and resistors, coupled to a variable reactance produced by means of a Schottky diode or a reverse junction PN junction.  10 / - Device according to one of claims 1 to 7, for the modulation of an incident wave of frequency F greater than 10 GHz, wherein the oscillating circuit comprises a combination of waveguides and obstacles coupled to a variable reactance achieved by  a Schottky diode or a reverse junction PN junction.