Transmission line impedance transformer and related methods

211831/2

TRANSMISSION LINE IMPEDANCE TRANSFORMER AND RELATED METHODS

Field of the Invention 5 [0001] The present invention relates to the field of transformers, and, more particularly, to radio frequency transmission line impedance transformers and related methods.

Background of the Invention [0002] Wireless communications devices are an integral part of society and 10 permeate daily life. The typical wireless communications device includes an antenna, and a transceiver coupled to the antenna. The transceiver and the antenna cooperate to transmit and receive communications signals.

[0003] A typical radio frequency (RF) transceiver includes a power amplifier for amplifying low amplitude signals for transmission via the antenna. Given that most 15 mobile communications devices operate on limited battery power, energy efficient power amplifiers may be desirable. More specifically and as will be appreciated by those skilled in the art, Class C and E power amplifiers are common in certain communications devices since they are efficient power amplifiers. These classes of power amplifiers are more efficient than Class A or B amplifiers, for example, but are 20 subject to performance tradeoffs. For example, they may be nonlinear over certain frequencies and may introduce greater amounts of distortion into the amplified signal (if the signal requires a linear amplifier).

[0004] As will be appreciated by those skilled in the art, in high power amplifier applications, amplifiers are typically used to amplify signals received via transmission 25 lines. In these applications, it may be necessary to transform the impedances of the transmission lines coupled to the input and output of the amplifier to match the load line impedance of the amplifier. As will be appreciated by those skilled in the art, the matched impedances provide greater efficiency with lower losses and greater bandwidth for the transmitted signal. - 1 - 02085339\61-01 211831/2 [0005] To improve the low end frequency response, magnetic materials, for example, ferrite may be added to the impedance transformer. For example, with reference to FIGS. 1-2, a ferrite impedance transformer 20 is now described. The ferrite impedance transformer 20 matches differing impedances between an input 25 and an 5 output 26, illustratively, a 1:4 ratio. The ferrite impedance transformer 20 illustratively includes a circuit board 21, a plurality of ferrite cores 23a-23b, 24a-24b mounted on the circuit board, and a pair of rigid coaxial cables 22a-22b wound through each of the ferrite cores.

[0006] This ferrite impedance transformer 20 may suffer from several drawbacks. 10 For example, the ferrite impedance transformer 20 may be difficult to manufacture, as the rigid coaxial cables 22a-22b are hard to manipulate. Moreover, the rigid coaxial cable 22a-22b may be expensive, and may be typically hand wound and hand soldered onto the circuit board 21. Further, given the manual labor-intensive manufacture process, the ferrite impedance transformer 20 may be subject to significant variation in 15 electrical performance.

[0007] Prior art is listed below. Acknowledgment of the following reference is not to be inferred as meaning that this is in any way relevant to the patentability of the invention disclosed herein.

[0008] EP 0539133 discloses a frequency selective transformer circuit integrated on 20 a multilayer printed board and operating at radio frequencies. To the input of the transformer circuit there is coupled a resonant circuit realized by microstrip conductors operating as a two-pole filter. With the aid of the microstrip transformer it is also possible to realize a mixer which attenuates the image frequency.

[0009] US 5808518 discloses a Guanella 1:4 balun which is made capable of 25 handling frequencies in the gigahertz range by printing its transmission lines on a microwave laminate. The resulting balun has a largest dimension substantially smaller than one free space wavelength throughout a 20:1 usable frequency range.

Summary of the Invention [0010] In view of the foregoing background, it is therefore an object of the present 30 invention to provide a transmission line impedance transformer that is readily manufactured. - 2 - 02085339\61-01 211831/2 [0011] This and other objects, features, and advantages in accordance with the present invention are provided by a transmission line impedance transformer. The transmission line impedance transformer includes a printed circuit board (PCB) comprising at least one dielectric layer and at least one electrically conductive layer 5 thereon defining a medial interconnection portion, and first and second lateral loop portions extending laterally outwardly from opposing first and second sides of the medial interconnection portion. The PCB also includes a first plurality of ferrite body receiving openings therein adjacent the first lateral loop portion, and a second plurality of ferrite body receiving openings therein adjacent the second lateral loop portion. The 10 transmission line impedance transformer also includes at least one first ferromagnetic body extending through the first plurality of ferrite body receiving openings to surround the first lateral loop portion, and at least one second ferromagnetic body extending through the second plurality of ferrite body receiving openings to surround the second lateral loop portion. Advantageously, the transmission line impedance transformer may 15 be planar and may be manufactured without the typical wound rigid coaxial cables. [0012] More particularly, the medial interconnection portion may define an input and an output. For example, the input and the output may have different impedances. The electrically conductive layer may comprise a pair thereof, and the medial interconnection portion may comprise at least one electrically conductive via extending 20 between the pair of electrically conductive layers.

