RADIOFREQUENCY TRANSMISSION DEVICE

Abstract

A radiofrequency transmission device (D) includes: a transmission unit (10) for transmitting a voltage signal (S) in pulsed form; a radiofrequency antenna (A); filtering elements (30); and a voltage source (Vcc), wherein the filtering elements (30) include: n coils (B.sub.1, B.sub.2 . . . B.sub.n), electrically connected in series, of which (n1) coils each have a natural resonance frequency such that:

f.sub.RLi=if.sub.F each having an inductance (L.sub.i) such that, at the predetermined fundamental frequency:

L.sub.TOT=L.sub.1+L.sub.2+ . . . L.sub.i+ . . . L.sub.n=Z.sub.TOT=Z.sub.1+Z.sub.2+ . . . Z.sub.i+ . . . Z.sub.n

and

L.sub.i=Z.sub.i

where f.sub.RLi Is the natural resonance frequency of the i-th coil i is a number varying from 2 to n, L.sub.TOT is the total inductance of the n coils, L.sub.i is the inductance of the i-th coil, Z.sub.TOT is the total impedance of the n coils, Z.sub.i is the impedance of the i-th coil, n is an integer greater than zero.

Claims

1. A radiofrequency transmission device (D) comprising: a transmission unit (10) for transmitting a pulsed voltage signal (S) at a predetermined fundamental frequency (f.sub.F), generating parasitic voltage signals at frequencies which are multiples of the predetermined fundamental frequency (f.sub.F), a radiofrequency antenna (A), matching means (M1) for matching the transmission frequency of the antenna (A) to the value of the predetermined fundamental frequency (f.sub.F), filtering means (30) for filtering the parasitic voltage signals, a voltage source (Vcc), connected to the matching means (M1) and supplying voltage to the transmission unit (10), wherein: the filtering means (30) are electrically connected to the transmission unit (10), and to the matching means (M1), and said filtering means (30) comprise: a number n of coils (B.sub.1, B.sub.2, B.sub.i . . . B.sub.n), electrically connected in series with each other, of which (n1) coils (B.sub.2, B.sub.3, B.sub.4, . . . B.sub.n) have a natural resonance frequency (f.sub.RLi) such that:
f.sub.RLi=if.sub.F where f.sub.RLi is the natural resonance frequency of the i-th coil (B.sub.i) , i is a number varying from 2 to n, f.sub.F is the predetermined fundamental frequency. each having an inductance (L.sub.i) such that, at the predetermined fundamental frequency (f.sub.F):
L.sub.TOT=L.sub.1+L.sub.2+ . . . L.sub.i+ . . . L.sub.n=Z.sub.TOT=Z.sub.1+Z.sub.2+ . . . Z.sub.i+ . . . Z.sub.n
and
L.sub.i=Z.sub.i where L.sub.TOT is the total inductance of the n coils, L.sub.i is the inductance of the i-th coil (B.sub.i), Z.sub.TOT is the total impedance of the n coils, Z.sub.i is the impedance of the i-th coil (B.sub.i), n is an integer greater than zero.

2. The radiofrequency transmission device (D) as claimed in claim 1, wherein the filtering means (30) comprise three coils (B.sub.1, B.sub.2, B.sub.3).

3. The radiofrequency transmission device (D) as claimed in claim 1, wherein the predetermined fundamental frequency (f.sub.F) is in the range from 310 MHz to 434 MHz.

4. A wheel unit, which comprises a radiofrequency transmission device (D) as claimed in claim 1.

5. A hands free access badge for accessing a motor vehicle, wherein the hands free access badge comprises a radiofrequency transmission device (D) as claimed in claim 1.

6. A motor vehicle, which comprises a radiofrequency transmission device (D) as claimed in claim 1.

7. The radiofrequency transmission device (D) as claimed in claim 2, wherein the filtering means (30) comprise three coils (B.sub.1, B.sub.2, B.sub.3).

8. A wheel unit, which comprises a radiofrequency transmission device (D) as claimed in claim 2.

9. A wheel unit, which comprises a radiofrequency transmission device (D) as claimed in claim 3.

10. A hands free access badge for accessing a motor vehicle, wherein the hands free access badge comprises a radiofrequency transmission device (D) as claimed in claim 2.

11. A hands free access badge for accessing a motor vehicle, wherein the hands free access badge comprises a radiofrequency transmission device (D) as claimed in claim 3.

