A flexible planar transmission line and common mode suppression balun structure

Abstract

The present invention discloses a flexible planar transmission line and a common-mode suppression balun structure, which are made of a flexible printed circuit board, wherein the flexible printed circuit board includes a flexible insulating dielectric layer, an upper copper-clad layer located on the upper surface of the flexible insulating dielectric layer, and a lower copper-clad layer located on the lower surface of the flexible insulating dielectric layer; the upper and lower copper-clad layers are partially overlapped, and the overlapped upper and lower copper-clad layers form a flexible planar capacitor together with the flexible insulating dielectric layer in between; the conductor of the flexible planar transmission line is composed of the upper and lower copper-clad layers of the flexible printed circuit board; the common-mode suppression balun includes a flexible planar transmission line and a flexible planar capacitor. The flexible planar transmission line and the common-mode suppression balun structure of the present invention are made of a flexible printed circuit board, do not require any solder joints and discrete components, can be made into a completely planar structure, are very thin, have very good flexibility, are low in cost, have good reliability, and are very suitable for use in a magnetic resonance flexible radio frequency coil.

Description

Technical Field

[0001] The present invention relates to the technical field of radio frequency signal transmission and common mode suppression, and in particular to a flexible planar transmission line and a common mode suppression balun structure. Background Art

[0002] Magnetic resonance imaging is an advanced non-destructive imaging technology for the human body, which is widely used in the diagnosis of diseases in various parts of the human body. The MRI radio frequency coil is an important part of the MRI system, and its performance directly determines the quality of MRI.

[0003] The frequency of magnetic resonance signals is usually between tens of MHz and hundreds of MHz, and coaxial cables are usually used to transmit signals. At the same time, since the electromagnetic environment in which the magnetic resonance radio frequency coil works is very complex, a balun is needed to suppress the common mode current on the coaxial line, otherwise the signal-to-noise ratio of the magnetic resonance radio frequency coil will be significantly reduced, and it is even possible that during the radio frequency transmission of the magnetic resonance system, a huge radio frequency current will be induced on the cable line, causing burns to the patient.

[0004] See also Figure 1 The commonly used common-mode current suppression balun is to wind the coaxial line 1' into one or several turns of the solenoid 2', and peel off the outer insulating skin 22' at both ends to expose the shielding layer 21', weld the shielding layers at both ends to the hard printed circuit board 4', and weld a ceramic capacitor 3' in parallel through the copper-clad conductor on the hard printed circuit board 4', so that the shielding layer 21' and the capacitor 3' of this section of the coaxial line can form a parallel resonant circuit, presenting high resistance, thereby being able to suppress the common-mode current on the shielding layer.

[0005] With the development and progress of magnetic resonance technology, especially in recent years, flexible RF coils have become more and more popular among doctors and patients due to their advantages such as lightness, ease of use, and high patient comfort. However, the traditional common-mode suppression balun, whose structure is a coaxial solenoid in parallel with a ceramic capacitor, is very large in size due to the need to wind the solenoid and weld a high-voltage ceramic capacitor. It also usually requires independent plastic shell protection, so it is usually large and thick. Moreover, due to the increasing number of RF coil channels, there are more and more 16-channel, 32-channel and even 64-channel flexible RF coils, and each channel requires at least one balun. Therefore, this traditional balun structure of a coaxial line wound into a solenoid has become increasingly unable to meet the development needs of high-density flexible RF coils, and has become a bottleneck factor restricting the further softening of flexible coils. Summary of the invention

[0006] The purpose of the present invention is to overcome the defects of the prior art and provide a flexible planar transmission line and common-mode suppression balun structure, which is made of a flexible printed circuit board, does not require any solder joints and discrete components, can be made into a completely planar structure, has a very thin thickness, very good flexibility, low cost, good reliability, and is very suitable for use in a magnetic resonance flexible radio frequency coil.