[0013] In some embodiments, each of the first and second lateral loop portions may comprise at least one U-shaped conductive trace. Additionally, the first ferromagnetic body may comprise a first plurality thereof for surrounding the first lateral loop portion, and the at least one second ferromagnetic body may comprise a second plurality thereof 25 for surrounding the second lateral loop portion.

[0014] Moreover, the at least one first ferromagnetic body may comprise a respective first pair of joined together segments, and the at least one second ferromagnetic body may also comprise a respective second pair of joined together segments. 30 [0015] Further, in some embodiments, each of the at least one first and at least one second ferromagnetic bodies may comprise a respective tubular ferromagnetic body. - 3 - 02085339\61-01 211831/2

For example, the PCB and the at least one first and second ferromagnetic bodies may define an impedance transformer operable over a frequency range of 2 to 500 MHz. [0016] Another aspect is directed to a method of making a transmission line impedance transformer. The method includes providing a printed circuit board (PCB) 5 comprising at least one dielectric layer, and forming at least one electrically conductive layer on the PCB defining a medial interconnection portion, and first and second lateral loop portions extending laterally outwardly from opposing first and second sides of the medial interconnection portion. The method also includes forming a first plurality of ferrite body receiving openings in the PCB adjacent the first lateral loop portion and 10 forming a second plurality of ferrite body receiving openings in the PCB adjacent the second lateral loop portion. The method also includes positioning at least one first ferromagnetic body to extend through the first plurality of ferrite body receiving openings and to surround the first lateral loop portion, and positioning at least one second ferromagnetic body to extend through the second plurality of ferrite body 15 receiving openings and to surround the second lateral loop portion.

Brief Description of the Drawings [0017] FIG. 1 is a perspective view of a transmission line transformer, according to the prior art.

[0018] FIG. 2 is a schematic circuit diagram of the transmission line transformer of 20 FIG. 1.

[0019] FIG. 3 is a side elevational view of a transmission line transformer, according to the present invention.

[0020] FIG. 4 is a bottom view of the transmission line transformer of FIG. 3.

[0021] FIG. 5 is a top view of the transmission line transformer of FIG. 3. 25 [0022] FIG. 6a is a cross-sectional view of the transmission line transformer of FIG. 3 along lines 6-6.

[0023] FIG. 6b is a perspective view of a single ferromagnetic body from the transmission line transformer of FIG. 3.

[0024] FIG. 7 is a view of the top side conductive layer from the transmission line 30 transformer of FIG. 3. - 4 - 02085339\61-01 211831/2 [0025] FIG. 8 is a view of the bottom side conductive layer from the transmission line transformer of FIG. 3.

[0026] FIG. 9 is a measurement setup for measuring the transmission line transformer of FIG. 3. 5 [0027] FIG. 10 is a diagram illustrating electrical performance of the transmission line transformer of FIG. 3.

Detailed Description of the Preferred Embodiments [0028] The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the 10 invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. 15 [0029] Referring initially to FIGS. 3-8, a transmission line impedance transformer 30 according to the present invention is now described. The transmission line impedance transformer 30 illustratively includes a printed circuit board (PCB) 31 comprising a dielectric layer and a pair of electrically conductive layers 34-35 on the major surfaces of the PCB. In the illustrated embodiment, the PCB 31 is planar in 20 shape, but may have other shapes in other embodiments, for example, a curved shape. In other embodiments, the PCB 31 may include multiple dielectric layers and multiple electrically conductive layers.