12. A motor vehicle, which comprises a radiofrequency transmission device (D) as claimed in claim 2.

13. A motor vehicle, which comprises a radiofrequency transmission device (D) as claimed in claim 3.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Other characteristics and advantages of the invention will be apparent from a reading of the following description and from an examination of the appended drawings, in which:

[0050] FIG. 1, explained above, shows schematically the radiofrequency transmission device D according to the prior art,

[0051] FIG. 2, explained above, shows schematically the first matching unit 20,

[0052] FIG. 3 shows schematically the radiofrequency transmission device D according to the invention,

[0053] FIG. 4 shows schematically the filtering means 30 according to the invention,

[0054] FIG. 5 shows in graphic form the attenuation of the intensity of radiofrequency transmission at the resonance frequencies corresponding to the inductances of the coil, according to the invention,

[0055] FIG. 6 shows in graphic form the resonance frequency of the coils according to the value of the inductance of said coils, according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0056] The invention proposes a radiofrequency transmission device D, shown in FIG. 3.

[0057] The radiofrequency transmission device D comprises: [0058] a voltage supply source Vcc, which may be, for example, a battery, or the voltage drawn from the vehicle battery, [0059] a transmission unit 10 for transmitting a voltage signal S, comprising an oscillator and a power amplifier, [0060] matching means M1 for matching the transmission frequency of the antenna A to the fundamental frequency f.sub.F, supplied by a voltage source Vcc, [0061] and a radiofrequency antenna A connected to the matching means M1.

[0062] As explained above, the transmission unit 10 is supplied with voltage by the voltage supply source Vcc, and generates a voltage signal S (see FIG. 3) in the form of successive pulses, that is to say a pulsed voltage signal S, accompanied by the parallel generation of what are known as harmonic parasitic currents, that is to say periodic parasitic voltage signals whose frequencies are multiples of the predetermined fundamental frequency f.sub.F, that is to say frequencies equal to 2, 3, 4, and 5 times the predetermined fundamental frequency f.sub.F.

[0063] In order to overcome this drawback, the radiofrequency transmission device D according to the invention also comprises filtering means 30, electrically connected, on the one hand, to the transmission unit 10 and, on the other hand, to transmission frequency matching means M1.

[0064] According to the invention, the matching means M1 comprise: [0065] a first matching unit 20 for matching the frequency of the power amplifier of the transmission unit 10 to the fundamental frequency f.sub.F, supplied by the voltage source Vcc and connected, on the one hand, to said filtering means 30, and, on the other hand, to a second matching unit 40. Said first matching unit 20 generally comprises (see FIG. 2), as in the prior art, at least a first matching capacitor C1 connected to the supply source Vcc and to the ground, a coil Lx connected to the supply source Vcc and to a junction point J, a second matching capacitor C2 connected to the junction point J and to the ground, and a third capacitor C3 connected to the junction point J and to the input of the second matching unit 40, [0066] a second matching unit 40 for matching the transmission frequency of the antenna A, connected to the output of the first matching unit 20, comprising one or more matching capacitors (not shown), and connected electrically to the radiofrequency antenna A.

[0067] The filtering means 30 according to the invention comprise n coils B.sub.1, B.sub.2, . . . B.sub.i, . . . B.sub.n, electrically connected in series (see FIG. 4) with each other.

[0068] Advisably, according to the invention, (n1) coils, for example B.sub.2, B.sub.3, . . . B.sub.i, . . . B.sub.n, each have an inductance L.sub.i having a natural resonance frequency f.sub.RLi corresponding to a harmonic of the predetermined fundamental frequency f.sub.F.

More particularly:

f.sub.RLi=if.sub.F

where [0069] f.sub.RLi is the natural resonance frequency of the i-th inductance, [0070] i is a number varying from 2 to n, [0071] f.sub.F is the predetermined fundamental frequency.

[0072] In other words, each of the (n1) coils resonates at a frequency equal to a harmonic of the predetermined fundamental frequency f.sub.F, that is to say at one of the parasitic frequencies. This has the effect of attenuating the intensity of said harmonics.

[0073] Advantageously, each of the n coils has an impedance value Zi such that the total sum Z.sub.TOT of the impedance values (Zi) of the filtering means 30 measured at the predetermined fundamental frequency f.sub.F is equal to the total sum of the values of inductance L.sub.TOT:

Z.sub.TOT=Z.sub.1+Z.sub.2+ . . . Z.sub.i+ . . . Z.sub.n=L.sub.TOT=L.sub.1+L.sub.2+ . . . L.sub.i+ . . . L.sub.n

where [0074] Z.sub.TOT is the total impedance of the n coils, measured at the predetermined fundamental frequency f.sub.F, [0075] Z.sub.i is the impedance of the i-th coil B.sub.i, [0076] L.sub.TOT is the total inductance of the n coils, [0077] L.sub.i is the inductance of the i-th coil B.sub.i.

[0078] The n1 coils B.sub.2, B.sub.3, B.sub.4 . . . B.sub.n then serve to filter the parasitic frequencies without modifying the value of the total impedance Z.sub.TOT of the network formed by the n coils of the filtering means M1, which remains equal to the impedance Z.sub.TOT measured at the predetermined fundamental frequency f.sub.F.