[0007] The technical solution to achieve the above object is: a flexible planar transmission line and common mode suppression balun structure, made of a flexible printed circuit board, wherein:

[0008] The flexible printed circuit board comprises a flexible insulating dielectric layer, an upper copper-clad layer located on the upper surface of the flexible insulating dielectric layer, and a lower copper-clad layer located on the lower surface of the flexible insulating dielectric layer; the upper and lower copper-clad layers are partially overlapped, and the overlapped upper and lower copper-clad layers and the flexible insulating dielectric layer between the two together form a flexible planar capacitor;

[0009] The conductor of the flexible planar transmission line is formed by the upper and lower copper-clad layers of the flexible printed circuit board;

[0010] The common-mode suppression balun includes a flexible planar transmission line and a flexible planar capacitor; the upper copper layer or the lower copper layer of the flexible planar transmission line forms an inductor, and the two ends of the inductor are connected in parallel with the flexible planar capacitor to form a parallel resonant circuit, and the parallel resonant circuit forms the common-mode suppression balun.

[0011] The above-mentioned flexible planar transmission line and common-mode suppression balun structure, wherein the flexible printed circuit board is a double-layer flexible printed circuit board, the upper copper-clad layer and the lower copper-clad layer are respectively single copper-clad layers; the upper copper-clad layer constitutes a top conductor, and the lower copper-clad layer constitutes a bottom conductor;

[0012] The flexible planar transmission line comprises two conductors, one conductor is a top conductor of a double-layer flexible printed circuit board, and the other conductor is a bottom conductor of the double-layer flexible printed circuit board, wherein the bottom conductor has a width greater than that of the top conductor.

[0013] The above-mentioned flexible planar transmission line and common-mode suppression balun structure, wherein the flexible printed circuit board is a double-layer flexible printed circuit board, the upper surface of the flexible insulating dielectric layer is provided with a plurality of upper copper layers parallel to each other, each upper copper layer forms a top conductor; the lower surface of the flexible insulating dielectric layer is a lower copper layer, and the lower copper layer constitutes a bottom conductor;

[0014] The flexible planar transmission line includes a plurality of top conductors and a bottom conductor, and the bottom conductor has a width greater than a width of each of the top conductors.

[0015] The above-mentioned flexible planar transmission line and common-mode suppression balun structure, wherein the top conductor and the bottom conductor of the flexible planar transmission line are both straight lines, and a flexible planar capacitor is connected in parallel at both ends of a section of the bottom conductor of the flexible planar transmission line; the section of the bottom conductor connected in parallel with the flexible planar capacitor forms an inductor;

[0016] The top conductor of the flexible planar capacitor is connected to one end of the inductor through a via-hole connecting wire conductor, and the bottom conductor of the flexible planar capacitor is connected to the other end of the inductor through a connecting wire conductor;

[0017] The inductor and the flexible plane capacitor form a parallel resonant circuit, and the parallel resonant circuit forms the common mode suppression balun.

[0018] The above-mentioned flexible planar transmission line and common-mode suppression balun structure, wherein the flexible planar transmission line is wound into an 8-shape, and a plurality of vias are provided in the overlapping portion of the 8-shape. The top conductor of the flexible planar transmission line extends to the lower surface of the flexible insulating dielectric layer through the vias, or the bottom conductor of the flexible planar transmission line extends to the upper surface of the flexible insulating dielectric layer through the vias; after the top conductor of the flexible planar transmission line passes over the overlapping portion, it passes through the vias to return to the upper surface of the flexible insulating dielectric layer, or after the bottom conductor of the flexible planar transmission line passes over the overlapping portion, it passes through the vias to return to the lower surface of the flexible insulating dielectric layer.