[0030] The pair of electrically conductive layers 34-35 defines a medial interconnection portion 36, and first 41a, 42a and second 41b, 42b lateral loop portions 25 extending laterally outwardly from opposing first and second sides of the medial interconnection portion. More particularly, the medial interconnection portion 36 illustratively defines an input 48a-48b and an output 49a-49b, the input and output having different impedances. Furthermore, the medial interconnection portion 36 illustratively includes a plurality of electrically conductive vias 40a-40b extending 30 between the pair of electrically conductive layers 34-35 and coupling the layers together. - 5 - 02085339\61-01 211831/2 [0031] The PCB 31 illustratively includes a first plurality of ferrite body receiving openings 39a-39d therein adjacent the first lateral loop portions 41a, 42a (FIG. 6a) and a second plurality of ferrite body receiving openings 39a-39d (FIG. 6a) therein adjacent the second lateral loop portions 41b, 42b. In the illustrated embodiment, the first and 5 second ferrite body receiving openings 39a-39d are rectangular in shape, but could have other shapes in other embodiments. The transmission line impedance transformer 30 illustratively includes a plurality of first ferromagnetic bodies 33a-33b, 37a-37b extending through the first plurality of ferrite body receiving openings 39a-39d to surround the first lateral loop portion 41a, 42a, and a plurality of second ferromagnetic 10 bodies 32a-32b, 38a-38b extending through the second plurality of ferrite body receiving openings to surround the second lateral loop portions 41b, 42b.

[0032] In the illustrated embodiment, each of the first 41a, 42a and second 41b, 42b lateral loop portions are a U-shaped conductive trace. Further, in the illustrated embodiment and as perhaps best seen in FIG. 6a, the first 33a-33b, 37a-37b and second 15 32a-32b, 38a-38b ferromagnetic bodies illustratively comprise respective pairs of joined together segments. In other embodiments, the first 33a-33b, 37a-37b and second 32a-32b, 38a-38b ferromagnetic bodies may be integral. Additionally, in the illustrated embodiment, each of the first ferromagnetic bodies 33a-33b, 37a-37b and the second ferromagnetic bodies 32a-32b, 38a-38b are tubular in shape. In other embodiments, the 20 first 33a-33b, 37a-37b and second 32a-32b, 38a-38b ferromagnetic bodies may have other shapes, for example, rectangular. For example, the PCB and the at least one first 33a-33b, 37a-37b and second 32a-32b, 38a-38b ferromagnetic bodies may define an impedance transformer operable over a frequency range of 2 to 500 MHz. Of course, as will be appreciated by those skilled in the art, the transmission line impedance 25 transformer 30 may be modified to operate over a wide variety of frequencies.

[0033] Referring now additionally to FIG. 9, a diagram 60 illustrates operation of the transmission line impedance transformer 30. Illustratively, the transmission line impedance transformer 30 transforms an input 61 impedance of 12.5ξ into an output 62 impedance of 50.0ξ, an illustrative transformation ratio of 1:4. Of course, as appreciated 30 by those skilled in the art, the transmission line impedance transformer 30 may be modified to have other impedance transformation ratios. Nonetheless, the PCB 31 would be modified accordingly. - 6 - 02085339\61-01 211831/2 [0034] Referring now to FIG. 10, which includes a chart 50 illustrating the electrical performance of the transmission line impedance transformer 30 described above. In particular, the chart 50 includes an x-axis plot for frequency, a left y-axis for insertion loss in decibels, and a right y-axis plot for return loss in decibels (return loss 5 corresponding to how close the impedance looking into the terminal is to the intended design impedance. In the illustrated example, one side should like 50ξ, and the other side should show 12.5ξ). Curve 51 illustrates the insertion loss, which maintains a desirable value of less than 0.5 dB over the operating frequency range, see, for example, points M1 Frequency = 2.100 MHz, db(S(2,1))=-0.448; M2 Frequency = 188.1 MHz, 10 db(S(2,1))=-0.193; M3 Frequency = 341.1 MHz, db(S(2,1))=-0.251; and M4 Frequency = 505.1 MHz, db(S(2,1))=-3.032. Curves 52-53 illustrate the return loss for the transmission line impedance transformer 30, which is better than -15 decibels over the operating range of 2 to 500 MHz.

[0035] Advantageously, the above described transmission line impedance 15 transformer 30 is toroidal and well suited for high frequency/high power applications yet may be manufactured without cumbersome hand wound rigid coaxial cables, as in the prior art. In other words, the transmission line impedance transformer 30 may be manufactured without intensive manual labor. Indeed, the transmission line impedance transformer 30 uses no soldering for assembly and may be manufactured before any 20 wave soldering process is used. Helpfully, the transmission line impedance transformer 30 uses no external assemblies and is more mechanically robust than the typical rigid coaxial cable type transmission line transformer. Moreover, the transmission line impedance transformer 30 is readily manufactured with repeatable and consistent electrical performance since the manual manufacturing component of the typical 25 transmission line transformer is removed. Also, since the transmission line impedance transformer 30 need not use expensive rigid coaxial cable, the cost of manufacture is reduced. 02085339\61-01 - 7 -