WO2016202370A1

WO2016202370A1 - A radio frequency transformer for transforming an input radio frequency signal into an output radio frequency signal

Info

Publication number: WO2016202370A1
Application number: PCT/EP2015/063510
Authority: WO
Inventors: Udo Karthaus; Karl-Frederik BINK
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2015-06-17
Filing date: 2015-06-17
Publication date: 2016-12-22
Also published as: CN112614674A; CN107548511B; CN107548511A

Abstract

The invention relates to a radio frequency transformer (100) for transforming an input radio frequency signal into an output radio frequency signal, the radio frequency transformer (100) comprising a primary winding (101) having a first input terminal and a second input terminal, wherein the primary winding (101) comprises a first portion (L1Pa), a second portion (L1Pb), a third portion (L1Na), and a fourth portion (L1Nb), wherein the first portion (L1Pa) is electromagnetically coupled to the fourth portion (L1Nb), and wherein the second portion (L1Pb) is electromagnetically coupled to the third portion (L1Na), and a secondary winding (103) having a first output terminal and a second output terminal, wherein the secondary winding (103) is electromagnetically coupled to the first portion (L1Pa), the second portion (L1Pb), the third portion (L1Na), and the fourth portion (L1Nb) of the primary winding (101).

Description

DESCRIPTION

A radio frequency transformer for transforming an input radio frequency signal into an output radio frequency signal

TECHNICAL FIELD

The invention relates to the field of radio frequency transformers, in particular to radio frequency transformers for transforming between balanced and unbalanced terminals.

BACKGROUND

Radio frequency transformers are major components in radio frequency (RF) systems, such as mobile communication systems or radar systems, and are used for a large variety of applications. Typically, radio frequency transformers are employed for impedance transformation between primary windings and secondary windings and for transforming between balanced and unbalanced terminals.

In particular, radio frequency transformers are commonly used within radio frequency transmitters and can have a major impact on their performance. For example, digital radio frequency transmitters using direct digital radio frequency modulators (DDRMs) or analog radio frequency transmitters using 25% duty cycle mixers desire low common mode impedances in order to terminate strong common mode currents at twice the frequency of an input radio frequency signal. Common radio frequency transformers, however, have a common mode shunt resonance approximately at twice their differential mode resonance frequency, thereby reducing the performance of radio frequency transmitters.

SUMMARY It is an object of the invention to improve a radio frequency transformation.

This object is achieved by the features of the independent claims. Further implementation forms are apparent from the dependent claims, the description and the figures. The invention is based on the finding that portions within a primary winding of a radio frequency transformer can be electromagnetically coupled in order to adjust the common mode impedance independently from the differential mode impedance. By tightly coupling two halves of the primary winding to each other, e.g. the positive and negative halves of the primary winding, a common mode inductance can be lowered and the common mode resonance frequency of the radio frequency transformer can be shifted towards higher frequencies away from twice the frequency of an input radio frequency signal.

The radio frequency transformer can be used within a radio frequency transmitter in conjunction with a modulator, e.g. a direct digital radio frequency modulator (DD M), wherein the radio frequency transmitter takes advantage of the specific characteristics of the radio frequency transformer. The radio frequency transmitter allows for an improved performance, e.g. a reduced 2^nd harmonic voltage swing on drains of the modulator, an increased 1 dB compression point (P1dB), an improved counter third order intermodulation (C-IM3) performance, and a lowered 2^nd harmonic emission.

The radio frequency transformer and/or the radio frequency transmitter can be used in any kind of radio frequency system, such as a mobile communication system or a radar system.

According to a first aspect, the invention relates to a radio frequency transformer for transforming an input radio frequency signal into an output radio frequency signal, the radio frequency transformer comprising a primary winding having a first input terminal and a second input terminal, wherein the first input terminal and the second input terminal are configured to handle the input radio frequency signal, wherein the primary winding comprises a first portion, a second portion, a third portion, and a fourth portion, wherein the first portion is electromagnetically coupled to the fourth portion, and wherein the second portion is electromagnetically coupled to the third portion, and a secondary winding having a first output terminal and a second output terminal, wherein the first output terminal and the second output terminal are configured to provide the output radio frequency signal, and wherein the secondary winding is electromagnetically coupled to the first portion, the second portion, the third portion, and the fourth portion of the primary winding. Thus, an efficient concept for transforming an input radio frequency signal into an output radio frequency signal is realized.

The primary winding and/or the secondary winding can have further terminals, e.g. for providing a supply voltage. Accordingly, the primary winding and/or the secondary winding can have at least two terminals. The number of turns of the primary winding and the number of turns of the secondary winding can be chosen arbitrarily, i.e. the turn ratio between the primary winding and the secondary winding can be chosen arbitrarily. The electromagnetic coupling can comprise an inductive coupling and/or a capacitive coupling. The presence of an electromagnetic coupling can refer to a presence of a high coupling factor, e.g. 0.5 to 1. In a first implementation form of the radio frequency transformer according to the first aspect as such, the first portion is electromagnetically decoupled from the second portion, and the third portion is electromagnetically decoupled from the fourth portion. Thus, the common mode resonance frequency of the radio frequency transformer can be shifted more efficiently towards higher frequencies.