[0019] The above-mentioned flexible planar transmission line and common-mode suppression balun structure, wherein two ends of a section of bottom conductor of the 8-shaped flexible planar transmission line are connected in parallel with a flexible planar capacitor; the section of bottom conductor connected in parallel with the flexible planar capacitor forms an inductor;

[0020] The top conductor of the flexible planar capacitor is connected to one end of the inductor through a via-hole connecting line conductor, and the bottom conductor of the flexible planar capacitor is connected to the other end of the inductor;

[0021] The inductor and the flexible plane capacitor form a parallel resonant circuit, and the parallel resonant circuit forms the common mode suppression balun.

[0022] The above-mentioned flexible planar transmission line and common-mode suppression balun structure, wherein the flexible insulating dielectric layer of the flexible printed circuit board is polyimide, and a polyimide protective layer is respectively arranged on the topmost upper copper layer and under the bottommost lower copper layer.

[0023] In the above-mentioned flexible planar transmission line and common-mode suppression balun structure, the flexible printed circuit board adopts a multi-layer flexible printed circuit board.

[0024] The flexible planar transmission line and common-mode suppression balun structure of the present invention no longer use the traditional coaxial line for signal transmission, but use a flexible printed circuit board to make a flexible planar transmission line. The ceramic capacitor required for the common-mode suppression balun is also replaced by a flexible planar capacitor formed by the top conductor of the flexible printed circuit board, the bottom conductor and the insulating medium layer between the two. Moreover, the flexible planar transmission line and the flexible planar capacitor are also directly connected through the printed circuit board conductor, and welding is no longer required. The advantages of this invention are:

[0025] (1) Both the transmission line and balun are planar structures with a thickness of less than 0.2 mm, which is completely flexible and will not affect the thickness and flexibility of the flexible RF coil at all;

[0026] (2) Since no welding process is required, the process is simple, it is not afraid of bending, and has high reliability;

[0027] (3) No coaxial line and ceramic capacitors are required, and the coil loop can even be made on the same flexible printed circuit board, which greatly reduces the cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 The structure diagram of the traditional transmission line and common mode rejection balun;

[0029] Figure 2 It is a schematic diagram of a flexible planar transmission line and a common-mode suppression balun structure of Embodiment 1;

[0030] Figure 3 It is a schematic diagram of a flexible planar transmission line and a common-mode suppression balun structure of Embodiment 2;

[0031] Figure 4 It is a schematic diagram of a flexible planar transmission line and a common-mode suppression balun structure of Example 3;

[0032] Figure 5 This is a diagram showing the simulation calculation results of the common-mode attenuation effect of the common-mode suppression balun using the structure of the present invention. DETAILED DESCRIPTION

[0033] In order to enable those skilled in the art to better understand the technical solution of the present invention, the specific implementation methods thereof are described in detail below in conjunction with the accompanying drawings:

[0034] Embodiment 1:

[0035] See also Figure 2A flexible planar transmission line and a common-mode suppression balun structure are made of a flexible printed circuit board, the flexible printed circuit board includes a flexible insulating dielectric layer 13, an upper copper layer located on the upper surface of the flexible insulating dielectric layer, and a lower copper layer located on the lower surface of the flexible insulating dielectric layer; the upper and lower copper layers are partially overlapped, and the overlapped upper and lower copper layers and the flexible insulating dielectric layer in between form a flexible planar capacitor 3 together; the flexible insulating dielectric layer 13 of the flexible printed circuit board is polyimide, and a polyimide protective layer is respectively arranged on the topmost upper copper layer and under the bottommost lower copper layer.

[0036] The flexible printed circuit board is a double-layer flexible printed circuit board, wherein the upper copper clad layer and the lower copper clad layer are respectively single copper clad layers; the upper copper clad layer constitutes a top conductor 11 , and the lower copper clad layer constitutes a bottom conductor 12 .

