Method for transforming the impedance of a radio-frequency transmission line of a printed circuit and printed circuit thereof

Abstract

A method for transforming the impedance of a radio-frequency transmission line of a printed circuit from a first impedance value to a second impedance value, the radio-frequency transmission line being adapted to transport a radio-frequency signal at a frequency value comprised in a frequency range defined between a minimum frequency value and a maximum frequency value, wherein the following steps are envisaged:—dividing the radio-frequency transmission line into a plurality of circuit sections each one of the circuit sections including a first and a second impedance connected in parallel with each other by two circuit branches placed at a maximum distance (d.sub.max) from each other, wherein the circuit sections have respective third impedance values that gradually increase, respectively decrease, from the first impedance value to the second impedance value;—determining the maximum distance between the circuit branches in such a way as to avoid any undesired frequency values within the frequency range;—setting a fourth impedance value of one of the two impedances;—calculating a fifth impedance value of the other one of the two impedances, such that the impedance value of the circuit section is the third respective impedance value.

Claims

1. A method for transforming the impedance of a radio-frequency transmission line of a printed circuit from a first impedance value to a second impedance value, said radio-frequency transmission line being adapted to transport a radio-frequency signal at a frequency value comprised in a frequency range defined between a minimum frequency value and a maximum frequency value, said method being characterized in that it comprises the steps of: dividing said radio-frequency transmission line into a plurality of circuit sections, each one of said circuit sections comprising a first and a second impedance connected in parallel with each other by two circuit branches placed at a maximum distance (d.sub.max) from each other, wherein said circuit sections have respective third impedance values that gradually increase, respectively decrease, from said first impedance value to said second impedance value; determining said maximum distance (d.sub.max) between said circuit branches in such a way as to avoid any undesired frequency values within said frequency range; setting a fourth impedance value of one of said two impedances; calculating a fifth impedance value of the other one of said two impedances, such that the impedance value of said circuit section is said third respective impedance value.

2. The method according to claim 1, wherein said maximum distance (d.sub.max) is calculated in such a way that at no frequency within said frequency range it is equal to λ/2 or an integer multiple thereof, wherein λ is the wavelength of a signal going through said radio-frequency circuit line.

3. The method according to claim 1, wherein said first and said second impedances are implemented through respective tracks of said printed circuit, arranged in distinct planes.

4. The method according to claim 3, wherein said respective tracks are made on opposite sides of said printed circuit.

5. The method according to claim 2, wherein said circuit sections have a length equal to λ/4.

6. A printed circuit comprising a radio-frequency transmission line having a first node at a first impedance value and a second node at a second impedance value, said radio-frequency transmission line being adapted to transport a radio-frequency signal at a frequency value comprised in a frequency range defined between a minimum frequency value and a maximum frequency value, wherein: said radio-frequency transmission line is divided into a plurality of circuit sections, each one of said circuit sections comprising a first and a second impedance connected in parallel with each other by two circuit branches placed at a maximum distance (d.sub.max) from each other, wherein said circuit sections have respective third impedance values that gradually increase, respectively decrease, from said first impedance value to said second impedance value; a maximum distance (d.sub.max) between said circuit branches is set in such a way as to avoid any undesired frequency values within said frequency range; one of said two impedances has a predetermined fourth impedance value; the other one of said two impedances has a fifth impedance value, such that the impedance value of said circuit section is said third respective impedance value.

7. The printed circuit according to claim 6, wherein said maximum distance (d.sub.max) is calculated in such a way that at no frequency within said frequency range it is equal to λ/2 or an integer multiple thereof, wherein λ is the wavelength of a signal going through said radio-frequency circuit line.