The electromagnetic decoupling can comprise an inductive decoupling and/or a capacitive decoupling. The presence of an electromagnetic decoupling can refer to a presence of a low coupling factor, e.g. 0 to 0.5. In a second implementation form of the radio frequency transformer according to the first aspect as such or the first implementation form of the first aspect, the input radio frequency signal is a differential radio frequency signal, and the output radio frequency signal is a single-ended radio frequency signal. Thus, a balanced-to-unbalanced (bal-un) transformation is realized efficiently. The radio frequency transformer can consequently be used as a radio frequency balun.

In a third implementation form of the radio frequency transformer according to the first aspect as such or the first implementation form of the first aspect, the input radio frequency signal is a differential radio frequency signal, and the output radio frequency signal is a differential radio frequency signal. Thus, a balanced-to-balanced (bal-bal) transformation is realized efficiently.

In a fourth implementation form of the radio frequency transformer according to the first aspect as such or the first implementation form of the first aspect, the input radio frequency signal is a single-ended radio frequency signal, and the output radio frequency signal is a differential radio frequency signal. Thus, an unbalanced-to-balanced (un-bal) transformation is realized efficiently.

In a fifth implementation form of the radio frequency transformer according to the first aspect as such or the first implementation form of the first aspect, the input radio frequency signal is a single-ended radio frequency signal, and the output radio frequency signal is a single- ended radio frequency signal. Thus, an unbalanced-to-unbalanced (un-un) transformation is realized efficiently.

In a sixth implementation form of the radio frequency transformer according to the first aspect as such or any preceding implementation form of the first aspect, the input radio frequency signal comprises a signal component at an input frequency, wherein a common mode resonance frequency of the radio frequency transformer is greater than twice the input frequency. Thus, a common mode impedance of the radio frequency transformer can efficiently be lowered at twice the input frequency.

In a seventh implementation form of the radio frequency transformer according to the first aspect as such or any preceding implementation form of the first aspect, the first portion and the second portion of the primary winding are connected in parallel, and/or the third portion and the fourth portion of the primary winding are connected in parallel. Thus, an

electromagnetic coupling and/or electromagnetic decoupling is realized efficiently.

In an eighth implementation form of the radio frequency transformer according to the first aspect as such or the first implementation form to the sixth implementation form of the first aspect, the first portion and the second portion of the primary winding are connected in series, and/or the third portion and the fourth portion of the primary winding are connected in series. Thus, an electromagnetic coupling and/or electromagnetic decoupling is realized efficiently.

In a ninth implementation form of the radio frequency transformer according to the first aspect as such or any preceding implementation form of the first aspect, the radio frequency transformer is arranged on a semiconductor substrate, wherein the first portion comprises a first conducting line, wherein the second portion comprises a second conducting line, wherein the third portion comprises a third conducting line, and wherein the fourth portion comprises a fourth conducting line. Thus, the radio frequency transformer is efficiently provided as a radio frequency integrated circuit ( FIC). The radio frequency transformer can be regarded as an on-chip radio frequency transformer.

In a tenth implementation form of the radio frequency transformer according to the ninth implementation form of the first aspect, a portion of the first conducting line is adjacent to a portion of the fourth conducting line, and/or a portion of the second conducting line is adjacent to a portion of the third conducting line. Thus, an electromagnetic coupling between neighboring conducting lines is realized efficiently. The portions of the conducting lines can be regarded to be adjacent, when no further conducting line is arranged in between. Consequently, a high coupling factor can be provided.

In an eleventh implementation form of the radio frequency transformer according to the ninth implementation form or the tenth implementation form of the first aspect, a portion of the first conducting line and a portion of the fourth conducting line are arranged within different layers on the semiconductor substrate, and/or a portion of the second conducting line and a portion of the third conducting line are arranged within different layers on the semiconductor substrate. Thus, an electromagnetic coupling between conducting lines in different layers is realized efficiently.

The layers can be conductive metal layers. The layers can be stacked on the semiconductor substrate, thereby realizing a multi-layer integrated circuit. Consequently, a high coupling factor can be provided.

In a twelfth implementation form of the radio frequency transformer according to the first aspect as such or any preceding implementation form of the first aspect, the primary winding is connected to a supply voltage source. Thus, a supply voltage can be provided efficiently.

The supply voltage source can e.g. supply components for generating the input radio frequency signal, e.g. a modulator. The supply voltage source can be connected to the primary winding using a further terminal.

