FR2527870A1 - Microwave transponder for transceiver interrogation - includes microwave circuits built on dielectric surfaces to provide unique response coded signal - Google Patents

Abstract

The transponder which is intended to receive an interrogation signal transmitted by a transmitter-receiver, returns to the transmitter-receiver an echo signal which gives a code characteristic of the transponder. The latter includes a card consisting of a plate of dielectric on the surface of which is a single radiating surface formed using microband techniques. The plate serves as a receiving aerial for interrogation signals, and a transmitting aerial for the double frequency response signal. Another plate is covered with conducting material and resides immediately behind the first to serve as an earth plate. A third plate (behind again) carries microwage circuits for rectification, filtering, frequency conversion, modulation and coding.

Description

Coded answering card, intended to be associated with a microwave transceiver in particular, for the identification of objects.

The invention relates to answering machines intended to receive an interrogation signal transmitted by a transceiver and to send an echo signal to the latter capable of supplying the interrogation transceiver with a code characteristic of the answering machine. . These responders can be used to identify fixed or mobile objects carrying the responder.

In the first case, the interrogator is mobile and the applications concerned are, for example, the identification of people or the communication of various information to motor vehicles equipped with the interrogator, from answering machines placed in the roadway.

In the second case, the interrogator is fixed and the applications concerned are, for example, sorting postal parcels carried by rolling conveyors, identifying a vehicle in road traffic, or an object on a chain mass production of a human being entering a dangerous or prohibited area, etc.

The responder's code can be stored in a computer and control a process, and therefore the responder must be able to provide a significant amount of information in digital form. In addition, the answering machine must not contain any source of clean energy, such as battery or accumulator. In certain cases, it has been proposed to associate the answering machine with an interrogator emitting a frequency divided into brief periodic rectangular signals and to make the answering machine comprise a non-linear element which generates, from the signal a: 'interrogation, a harmonic, of preferably the two harmonic, of said frequency and a modulator, controlled by a coding device, which modulates said harmonic to give a response signal itself divided into rectangular signals.

It has also been proposed to supply the coding device and the modulator by rectifying the interrogation signal in the answering machine.

However, no satisfactory practical implementation of the abovementioned designs seems to have been made and the problem therefore arises of producing an answering machine which fulfills all of the conditions set out above and is in the form of a weak card. dimensions, easy to place on the object, having a practically unlimited lifespan and requiring no maintenance.

The subject of the invention is such a passive responder card, which is simple to make, compatible with mass production, capable of responding to interrogation signals of low power (for example 10 to 15 milliwatts during the duration of each signal rectangular) and to send response signals with a carrier frequency twice that of the interrogation signals having a satisfactory amplitude.

According to the invention, these results are mainly obtained by constituting the transponder in the form of a card composed of a first dielectric plate on the external surface of which is produced, in the form of microstrip, a rectangular radiating surface. single arranged to constitute an antenna for receiving the frequency of the interrogation signal and for transmitting the double frequency of the response signal, a second and a third dielectric plate, the second plate being sandwiched between the other two and covered with a conductive layer forming a ground conductor, the third plate comprising microwave rectifying, filtering, frequency conversion, modulation and coding circuits.

According to a preferred embodiment, the same circuit performs both the frequency conversion and the modulation of the response signal and comprises an input bandpass filter centered on the frequency of the interrogation signal, an electronic component not linear, an output bandpass filter centered on the double frequency and means for applying the coding signals to said component, so that they constitute a bias voltage varying the conversion losses of said component.

Other features, as well as the advantages of the invention, will become apparent in the light of the description below.

In the attached drawing
Figure 1 is an exploded view of a transponder card
conforms to a preferred embodiment of the invention
tion
Figure 2 is an equivalent diagram, in elements to
localized constants, of the rectification circuit that
has said card
Figure 3 is an equivalent diagram, in elements to
localized constants, of the conversion circuit of
frequency and modulation; and
FIG. 4 represents a preferred embodiment of the
coding circuit.

In Figure 1, there is shown a card formed by the sandwich assembly of three plates of "dielectric (eg alumina) 1, 2 3. The total thickness of the card does not exceed 4 mm, its surface 9 x 4 cm.