8. A Doherty amplifier comprising: a signal source adapted to generate an input signal; a hybrid coupler adapted to receive said input signal and divide it into first and second output signals phase-shifted by 90°; a carrier amplifier adapted to receive as input said first output signal; a peak amplifier adapted to receive as input said second output signal; a line having an electric length equal to ¼ of a wave of the frequency of said input signal, arranged downstream of said carrier amplifier; a recombination node adapted to receive the output signals from said line and from said peak amplifier, said recombination node being connected to a delivery node towards a load by a transmission line, wherein said transmission line is adapted to transport a radio-frequency signal at a frequency value comprised in a frequency range defined between a minimum frequency value and a maximum frequency value, wherein: said radio-frequency transmission line is divided into a plurality of circuit sections, each one of said circuit sections comprising a first and a second impedance connected in parallel with each other by two circuit branches placed at a maximum distance (d.sub.max) from each other, wherein said circuit sections have respective third impedance values that gradually increase, respectively decrease, from said first impedance value to said second impedance value; a maximum distance between said circuit branches is set in such a way as to avoid any undesired frequency values within said frequency range; one of said two impedances has a predetermined fourth impedance value; the other one of said two impedances has a fifth impedance value, such that the impedance value of said circuit section is said third respective impedance value.

9. The Doherty amplifier according to claim 8, wherein said maximum distance (d.sub.max) is calculated in such a way that at no frequency within said frequency range it is equal to λ/2 or an integer multiple thereof, wherein λ is the wavelength of a signal going through said radio-frequency circuit line.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Such features as well as further advantages of the present invention will become more apparent from the following description of an embodiment thereof as shown in the annexed drawings, which are supplied by way of non-limiting example, wherein:

(2) FIG. 1 shows a block diagram of a Doherty amplifier according to the prior art;

(3) FIG. 2 shows a radio-frequency transmission line of a printed circuit according to the prior art;

(4) FIG. 3 shows a radio-frequency transmission line of a printed circuit according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(5) With reference to FIG. 3, there is shown a radio-frequency transmission line 20 comprising a plurality of, e.g., three, circuit sections S.sub.1,S.sub.2,S.sub.3.

(6) Each circuit section S.sub.1,S.sub.2,S.sub.3 corresponds to a transmission line tract having a footprint width that is inversely proportional to the impedance. Therefore, as the impedance grows, the width of the transmission line tract decreases.

(7) With reference, for simplicity's sake, only to the circuit section S.sub.1, this can be schematized as a first impedance Z.sub.1 placed in parallel with a second impedance Z2 and mutually connected by two circuit branches 13,13′ placed at a maximum distance d.sub.max from each other.

(8) The circuit sections S.sub.2,S.sub.3 show a similar circuit arrangement.

(9) Typically, the circuit sections S.sub.1,S.sub.2,S.sub.3 have each a length λ/4, where λ is the wavelength of the signal going through the radio-frequency transmission line 20. However, other length values should not be excluded.

(10) The radio-frequency transmission line 20 is adapted to transport a radio-frequency signal at a frequency value comprised within a frequency range defined between a minimum frequency value f.sub.min and a maximum frequency value f.sub.max.

(11) In a first step of the method according to the invention, in order to transform the impedance of the radio-frequency transmission line 20 from a first impedance value V.sub.1 to a second impedance value V.sub.2, the circuit designer sizes the total impedance of each circuit section S.sub.1,S.sub.2,S.sub.3 in accordance with the prior art, such that it gradually increases, respectively decreases, from the first impedance value V.sub.1 to the second impedance value V.sub.2.

(12) As aforementioned, if the first impedance value V.sub.1 is 2Ω and the second impedance value V.sub.2 is 50Ω, then the impedance values of the circuit sections S.sub.1,S.sub.2,S.sub.3 may be, for example, 10, 20 and 30Ω, respectively.

(13) In a second step, it is necessary to calculate a maximum distance d.sub.max at which the two circuit branches 13,13′ must be placed. In fact, this being a radio-frequency circuit, disturbances (notches) might arise at certain undesired frequency values, which should therefore be avoided.