[0079] This is because, for each coil B.sub.i, the impedance Z.sub.i of said coil tends toward infinity, when measured at the resonance frequency f.sub.RLi, which is natural to said coil; that is to say, Z.sub.i= measured at the resonance frequency f.sub.RLi of the coil B.sub.i.

[0080] However, at the predetermined fundamental frequency f.sub.F, which is the operating frequency of the filtering means 30, the impedance Z.sub.i of each coil B.sub.i is equal to its inductance L.sub.i, and therefore:

Z.sub.i=L.sub.i

[0081] For its part, the remaining coil, for example the first coil B.sub.1, does not have an inductance L.sub.1 whose resonance frequency is equal to a multiple of the predetermined fundamental frequency f.sub.F.

[0082] The remaining coil, that is to say the first coil B.sub.1, has an impedance Z.sub.1, such that it satisfies the equation

Z.sub.1=Z.sub.TOTZ.sub.2 . . . Z.sub.i . . . Z.sub.n

where [0083] Z.sub.TOT is the total impedance of the n coils, measured at the predetermined fundamental frequency f.sub.F, [0084] Zi is the impedance of the i-th coil, [0085] and an inductance L.sub.1 such that, at the predetermined fundamental frequency f.sub.F, there is an inductance L.sub.1 which is equal to the impedance Z.sub.1:

L.sub.1=Z.sub.1

[0086] Thus the parasitic frequencies are filtered by means of the (n1) coils B.sub.1 . . . B.sub.n of the filtering means 30, and are no longer propagated in the transmission device D, as was the case in the prior art.

[0087] With the radiofrequency transmission device D according to the invention, therefore, the radiofrequency antenna A transmits at the predetermined fundamental frequency f.sub.F, and does not transmit radio waves at the parasitic frequencies.

[0088] Additionally, the electronic circuit no longer transmits radio waves at the parasitic frequencies via the ground planes or the copper tracks of its constituent printed circuit, as was the case in the prior art.

[0089] FIG. 5 shows the frequency amplitude reduction of the coils B.sub.2, B.sub.3, B.sub.4 as a function of their inductance L.sub.2, L.sub.3 and L.sub.4.

[0090] The greatest frequency amplitude reduction is obtained at the resonance frequency f.sub.RL2, f.sub.RL3, f.sub.RL4 of said coils B.sub.2, B.sub.3, B.sub.4.

[0091] According to the invention, the inductances L.sub.2, L.sub.3, L.sub.4 of the coils are selected in such a way that their natural resonance frequencies f.sub.RL2, f.sub.RL3, f.sub.RL4 are substantially equal to the parasitic frequencies transmitted by the transmission unit 10.

[0092] FIG. 6 shows the curve of the resonance frequency f.sub.R of the coils as a function of their inductance L.

[0093] For example, in the case where n=3, for the second coil B.sub.2, the inductance L.sub.2 is selected in such a way that the resonance frequency of said second coil B.sub.2 is equal to twice the predetermined fundamental frequency f.sub.F, and therefore:

f.sub.RL2=2f.sub.F

[0094] Similarly, for the third coil B.sub.3, the inductance L.sub.3 is selected in such a way that the resonance frequency of the third coil B.sub.3 is equal to three times the predetermined fundamental frequency f.sub.F, i.e.:

f.sub.RL3=3f.sub.F

[0095] For a given inductance L.sub.i, the impedance Z.sub.i is determined as follows: Z.sub.i=L.sub.i when the impedance Z.sub.i is measured at the predetermined fundamental frequency f.sub.F.

[0096] Therefore Z.sub.2=L.sub.2 at the predetermined fundamental frequency f.sub.F and [0097] Z.sub.3=L.sub.3 at the predetermined fundamental frequency f.sub.F.

[0098] Then, a predetermined fundamental frequency f.sub.F corresponds to a total impedance Z.sub.TOT of the filtering means M1, that is to say the total impedance of the three-coil network, where Z.sub.TOT=L.sub.TOT at said predetermined fundamental frequency f.sub.F.

[0099] The impedance Z.sub.1 of the first coil B.sub.1 is then selected in such a way that:

Z.sub.1=Z.sub.TOTZ.sub.2Z.sub.3

[0100] The inductance L.sub.1 of the first coil B.sub.1 is then a function of the impedance Z.sub.1, where L.sub.1=Z.sub.1 at the predetermined fundamental frequency f.sub.F.

[0101] Thus, with the filtering means 30 according to the invention, a network of n coils can be used to filter the parasitic frequencies transmitted by the transmission unit 10 by a careful selection of the characteristics (impedance, inductance) of said coils as a function of the predetermined fundamental frequency.

[0102] The transmission device D according to the invention no longer resonates at the parasitic frequencies, and the antenna A transmits radio waves at the desired transmission frequency only.