The plate 1 carries on its outer face a metallic layer forming, with the dielectric and the metallic layer 20 carried by the corresponding face of the plate 2, a parallelepiped resonant cavity 10, an impedance matching line 11 for the signal d interrogation received at frequency f and an impedance matching line 12 for the response signal retransmitted at frequency 2f.

The two narrow side faces of the cavity corresponding to the short sides of the rectangle 10 constitute a receiving dipole of the frequency f, while the other two narrow side faces, corresponding to the long sides of the rectangle 10, constitute a transmission dipole of the frequency 2f. The use of the same resonant cavity for the reception and re-emission of the signal limits the space requirement on the card. No adjustment of such a dual frequency antenna is necessary, once it has been properly calculated. The antenna has linear polarization. It has a relatively low gain (for example, 2.5 dB for f = 1.250 MHz and 6 dB for f = 2.500 MHz). Line 11 is connected, by means of a conductor 110, to a rectification circuit formed by a metal layer carried by the internal face of the plate 3 and by a diode 30 incorporated in said plate, while the line 12 is connected, by means of a conductor 120, to a frequency conversion and modulation circuit formed by a metal layer carried by the internal face of the plate 3 and by a diode 31 incorporated in said plate.

The rectification circuit is the transposition, in elements with distributed constants, of the block diagram of FIG. 2. I1 essentially comprises a bandpass filter, centered on the frequency f (Selfs L1, L2 capacitance
C1); diode 30; a low-pass filter (L3, L4, C2) and an input impedance matching circuit comprising a transformer T1. The load, of impedance Zc, is constituted by the coding circuit, represented at 4 in FIG. 1, and which can be incorporated in the plate 3.

We find, in Figure 1, the equivalent elements, assigned the same reference symbols. The line sections Tlo and T11 are equivalent to the transformer T1 which allows the circuit to behave, seen from the input, like a characteristic impedance ZR = 15 ohms for the frequency f. It will be noted that the wide lines, therefore with low characteristic impedance, are equivalent to a shunt capacitance, while the narrow lines, with high characteristic impedance, are equivalent to a self in series.

The role of the bandpass filter centered on the frequency f is to reflect the harmonics generated by the diode, which are thus returned to the latter. Its impedance, placed as a branch on the line, must therefore be very large for the signal of frequency f, and tend towards a short circuit for the other frequencies. The low-pass filter must prevent the output of the signal of frequency f and its harmonics and be calculated to give a minimum of ripple at the output.

The values f = 1,250 MHz and f = 2,440 MHz have been given above, by way of example. In practice, the transmission frequency will be chosen from this range of decimeter waves, for which the micro-ribbon antennas and the diode tuning circuits will have a small footprint and the directivity of the antennas will be sufficient to facilitate localization, while the frequency doubling conversion losses remain relatively low. It is also to avoid increasing the conversion losses that we chose the harmonic two, rather than harmonics of higher rank, to facilitate the extraction, by the receiver, of the useful signal in the middle of the signals at frequency f, reflected by various obstacles.

In practice, it is preferable to use an interrogation signal having a peak power of between 10 to 100 watts, cut into rectangular signals having a width e of the order of fifty microseconds, with a recurrence period. of the order of 50 ms for example. We arrange to work with a peak power and at a distance between interrogator and responder such that the power of the signal received by the responder at frequency f is between 10 and 15 milliwatts for the duration 0.

The diode 30 receives approximately 8/10 of this power, and the rectification circuit is calculated to obtain a maximum efficiency (of 85% for example) under these conditions. There is thus obtained, at the output of the rectification circuit, a direct voltage of 4 volts, with a power largely sufficient to supply the coding circuits for the duration e.

The frequency conversion and modulation circuit is the transposition, in elements with distributed constants, of the block diagram of FIG. 3. I1 essentially comprises a bandpass filter L5, C3, L6, centered on the frequency f, a filter bandpass L7, C4, L8, C5, centered on the frequency 2f and having a high impedance for this frequency and reflecting towards the diode 31 all the other frequencies, and a low-pass filter Lg, C6 allowing the coding signals, applied to its input 32-33, to be transmitted to the diode, while preventing signals at frequencies f or 2f from reaching the coding circuits.