In a thirteenth implementation form of the radio frequency transformer according to the twelfth implementation form of the first aspect, the supply voltage source is connected to the primary winding between the first portion and the third portion, and/or the supply voltage source is connected to the primary winding between the second portion and the fourth portion. Thus, an efficient symmetrical center tapping of the primary winding is realized.

In a fourteenth implementation form of the radio frequency transformer according to the first aspect as such or any preceding implementation form of the first aspect, the number of turns of the primary winding equals the number of turns of the secondary winding. Thus, an efficient impedance transformation between the primary winding and the secondary winding is realized. The turn ratio between the primary winding and the secondary winding can e.g. be 2:2, 4:4, or 8:8. Due to imperfections, an effective turn ratio comprising fractional numbers of turns can be obtained, e.g. an effective turn ratio of 2:1.9. According to a second aspect, the invention relates to a radio frequency transmitter comprising a modulator being configured to generate an input radio frequency signal, and a radio frequency transformer according to the first aspect as such or any implementation form of the first aspect, wherein the radio frequency transformer is configured to transform the input radio frequency signal into an output radio frequency signal. Thus, an efficient radio frequency transmitter is provided.

The radio frequency transmitter has a reduced 2^nd harmonic voltage swing on drains of the modulator, an increased 1 dB compression point (P1dB), an improved counter third order intermodulation (C-IM3) performance, and a lowered 2^nd harmonic emission.

The output radio frequency signal can directly be provided to a further component, such as a power amplifier, a radio frequency switch, a duplex filter, an antenna tuner, or an antenna.

In a first implementation form of the radio frequency transmitter according to the second aspect as such, the modulator is configured to generate the input radio frequency signal by pulling a radio frequency current out of the radio frequency transformer. Thus, the input radio frequency signal can be generated efficiently.

The source impedance of the modulator can be greater than the load impedance of the radio frequency transformer. The input radio frequency signal can be formed by the radio frequency current.

In a second implementation form of the radio frequency transmitter according to the second aspect as such or the first implementation form of the second aspect, the modulator is a direct digital radio frequency modulator (DD M). Thus, the input radio frequency signal is directly generated without an intermediate mixing stage by using switched transistors.

In a third implementation form of the radio frequency transmitter according to the second aspect as such, the first implementation form of the second aspect, or the second implementation form of the second aspect, the radio frequency transmitter further comprises a power amplifier being configured to amplify the output radio frequency signal. Thus, an efficient radio frequency transmitter is provided. According to a third aspect, the invention relates to a method for transforming an input radio frequency signal into an output radio frequency signal using a radio frequency transformer, wherein the radio frequency transformer comprises a primary winding and a secondary winding, wherein the primary winding has a first input terminal and a second input terminal, and wherein the secondary winding has a first output terminal and a second output terminal, the method comprising handling the input radio frequency signal by the first input terminal and the second input terminal of the primary winding, electromagnetically coupling a first portion of the primary winding to a fourth portion of the primary winding, electromagnetically coupling a second portion of the primary winding to a third portion of the primary winding, electromagnetically coupling the secondary winding to the first portion, the second portion, the third portion, and the fourth portion of the primary winding, and providing the output radio frequency signal by the first output terminal and the second output terminal of the secondary winding. Thus, an efficient concept for transforming an input radio frequency signal into an output radio frequency signal is realized.

The method can be performed by the radio frequency transformer and/or the radio frequency transmitter. Further features of the method directly result from the functionality of the radio frequency transformer and/or the radio frequency transmitter.

BRIEF DESCRIPTION OF EMBODIMENTS

Embodiments of the invention will be described with respect to the following figures, in which: Fig. 1 shows a diagram of a radio frequency transformer for transforming an input radio frequency signal into an output radio frequency signal according to an embodiment;

Fig. 2 shows a diagram of a radio frequency transmitter comprising a modulator and a radio frequency transformer according to an embodiment;

Fig. 3 shows a diagram of a method for transforming an input radio frequency signal into an output radio frequency signal according to an embodiment;

Fig 4 shows a diagram of a radio frequency transformer for transforming an input radio frequency signal into an output radio frequency signal according to an embodiment; Fig 5 shows a diagram of a radio frequency transformer for transforming an input radio frequency signal into an output radio frequency signal according to an embodiment;

Fig 6 shows a diagram of a radio frequency transformer for transforming an input radio frequency signal into an output radio frequency signal according to an embodiment;

Fig 7 shows a diagram of a radio frequency transformer for transforming an input radio frequency signal into an output radio frequency signal according to an embodiment; Fig. 8 shows a diagram of circuit comprising a primary winding of a radio frequency transformer according to an embodiment;

Fig. 9 shows a diagram of a common mode load impedance of a radio frequency transformer over frequency in dependence of a coupling factor according to an embodiment;

Fig. 10 shows a diagram of a radio frequency transmitter comprising a modulator and a radio frequency transformer according to an embodiment;

Fig. 1 1 shows a diagram of an output power and a counter third order intermodulation performance of a radio frequency transmitter according to an embodiment;

Fig. 12 shows a diagram of a reference radio frequency transformer comprising a primary winding and a secondary winding; and Fig. 13 shows a diagram of a reference radio frequency transformer comprising a primary winding and a secondary winding.