(14) The Applicant has experimentally verified that such certain frequencies can be obtained with the formula f.sub.k=(f.sub.TL1.Math.λ/2.Math.k)/EL.sub.TL1, where f.sub.TL1 is the intermediate frequency (band center frequency) between said minimum frequency f.sub.min and said maximum frequency f.sub.max, λ is the wavelength of a signal going through said radio-frequency transmission line 20, k is an integer number belonging to the set of natural numbers, greater than or equal to one, and EL.sub.TL1 is the electric length of the transmission line tract between the two circuit branches 13,13′, expressed in multiples of λ at the frequency f.sub.TL1.

(15) To make sure that such disturbances will fall outside said frequency range, it is therefore sufficient to impose that at no frequency within the frequency range the maximum distance d.sub.max between the two circuit branches 13,13′ is equal to λ/2 or an integer multiple thereof.

(16) In a third step, the designer sets the value of the first impedance Z.sub.1 and determines the value of the second impedance Z.sub.2 such that the impedance value of the circuit section S.sub.1 is the one fixed beforehand.

(17) When implementing the transmission line 20 on a printed circuit, it can be observed that, advantageously, the impedance Z.sub.1 is twice the impedance P.sub.1 of the prior-art circuit of FIG. 1, and also that the impedance Z.sub.2 is approximately equal to Z.sub.1 (approximately equal because Z.sub.1 must be determined in such a way as to compensate for the effect of the two vertical branches 13,13′). It is thus possible to create two transmission line tracts having a footprint width equal to approximately half the width required by the corresponding transmission line tract of the circuit of FIG. 2. This is because track width is inversely proportional to impedance, and hence the footprint width decreases as the impedance increases.

(18) Advantageously, by implementing the tracks that constitute each one of the two transmission line tracts with impedance Z.sub.1 and Z.sub.2 in distinct planes, e.g., on the opposite sides of a suitable printed circuit board, it is possible to reduce by half the necessary footprint area in comparison with the prior-art circuit of FIG. 2.

(19) It should be observed that the embodiment of FIG. 3 also solves possible multiple-path problems that are typical of radio-frequency circuits. In fact, the connection between each circuit section S.sub.1,S.sub.2,S.sub.3 consists of a single line tract 15.

(20) The radio-frequency transmission line 20 illustrated in FIG. 3 may advantageously constitute the transmission line that connects the recombination node 9 to the delivery node 11 of a Doherty amplifier, as shown in FIG. 1.

(21) Therefore, the present invention advantageously makes it possible to obtain narrower circuit lines that take less space or, as an alternative, to reduce the losses of an amplifier in Doherty configuration while leaving its dimensions unchanged.

(22) This latter advantage can be further clarified by means of a numerical example.

(23) Let us assume that a first track having an impedance of 50Ω is available. As previously explained, when the footprint width of the track is doubled, the impedance is halved to a value of 25Ω. By acting upon the material of the support of the printed circuit, it is possible to bring the impedance back to 50Ω. By dividing the first track into two second tracks (e.g., like those represented by the impedances Z.sub.1 and Z.sub.2 of the circuit section S.sub.1 of FIG. 3), a total impedance of 100Ω is obtained. If the two second tracks thus obtained are arranged in a symmetrical and parallel manner, the impedance will return to the original value of 50Ω (of the first track), but the current flowing through each one of the two second tracks will now be halved. Since the losses follow a law that depends on the square of the current, it is apparent that, when the currents are halved, the losses will also be reduced accordingly.

(24) The method for transforming the impedance of a radio-frequency transmission line of a printed circuit and the printed circuit thereof described herein by way of example may be subject to many possible variations without departing from the novelty spirit of the inventive idea; it is also clear that in the practical implementation of the invention the illustrated details may have different shapes or be replaced with other technically equivalent elements.

(25) It can therefore be easily understood that the present invention is not limited to a method for transforming the impedance of a radio-frequency transmission line of a printed circuit and a printed circuit thereof described herein by way of example, but may be subject to many modifications, improvements or replacements of equivalent parts and elements without departing from the inventive idea, as clearly specified in the following claims.