[0037] The flexible planar transmission line includes two conductors, one conductor is the top conductor 11 of the double-layer flexible printed circuit board, and the other conductor is the bottom conductor 12 of the double-layer flexible printed circuit board, and the width of the bottom conductor 12 is greater than the width of the top conductor 11. In this way, the conductor of the flexible planar transmission line is composed of the upper and lower copper layers of the flexible printed circuit board; the top conductor and the bottom conductor of the flexible planar transmission line are both straight lines, and a flexible planar capacitor 3 is connected in parallel to both ends of a section of the bottom conductor of the flexible planar transmission line; the section of the bottom conductor connected in parallel with the flexible planar capacitor 3 forms an inductor; the top conductor of the flexible planar capacitor 3 is connected to one end of the inductor through a via-hole connecting line conductor 4, and the bottom conductor of the flexible planar capacitor 3 is connected to the other end of the inductor through a connecting line conductor 5; the inductor and the flexible planar capacitor 3 form a parallel resonant circuit, and the parallel resonant circuit forms the common-mode suppression balun.

[0038] In this embodiment, a section of flexible planar transmission line is used to replace the commonly used coaxial line. The top conductor 11 is thinner and functions similarly to the core wire of the coaxial line; the bottom conductor 12 is wider and can be solid or grid-shaped copper, and functions similarly to the shielding wire of the coaxial line. One end of a certain section of the bottom conductor 12 is connected to the flexible planar capacitor 3 through two sections of capacitor connecting line conductors. The flexible planar capacitor 3 is not a traditional ceramic capacitor, and its capacitance is determined by the following formula:

[0039] C=ε0*ε r *A / d

[0040] Where C is the capacitance value, ε0 is the vacuum dielectric constant, ε r is the relative dielectric constant of the flexible insulating dielectric layer, A is the overlapping area, and d is the thickness of the flexible insulating dielectric layer.

[0041] A section of the bottom conductor of the flexible planar transmission line and the flexible planar capacitor 3 are connected in parallel to resonate. The inductance of the inductor can be adjusted by adjusting the length, width and shape of the bottom conductor 12, the connecting line conductor 5 and the via connecting line conductor 4. The capacitance of the capacitor formed by them can be adjusted by adjusting the overlapping area of ​​the top conductor and the bottom conductor of the flexible planar capacitor 3, the thickness of the dielectric layer and the dielectric constant of the dielectric material. Finally, the resonant frequency of the inductor and the capacitor can be adjusted, and this resonant frequency can be adjusted to the frequency of the magnetic resonance signal, thereby forming a common-mode suppression balun, which can effectively suppress the common-mode current on the transmission line without having any effect on the differential-mode signal.

[0042] Embodiment 2:

[0043] See also Figure 3 , a flexible planar transmission line and a common-mode suppression balun structure. Based on the first embodiment, the flexible printed circuit board adopts a double-layer flexible printed circuit board, and the upper surface of the flexible insulating dielectric layer is provided with multiple upper copper layers parallel to each other, and each upper copper layer forms a top conductor; the lower surface of the flexible insulating dielectric layer is a lower copper layer, and a lower copper layer constitutes a bottom conductor.

[0044] The flexible planar transmission line includes a plurality of top conductors 11 and a bottom conductor 12, and the width of the bottom conductor 12 is greater than the width of each top conductor 11. In this embodiment, there are two top conductors. In the flexible planar transmission line of this structure, the two top conductors 11 can transmit two RF signals, or one RF signal and one DC signal. The working mode and effect of the common mode suppression balun 2 formed by a section of the bottom conductor 12 and the flexible planar capacitor 3 in this flexible planar transmission line remain unchanged.

[0045] Embodiment three:

[0046] See also Figure 4 A flexible planar transmission line and a common-mode suppression balun structure are provided. The flexible planar transmission line is made of the double-layer flexible printed circuit board in the first embodiment. The flexible planar transmission line is wound into an 8-shape. The flexible planar transmission line is provided with a plurality of vias 9 at the intersection A of the 8-shape. The top conductor 11 of the flexible planar transmission line extends to the lower surface of the flexible insulating dielectric layer through the vias 9, or the bottom conductor 12 of the flexible planar transmission line extends to the upper surface of the flexible insulating dielectric layer through the vias; after the top conductor 11 of the flexible planar transmission line passes over the overlapping portion, it passes through the vias 9 to return to the upper surface of the flexible insulating dielectric layer, or after the bottom conductor 12 of the flexible planar transmission line passes over the overlapping portion, it passes through the vias 9 to return to the lower surface of the flexible insulating dielectric layer.