DETAILED DESCRIPTION OF EMBODIMENTS Fig. 1 shows a diagram of a radio frequency transformer 100 for transforming an input radio frequency signal into an output radio frequency signal according to an embodiment.

The radio frequency transformer 100 comprises a primary winding 101 having a first input terminal and a second input terminal, wherein the first input terminal and the second input terminal are configured to handle the input radio frequency signal, wherein the primary winding 101 comprises a first portion L1 Pa, a second portion L1 Pb, a third portion L1 Na, and a fourth portion L1 Nb, wherein the first portion L1 Pa is electromagnetically coupled to the fourth portion L1 Nb, and wherein the second portion L1 Pb is electromagnetically coupled to the third portion L1 Na.

The radio frequency transformer 100 further comprises a secondary winding 103 having a first output terminal and a second output terminal, wherein the first output terminal and the second output terminal are configured to provide the output radio frequency signal, and wherein the secondary winding 103 is electromagnetically coupled to the first portion L1 Pa, the second portion L1 Pb, the third portion L1 Na, and the fourth portion L1 Nb of the primary winding 101.

Fig. 2 shows a diagram of a radio frequency transmitter 200 comprising a modulator 201 and a radio frequency transformer 100 according to an embodiment. The modulator 201 is configured to generate an input radio frequency signal. The radio frequency transformer 100 is configured to transform the input radio frequency signal into an output radio frequency signal.

The radio frequency transformer 100 comprises a primary winding 101 having a first input terminal and a second input terminal, wherein the first input terminal and the second input terminal are configured to handle the input radio frequency signal, wherein the primary winding 101 comprises a first portion L1 Pa, a second portion L1 Pb, a third portion L1 Na, and a fourth portion L1 Nb, wherein the first portion L1 Pa is electromagnetically coupled to the fourth portion L1 Nb, and wherein the second portion L1 Pb is electromagnetically coupled to the third portion L1 Na. The radio frequency transformer 100 further comprises a secondary winding 103 having a first output terminal and a second output terminal, wherein the first output terminal and the second output terminal are configured to provide the output radio frequency signal, and wherein the secondary winding 103 is electromagnetically coupled to the first portion L1 Pa, the second portion L1 Pb, the third portion L1 Na, and the fourth portion L1 Nb of the primary winding 101.

Fig. 3 shows a diagram of a method 300 for transforming an input radio frequency signal into an output radio frequency signal according to an embodiment. The method 300 can be performed using a radio frequency transformer, wherein the radio frequency transformer comprises a primary winding and a secondary winding, wherein the primary winding has a first input terminal and a second input terminal, and wherein the secondary winding has a first output terminal and a second output terminal. The radio frequency transformer can be the radio frequency transformer 100 as described in conjunction with Fig. 1 or Fig. 2.

The method 300 comprises handling 301 the input radio frequency signal by the first input terminal and the second input terminal of the primary winding, electromagnetically coupling 303 a first portion of the primary winding to a fourth portion of the primary winding, electromagnetically coupling 305 a second portion of the primary winding to a third portion of the primary winding, electromagnetically coupling 307 the secondary winding to the first portion, the second portion, the third portion, and the fourth portion of the primary winding, and providing 309 the output radio frequency signal by the first output terminal and the second output terminal of the secondary winding.

Further embodiments of the radio frequency transformer 100, the radio frequency transmitter 200, and the method 300 will be described in the following.

Fig 4 shows a diagram of a radio frequency transformer 100 for transforming an input radio frequency signal into an output radio frequency signal according to an embodiment. The diagram comprises a layout and a schematic circuit of the radio frequency transformer 100, wherein the radio frequency transformer 100 forms a possible implementation of the radio frequency transformer as described in conjunction with Fig. 1. The radio frequency transformer 100 has a primary winding 101 and a secondary winding 103, wherein a turn ratio between the primary winding 101 and the secondary winding 103 is 2:2.

The primary winding 101 has a first input terminal in_p and a second input terminal in_n, wherein the first input terminal in_p and the second input terminal in_n are configured to handle the input radio frequency signal. The secondary winding 103 has a first output terminal out and a second output terminal gnd, wherein the first output terminal out and the second output terminal gnd are configured to provide the output radio frequency signal. The input radio frequency signal is a differential radio frequency signal, and the output radio frequency signal is a single-ended radio frequency signal. The primary winding 101 is further connected to a supply voltage source using a further terminal vdd.