The two line sections T20 and T21 carry out an impedance adaptation and are equivalent to the transformer T2 of FIG. 3, of characteristic impedance ZD = 60 ohms. The ratio between the input power of the rectifier and the input power of the doubler is equal to

A line 34 (Figure 1) allows the two impedances to be adapted
ZD and ZR in parallel with the output impedance of the receiving antenna ZC = 50 ohms.

During the zero levels of the coding signal, the diode 31 is not polarized and the conversion loss is 9 dB, while it is polarized during levels one, so that the conversion loss is increased by at least minus 3 dB. I1 results in a modulation of the signal at frequency 2f re-emitted at terminal 35.

This modulation system leads to a particularly simple construction of the circuits.

The coding circuit, represented in FIG. 4, comprises a message generator, composed of six shift registers of eight bits each, 41 to 46. Programming pads with eight fuses each (410 to 460) make it possible to program these registers. The coding circuit further comprises a Manchester coder composed of a flip-flop 47 and three doors
NAND AND 48 to 50, an oscillator, composed of two inverters 51-52 in series and a flip-flop D mounted as a divider 53, which performs the shaping of the clock signals supplied by the oscillator.

On power-up, provided by the rectified voltage Vcc supplied at L4 during the intervals 0 of the interrogation signal, the oscillator starts and, moreover, the "parallel-series" line P / S is maintained for a certain time at logic level one by circuit 54. During this time, the information programmed on the pads is loaded into the shift registers. Then, the P / S line goes to level zero, which bridges the serial transmission of the message to the output 500, connected to the coding input 32 of FIG. 3.

 It goes without saying that various modifications can be made to the answering machine described and shown, without departing from the spirit of the invention.

Claims (6)

Patent claims  1. Responder intended to receive an interrogation signal emitted by a transceiver and to send an echo signal to the latter capable of supplying the interrogation transceiver with a code characteristic of the responder, characterized in that said responder mainly consists of a card composed of a first dielectric plate (1) on the external surface of which is produced, in the form of micro-tape, a single rectangular radiating surface (10) arranged to constitute an antenna reception of the frequency of the interrogation signal and emission of the double frequency of the response signal, a second (2) and a third (3) dielectric plate, the second plate being sandwiched between the two others and covered with a conductive layer (20) forming a ground conductor, the third plate (3) comprising microwave rectifying, filtering, frequency conversion, modulation and coding circuits.  2. Responder according to claim 1, characterized by a same circuit which performs both the frequency conversion and the modulation of the response signal and comprises an input bandpass filter (L5, C3, L6) centered on the frequency of the interrogation signal, a non-linear electronic component (31), an output band filter (L7, C4, L8, C5), centered on the double frequency and means (L9, C63 of applying said coding signals to said component, so that they constitute a bias voltage varying the conversion losses of said component.  3. Responder according to claim 1 or 2, characterized in that said rectification circuit comprises a bandpass filter (L1, L2, C1) centered on the frequency of the interrogation signal, a non-linear electronic component (30) and a low-pass filter (L3, C2, L4) arranged to prevent the transmission, at its output, of the frequency of said interrogation signal and of its harmonics.  4. Responder according to claim 2 or 3, characterized in that said means for applying the coding signals comprise a low-pass filter (L9, C6) preventing  transmitting the frequency of said interrogation signal and its harmonics to the coding circuit.  5. Answering machine according to one of claims 1 to 4, characterized in that the circuits and the antennas which it comprises are arranged to work with maximum efficiency with an interrogation signal in decimeter waves cut into rectangular signals having a duration of the order of a few tens of micro-seconds and a recurrence period of the order of a few tens of milliseconds, with powers received of the order of about ten milliwalts during said duration.  6. Responder according to one of claims 1 to 5, characterized by a logic coding circuit supplied by said rectification circuit and delivering to said low-pass filter the signals of the response message, said logic circuit comprising a message generator composed of '' a plurality of shift registers (41 to 46) programmable by means of fuse pads (410 to 460), a Manchester encoder composed of a rocker (47) and three NAND gates (48 to 50) and a generator clock (51-52).