[0047] A flexible planar capacitor 3 is connected in parallel at both ends of a bottom conductor of a figure-8 flexible planar transmission line; the bottom conductor connected in parallel with the flexible planar capacitor forms an inductor; the top conductor of the flexible planar capacitor 3 is connected to one end of the inductor through two via-hole connecting line conductors, and the bottom conductor of the flexible planar capacitor 3 is connected to the other end of the inductor; the inductor and the flexible planar capacitor 3 form a parallel resonant circuit, and the parallel resonant circuit forms a common-mode suppression balun 2.

[0048] See also Figure 5 , a common-mode suppression balun for 3.0T magnetic resonance is made using the structure of the present invention, and the common-mode attenuation effect is simulated and calculated. The bottom conductor width of this common-mode suppression balun is 0.4mm, the top conductor width is 0.1mm, the width of the entire common-mode suppression balun does not exceed 10mm, and the length does not exceed 20mm. A polyimide flexible insulating dielectric layer is used, and the thickness of the flexible insulating dielectric layer is 50um. Therefore, the entire flexible planar transmission line and common-mode suppression balun are very thin, very soft, and very small in size, and can be used very conveniently in the magnetic resonance radio frequency coil. Figure 5 It can be seen that the common-mode attenuation of this common-mode suppression balun can reach more than 20dB, and the attenuation effect is very good. Moreover, since the flexible insulating dielectric layer of the commonly used flexible printed circuit board is polyimide, its voltage resistance is very high. The 50um thick polyimide can withstand thousands of volts without breakdown, so the safety is fully guaranteed. What's more clever is that since the size of the entire common-mode suppression balun is very small, when it is properly bent, such as when the bending radius is not less than 5cm, the change in its attenuation curve is not large, so if it is placed in a flexible magnetic resonance RF coil for use, as long as it is not bent to an extreme, the effect of the common-mode suppression balun is guaranteed.

[0049] In the present invention, the flexible printed circuit board can also be a multi-layer flexible printed circuit board. The double-layer flexible printed circuit board substrate is composed of an insulating dielectric layer and a copper-clad layer with two sides being a whole piece. The four-layer flexible printed circuit board is composed of three insulating dielectric layers and four copper-clad layers. In actual use, it is necessary to etch away the unnecessary copper-clad layers, leaving the circuit to form the corresponding conductor.

[0050] In summary, the flexible planar transmission line and common-mode suppression balun structure of the present invention are made of a flexible printed circuit board, do not require any solder joints and discrete components, can be made into a completely planar structure, are very thin, have very good flexibility, low cost, good reliability, and are very suitable for use in magnetic resonance flexible radio frequency coils.

[0051] Those skilled in the art should recognize that the above embodiments are only used to illustrate the present invention, and are not intended to limit the present invention. As long as they are within the spirit of the present invention, any changes or modifications to the above embodiments will fall within the scope of the claims of the present invention.