The primary winding 101 comprises a first portion L1 Pa, a second portion L1 Pb, a third portion L1 Na, and a fourth portion L1 Nb, wherein the first portion L1 Pa is electromagnetically coupled to the fourth portion L1 Nb, and wherein the second portion L1 Pb is

electromagnetically coupled to the third portion L1 Na. Furthermore, the first portion L1 Pa is electromagnetically decoupled from the second portion L1 Pb, and the third portion L1 Na is electromagnetically decoupled from the fourth portion L1 Nb. The secondary winding 103 is electromagnetically coupled to the first portion L1 Pa, the second portion L1 Pb, the third portion L1 Na, and the fourth portion L1 Nb of the primary winding 101. The electromagnetic coupling is illustrated in the schematic circuit by arrows.

The radio frequency transformer 100 is arranged on a semiconductor substrate, wherein the first portion L1 Pa comprises a first conducting line, wherein the second portion L1 Pb comprises a second conducting line, wherein the third portion L1 Na comprises a third conducting line, and wherein the fourth portion L1 Nb comprises a fourth conducting line. A portion of the first conducting line is adjacent to a portion of the fourth conducting line, and a portion of the second conducting line is adjacent to a portion of the third conducting line. The first portion L1 Pa and the second portion L1 Pb of the primary winding 101 are connected in parallel, and the third portion L1 Na and the fourth portion L1 Nb of the primary winding 101 are connected in parallel.

Fig 5 shows a diagram of a radio frequency transformer 100 for transforming an input radio frequency signal into an output radio frequency signal according to an embodiment. The diagram comprises a layout and a schematic circuit of the radio frequency transformer 100, wherein the radio frequency transformer 100 forms a possible implementation of the radio frequency transformer as described in conjunction with Fig. 1. The radio frequency transformer 100 has a primary winding 101 and a secondary winding 103, wherein a turn ratio between the primary winding 101 and the secondary winding 103 is 4:4.

Compared to a reference radio frequency transformer having portions L1 P and L1 N within a primary winding, each portion L1 P and L1 N can be regarded to be split into two parallel portions within the radio frequency transformer 100 as described in Fig. 4 and Fig. 5. For example, L1 P can be regarded to be split into L1 Pa (dotted) and L1 Pb (dashed), and L1 N can be regarded to be split into L1 Na (dash-dotted) and L1 Nb (dash-dot-dotted).

As the total width of adjacent portions, e.g. L1 Pa + L1 Nb or L1 Na + L1 Pb, can approximately be equal to the corresponding width of a reference radio frequency transformer, the differential inductance may not be changed. Also, the coupling factor between the primary winding and the secondary winding, the quality factors (Q-factors) of the primary winding and the secondary winding, and consequently the loss, may not be changed. Electromagnetic coupling between L1 P and L1 N may now be much tighter, as L1 Pa + L1 Nb as well as L1 Na + L1 Pb can be adjacent or direct neighbors. The coupling factor may e.g. be increased from approximately 0.5 to approximately 0.7 to 0.75. Thus, the common mode inductance seen into the first input terminal in_p and the second input terminal in_n may be reduced.

Fig 6 shows a diagram of a radio frequency transformer 100 for transforming an input radio frequency signal into an output radio frequency signal according to an embodiment. The diagram comprises a layout and a schematic circuit of the radio frequency transformer 100, wherein the radio frequency transformer 100 forms a possible implementation of the radio frequency transformer as described in conjunction with Fig. 1. The radio frequency transformer 100 has a primary winding 101 and a secondary winding 103, wherein a turn ratio between the primary winding 101 and the secondary winding 103 is 2:2.

The primary winding 101 has a first input terminal in_p and a second input terminal in_n, wherein the first input terminal in_p and the second input terminal in_n are configured to handle the input radio frequency signal. The secondary winding 103 has a first output terminal out and a second output terminal gnd, wherein the first output terminal out and the second output terminal gnd are configured to provide the output radio frequency signal. The input radio frequency signal is a differential radio frequency signal, and the output radio frequency signal is a single-ended radio frequency signal. Alternatively, the output radio frequency signal is a differential radio frequency signal, wherein the first output terminal can be referred to as out_p and the second output terminal can be referred to as out_n. The primary winding 101 is further connected to a supply voltage source using a further terminal vdd.

The radio frequency transformer 100 is arranged on a semiconductor substrate, wherein the first portion L1 Pa comprises a first conducting line, wherein the second portion L1 Pb comprises a second conducting line, wherein the third portion L1 Na comprises a third conducting line, and wherein the fourth portion L1 Nb comprises a fourth conducting line. A portion of the first conducting line is adjacent to a portion of the fourth conducting line, and a portion of the second conducting line is adjacent to a portion of the third conducting line. The first portion L1 Pa and the second portion L1 Pb of the primary winding 101 are connected in series, and the third portion L1 Na and the fourth portion L1 Nb of the primary winding 101 are connected in series.

Fig 7 shows a diagram of a radio frequency transformer 100 for transforming an input radio frequency signal into an output radio frequency signal according to an embodiment. The diagram comprises a layout and a schematic circuit of the radio frequency transformer 100, wherein the radio frequency transformer 100 forms a possible implementation of the radio frequency transformer as described in conjunction with Fig. 1. The radio frequency transformer 100 has a primary winding 101 and a secondary winding 103, wherein a turn ratio between the primary winding 101 and the secondary winding 103 is 4:4.

The radio frequency transformer 100 is arranged on a semiconductor substrate, wherein the first portion L1 Pa comprises a first conducting line, wherein the second portion L1 Pb comprises a second conducting line, wherein the third portion L1 Na comprises a third conducting line, and wherein the fourth portion L1 Nb comprises a fourth conducting line. A portion of the first conducting line is adjacent to a portion of the fourth conducting line, and a portion of the second conducting line is adjacent to a portion of the third conducting line. The first portion L1 Pa and the second portion L1 Pb of the primary winding 101 are connected in series, and the third portion L1 Na and the fourth portion L1 Nb of the primary winding 101 are connected in series. As shown in Fig. 6 and Fig. 7, no split within the primary winding 101 may be used. However, respective portions within the primary winding 101 may still be adjacent. This concept can be extended to any number of turns, e.g. by continuously alternating between two adjacent turns within the primary winding 101 and two adjacent turns within the secondary winding 103. Compared to the concept of split windings, less electromagnetic coupling between the primary winding 101 and the secondary winding 103 may occur, thereby increasing an insertion loss of the radio frequency transformer 100. Fig. 8 shows a diagram of circuit comprising a primary winding 101 of a radio frequency transformer according to an embodiment. The circuit comprises capacitors having capacitances Ci and C₂. The primary winding 101 is represented by inductors having inductances L₁.

The inductors are electromagnetically coupled and have a coupling factor k. A radio frequency current i₁ flows through the primary winding 101. Portions within the primary winding 101 , as well as the primary winding 101 and the secondary winding 103 as such be regarded as inductors.

The following exemplary calculations illustrate a reduction of the common mode inductance seen into the first input terminal and the second input terminal of the primary winding 101. The exemplary calculations are based on the following equations:

L_dW = 2{\ ₊ k L_CM = -^^X \ - k)L_x

C diff ^: - C, + C, CL = 2C,

2 ^{1 2}

C_dd_ii_ffffL_dd_ii_ffff (Ci + 2C₂ )(l + )

1 1

co_cm _ L 2C₂ W + k

diff V ^Ci V 1 - * The following table compares characteristics of an exemplary radio frequency transformer 100, as a calculation example, to a reference radio frequency transformer. Both radio frequency transformers are assumed to have a differential inductance of 2 nH, a differential resonance at a frequency of 1 GHz, and 15 % of the input capacitance being differential, i.e. kc = C2/C_diff = 0.15.

As can be seen, if the coupling factor k is increased from 0.5 to 0.7, which may not appear to be a huge change, the common mode inductance L_cm and its common mode impedance XL_cm(2RF) can be reduced by a factor of 1.89. Even more interesting is the fact, that the common mode resonance frequency is shifted away from 1.88 GHz, which is close to 2 x RF = 2 GHz, up to 2.58 GHz, further away from 2 x RF. Therefore, the impedance at 2 x RF is reduced by a factor of 5.67 in this example.

Fig. 9 shows a diagram of a common mode load impedance of a radio frequency transformer 100 over frequency in dependence of a coupling factor k according to an embodiment. The coupling factor k refers to the primary winding 101 of the radio frequency transformer 100. In the diagram, a finite quality factor Q of the capacitors and inductors is considered. In this example, a quality factor Q of 10 is assumed. As can be seen from the diagram, a benefit can e.g. be achieved starting from k=0.5 to k=0.7 by a factor of 6.0 Ohm / 2.4 Ohm = 2.5. The reduction of the factor of 2.5 can result in a significant C-IM3 improvement within a radio frequency transmitter 200.

The radio frequency transformer 100 can be used as a radio frequency balun providing a low common mode impedance at balanced terminals. The radio frequency transformer 100 can have a tight electromagnetic coupling between two halves of the primary winding 101. The primary winding 101 can be split into two parallel portions or windings.

The radio frequency transformer 100 can have at least one turn of adjacent conducting lines of the primary winding 101 and at least one turn of adjacent conducting lines of the secondary winding 103. Alternatively, the radio frequency transformer 100 can have at least 0.5 turns of adjacent conducting lines of the primary winding 101 and at least 0.5 turns of adjacent conducting lines of the secondary winding 103. The radio frequency transformer 100 can be implemented in a thick top-metal layer. A tight electromagnetic coupling can also be achieved by stacking two turns on top of each other, e.g. by using two different metal layers. The radio frequency transformer 100 may also be used in reverse direction, i.e. the primary winding 101 and the secondary winding 103 may be exchanged.