Claims

1. A flexible planar transmission line and a common mode suppression balun structure, characterized in that: Made of flexible printed circuit board, where: The flexible printed circuit board comprises a flexible insulating dielectric layer, an upper copper-clad layer located on the upper surface of the flexible insulating dielectric layer, and a lower copper-clad layer located on the lower surface of the flexible insulating dielectric layer; the upper and lower copper-clad layers are partially overlapped, and the overlapped upper and lower copper-clad layers and the flexible insulating dielectric layer between the two together form a flexible planar capacitor; The conductor of the flexible planar transmission line is composed of upper and lower copper clad layers of a flexible printed circuit board; The common-mode suppression balun includes a flexible planar transmission line and a flexible planar capacitor; the upper copper layer or the lower copper layer of the flexible planar transmission line forms an inductor, and the two ends of the inductor are connected in parallel with the flexible planar capacitor to form a parallel resonant circuit, and the parallel resonant circuit forms the common-mode suppression balun; The flexible insulating dielectric layer of the flexible printed circuit board is polyimide, and a polyimide protective layer is provided on the topmost upper copper layer and on the bottommost lower copper layer respectively; The flexible printed circuit board adopts a multi-layer flexible printed circuit board.

2. A flexible planar transmission line and common mode suppression balun structure according to claim 1, characterized in that: The flexible printed circuit board is a double-layer flexible printed circuit board, the upper copper-clad layer and the lower copper-clad layer are respectively single copper-clad layers; the upper copper-clad layer constitutes a top conductor, and the lower copper-clad layer constitutes a bottom conductor; The flexible planar transmission line comprises two conductors, one conductor is a top conductor of a double-layer flexible printed circuit board, and the other conductor is a bottom conductor of the double-layer flexible printed circuit board, wherein the bottom conductor has a width greater than that of the top conductor.

3. The flexible planar transmission line and common mode suppression balun structure according to claim 1, characterized in that: The flexible printed circuit board is a double-layer flexible printed circuit board, the upper surface of the flexible insulating dielectric layer is provided with a plurality of upper copper layers in parallel with each other, each upper copper layer forms a top conductor; the lower surface of the flexible insulating dielectric layer is a lower copper layer, and the lower copper layer constitutes a bottom conductor; The flexible planar transmission line includes a plurality of top conductors and a bottom conductor, and the bottom conductor has a width greater than a width of each of the top conductors.

4. A flexible planar transmission line and common mode suppression balun structure according to claim 2 or 3, characterized in that: The top conductor and the bottom conductor of the flexible planar transmission line are both straight lines, and two ends of a section of the bottom conductor of the flexible planar transmission line are connected in parallel with a flexible planar capacitor; the section of the bottom conductor connected in parallel with the flexible planar capacitor forms an inductor; The top conductor of the flexible planar capacitor is connected to one end of the inductor through a via-hole connecting wire conductor, and the bottom conductor of the flexible planar capacitor is connected to the other end of the inductor through a connecting wire conductor; The inductor and the flexible planar capacitor form a parallel resonant circuit, and the parallel resonant circuit forms the common-mode suppression balun.

5. The flexible planar transmission line and common mode suppression balun structure according to claim 2, characterized in that: The flexible planar transmission line is wound into an 8-shape, and a plurality of vias are provided at the overlapping portion of the 8-shape, and the top conductor of the flexible planar transmission line extends to the lower surface of the flexible insulating dielectric layer through the vias, or the bottom conductor of the flexible planar transmission line extends to the upper surface of the flexible insulating dielectric layer through the vias; After the top conductor of the flexible planar transmission line passes over the overlapping portion, it passes through the via hole to return to the upper surface of the flexible insulating dielectric layer, or after the bottom conductor of the flexible planar transmission line passes over the overlapping portion, it passes through the via hole to return to the lower surface of the flexible insulating dielectric layer.

6. The flexible planar transmission line and common mode suppression balun structure according to claim 5, characterized in that: Two ends of a bottom conductor of the 8-shaped flexible planar transmission line are connected in parallel with a flexible planar capacitor; the bottom conductor connected in parallel with the flexible planar capacitor forms an inductor; The top conductor of the flexible planar capacitor is connected to one end of the inductor through a via-hole connecting line conductor, and the bottom conductor of the flexible planar capacitor is connected to the other end of the inductor; The inductor and the flexible planar capacitor form a parallel resonant circuit, and the parallel resonant circuit forms the common-mode suppression balun.

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