Fig. 10 shows a diagram of a radio frequency transmitter 200 comprising a modulator 201 and a radio frequency transformer 100 according to an embodiment. The radio frequency transmitter 200 further comprises a power amplifier 1001 , a duplex filter 1003, an antenna tuner 1005, an antenna 1007, and a receiver 1009. The radio frequency transmitter 200 forms a possible implementation of the radio frequency transmitter 200 as described in conjunction with Fig. 2.

The modulator 201 is configured to generate an input radio frequency signal. The radio frequency transformer 100 is configured to transform the input radio frequency signal into an output radio frequency signal. The input radio frequency signal is a differential radio frequency signal, and the output radio frequency signal is a single-ended radio frequency signal. The radio frequency transformer 100 thus operates as a radio frequency balun.

The primary winding of the radio frequency transformer 100 is connected to a supply voltage source for supplying the modulator 201. The modulator 201 is configured to generate the input radio frequency signal by pulling a radio frequency current out of the radio frequency transformer 100. The modulator 201 is a direct digital radio frequency modulator (DD M) comprising a core with a plurality of switched transistors. The modulator 201 is operated using base band data, local oscillator signals, and/or clock signals.

The radio frequency transmitter 200 can e.g. be used in mobile communication systems. A purpose of the radio frequency transformer 100 is the conversion of the differential radio frequency signal provided by the modulator 201 into a single-ended radio frequency signal as an input to the power amplifier 1001. The power amplifier 1001 can be one of a plurality of (pre-) power amplifiers. A further purpose is the provision of the output nodes of the modulator 201 with a drain bias. This is realized by connecting a supply voltage source to the center tap of the primary winding of the radio frequency transformer 100. The radio frequency transmitter 200 can form a digital radio frequency transceiver.

Fig. 1 1 shows a diagram of an output power and a counter third order intermodulation (C- IM3) performance of a radio frequency transmitter 200 according to an embodiment. The diagram further shows an output power and a counter third order intermodulation (C-IM3) performance of a reference radio frequency transmitter. The output powers and counter third order intermodulation (C-IM3) performances are depicted in dependence of a tunable capacitor setting.

The output powers of the radio frequency transmitter 200 and the reference radio frequency transmitter both behave similarly. The radio frequency transformer 100 within the radio frequency transmitter 200 provides a slightly higher output power, thus having a lower loss.

At the capacitor setting for maximum output power, i.e. when properly tuned to the input frequency, the counter third order intermodulation (C-IM3) level of the radio frequency transmitter 200 is -64.0 dBc, as compared to -46.1 dBc using the reference radio frequency transmitter. Therefore, an improvement of 17.9 dB can be achieved in this example. This improvement of 17.9 dB may be realized, as a reduction of the common mode impedance by a factor of 2.5 can lower the 2 F common mode voltage swing by 20^*log(2.5) = 8 dB, and the counter third order intermodulation (C-IM3) performance is a third order inter-modulation effect.

Fig. 12 shows a diagram of a reference radio frequency transformer comprising a primary winding and a secondary winding. The diagram comprises a layout and a schematic circuit of the reference radio frequency transformer. The primary winding and the secondary winding have a turn ratio of 2:2.

The primary winding has a first input terminal in_p, a second input terminal in_n, and a further terminal vdd. The secondary winding has a first output terminal out, and a second output terminal gnd. Portions of the primary winding are referred to as L1 P and L1 N. The secondary winding is referred to as L2.

Fig. 13 shows a diagram of a reference radio frequency transformer comprising a primary winding and a secondary winding. The diagram comprises a layout and a schematic circuit of the reference radio frequency transformer. The primary winding and the secondary winding have a turn ratio of 4:4.

The reference radio frequency transformers described in Fig. 12 and Fig. 13 can be on-chip radio frequency transformers. The positive and negative sides of the primary windings can be referred to as L1 P and L1 N. The secondary winding can be referred to as L2. Due to the turn ratios of 2:2 and 4:4, the input impedances can be approximately equal to the load impedances in both cases. L1 P and L1 N are loosely coupled, as there is always one secondary turn in between primary turns. The coupling coefficient k between the two halves of the primary winding, L1 P and L1 N, can approximately be 0.5.

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1 . A radio frequency transformer (100) for transforming an input radio frequency signal into an output radio frequency signal, the radio frequency transformer (100) comprising: a primary winding (101 ) having a first input terminal and a second input terminal, wherein the first input terminal and the second input terminal are configured to handle the input radio frequency signal, wherein the primary winding (101 ) comprises a first portion (L1 Pa), a second portion (L1 Pb), a third portion (L1 Na), and a fourth portion (L1 Nb), wherein the first portion (L1 Pa) is electromagnetically coupled to the fourth portion (L1 Nb), and wherein the second portion (L1 Pb) is electromagnetically coupled to the third portion (L1 Na); and a secondary winding (103) having a first output terminal and a second output terminal, wherein the first output terminal and the second output terminal are configured to provide the output radio frequency signal, and wherein the secondary winding (103) is

electromagnetically coupled to the first portion (L1 Pa), the second portion (L1 Pb), the third portion (L1 Na), and the fourth portion (L1 Nb) of the primary winding (101 ).

2. The radio frequency transformer (100) of claim 1 , wherein the first portion (L1 Pa) is electromagnetically decoupled from the second portion (L1 Pb), and wherein the third portion

(L1 Na) is electromagnetically decoupled from the fourth portion (L1 Nb).

3. The radio frequency transformer (100) of claims 1 or 2, wherein the input radio frequency signal is a differential radio frequency signal, and wherein the output radio frequency signal is a single-ended radio frequency signal.

4. The radio frequency transformer (100) of any of the preceding claims, wherein the input radio frequency signal comprises a signal component at an input frequency, and wherein a common mode resonance frequency of the radio frequency transformer (100) is greater than twice the input frequency.

5. The radio frequency transformer (100) of any of the preceding claims, wherein the first portion (L1 Pa) and the second portion (L1 Pb) of the primary winding (101 ) are connected in parallel, or wherein the third portion (L1 Na) and the fourth portion (L1 Nb) of the primary winding (101 ) are connected in parallel.

6. The radio frequency transformer (100) of any of the preceding claims, wherein the radio frequency transformer (100) is arranged on a semiconductor substrate, wherein the first portion (L1 Pa) comprises a first conducting line, wherein the second portion (L1 Pb) comprises a second conducting line, wherein the third portion (L1 Na) comprises a third conducting line, and wherein the fourth portion (L1 Nb) comprises a fourth conducting line.

7. The radio frequency transformer (100) of claim 6, wherein a portion of the first conducting line is adjacent to a portion of the fourth conducting line, or wherein a portion of the second conducting line is adjacent to a portion of the third conducting line.

8. The radio frequency transformer (100) of claims 6 or 7, wherein a portion of the first conducting line and a portion of the fourth conducting line are arranged within different layers on the semiconductor substrate, or wherein a portion of the second conducting line and a portion of the third conducting line are arranged within different layers on the semiconductor substrate.

9. The radio frequency transformer (100) of any of the preceding claims, wherein the primary winding (101 ) is connected to a supply voltage source.

10. The radio frequency transformer (100) of claim 9, wherein the supply voltage source is connected to the primary winding (101 ) between the first portion (L1 Pa) and the third portion (L1 Na), or wherein the supply voltage source is connected to the primary winding (101 ) between the second portion (L1 Pb) and the fourth portion (L1 Nb).

1 1 . A radio frequency transmitter (200), comprising: a modulator (201 ) being configured to generate an input radio frequency signal; and a radio frequency transformer (100) according to any of the claims 1 to 10, wherein the radio frequency transformer (100) is configured to transform the input radio frequency signal into an output radio frequency signal.

12. The radio frequency transmitter (200) of claim 1 1 , wherein the modulator (201 ) is configured to generate the input radio frequency signal by pulling a radio frequency current out of the radio frequency transformer (100).

13. The radio frequency transmitter (200) of claims 1 1 or 12, wherein the modulator (201 ) is a direct digital radio frequency modulator.

14. The radio frequency transmitter (200) of claims 1 1 to 13, further comprising: a power amplifier (1001 ) being configured to amplify the output radio frequency signal.

15. A method (300) for transforming an input radio frequency signal into an output radio frequency signal using a radio frequency transformer (100), wherein the radio frequency transformer (100) comprises a primary winding (101 ) and a secondary winding (103), wherein the primary winding (101 ) has a first input terminal and a second input terminal, and wherein the secondary winding (103) has a first output terminal and a second output terminal, the method (300) comprising: handling (301 ) the input radio frequency signal by the first input terminal and the second input terminal of the primary winding (101 ); electromagnetically coupling (303) a first portion (L1 Pa) of the primary winding (101 ) to a fourth portion (L1 Nb) of the primary winding (101 ); electromagnetically coupling (305) a second portion (L1 Pb) of the primary winding (101 ) to a third portion (L1 Na) of the primary winding (101 ); electromagnetically coupling (307) the secondary winding (103) to the first portion (L1 Pa), the second portion (L1 Pb), the third portion (L1 Na), and the fourth portion (L1 Nb) of the primary winding (101 ); and providing (309) the output radio frequency signal by the first output terminal and the second output terminal of the secondary winding (103).