Filtering device and differential signal transmission circuit capable of suppressing common-mode noises upon transmission of a differential signal

A filtering device is capable of suppressing common mode noises upon transmission of a differential signal, and includes a differential transmission line, a grounding layer, a dielectric unit and a conductive structure. The differential transmission line has a pair of conductive traces spaced apart from each other. The grounding layer is spaced apart from the differential transmission line. The dielectric unit is disposed between the differential transmission line and the grounding layer. The conductive structure is embedded in the dielectric unit, is coupled electrically to the conductive traces and the grounding layer, and cooperates with the differential transmission line, the grounding layer and the dielectric unit to form a stacked structure that has an effective negative permittivity, thereby suppressing the common mode noises coupled to the conductive traces. A differential signal transmission circuit is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of Taiwanese Application No. 098126758, filed on Aug. 10, 2009.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a filtering device and a differential signal transmission circuit, more particularly to a filtering device and a differential signal transmission circuit capable of suppressing common-mode noises upon transmission of a differential signal.

2. Description of the Related Art

Differential signal transmission has been widely used in high-speed digital systems. However, a differential signal may accompany unwanted common-mode noises. For a high-speed data link, a cable is necessary to transmit the differential signal between two different electronic apparatuses. When the common-mode noises are coupled to the cable, the cable is excited to behave as an electromagnetic interference (EMI) antenna. Therefore, suppressing the common-mode noises upon transmission of the differential signal is necessary to solve the EMI problem associated with the cable.

Some conventional filtering devices capable of suppressing common-mode noises upon transmission of a differential signal employ patterned grounding structures, such as those disclosed in “An Embedded Common Mode Suppression Filter for GHz Differential Signals Using Periodic Defected Ground Plane,” IEEE Microwave and Wireless Components Letters, vol. 18, no. 4, pp. 248-250, April 2008 and “A Novel Wideband Common-Mode Suppression Filter for GHz Differential Signals Using Coupled Patterned Ground Structure,” IEEE Transactions on Microwave Theory and Technology, vol. 57, no. 4, pp. 848-855, April 2009. Although each of the aforesaid filtering devices has a relatively low cost, and is advantageous in terms of common-mode noises suppression over a wideband frequency range, it is disadvantageous in the following ways: a) it can not be miniaturized because one of the length and the width of the patterned grounding structure must be one half or one quarter of the wavelength of the differential signal, and b) its performance will be degraded with the inclusion of a shielding structure beneath the patterned ground structure.

SUMMARY OF THE INVENTION

Therefore, the object of the present invention is to provide a filtering device and a differential signal transmission circuit that can overcome the aforesaid drawbacks associated with the prior art.

According to one aspect of this invention, there is provided a filtering device capable of suppressing common-mode noises upon transmission of a differential signal. The filtering device comprises a differential transmission line, a grounding layer, a dielectric unit and a conductive structure. The differential transmission line has a pair of conductive traces spaced apart from each other. The grounding layer is spaced apart from the differential transmission line. The dielectric unit is disposed between the differential transmission line and the grounding layer. The conductive structure is embedded in the dielectric unit, is coupled electrically to the conductive traces and the grounding layer, and cooperates with the differential transmission line, the grounding layer and the dielectric unit to form a stacked structure that has an effective negative permittivity, thereby suppressing the common mode noises coupled to the conductive traces.

According to another aspect of this invention, there is provided a differential signal transmission circuit capable of suppressing common-mode noises upon transmission of a differential signal. The differential signal transmission circuit comprises:

an input terminal;

an output terminal;

a pair of mutually coupled first inductors, each of which has opposite first and second ends, and a node disposed between the first and second ends, the first ends of the first inductors serving as the input terminal, the second ends of the first inductors serving as the output terminal;

a mutual capacitor coupled between the nodes of the first inductors;

a series connection of two first capacitors coupled between the nodes of the first inductors; and

a parallel connection of a second capacitor and a second inductor coupled between a common node between the second capacitors, and a reference node.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before the present invention is described in greater detail with reference to the accompanying preferred embodiments, it should be noted herein that like elements are denoted by the same reference numerals throughout the disclosure.

Referring toFIGS. 1 and 2, the first preferred embodiment of a filtering device capable of suppressing common-mode noises upon transmission of a differential signal according to this invention is shown to include a differential transmission line1, a grounding layer2, a dielectric unit3and a conductive structure4. In this embodiment, the filtering device can be implemented in a three-layer printed circuit board (PCB).

The differential transmission line1has a pair of conductive traces11spaced apart from each other and symmetrical with respect to a centerline10defined therebetween and extending in a first direction (X). In this embodiment, the conductive traces11extend in the first direction (X), and are opposite to each other in a second direction (Y) traverse to the first direction (X).

The grounding layer2is spaced apart from the differential transmission line1in a third direction (Z) that is transverse to the first and second directions (X, Y).

The dielectric unit3is disposed between the differential transmission line1and the grounding layer2. In this embodiment, the dielectric unit3includes first and second substrates31,32stacked in the third direction (Z). The first substrate31is disposed above the second substrate32.

The conductive structure4is embedded in the dielectric unit3, is coupled electrically to the conductive traces11and the grounding layer2, and cooperates with the differential transmission line1, the grounding layer2and the dielectric unit3to form a stacked structure. The conductive structure4includes a conductive layer41and a plurality of via units. The conductive layer41is sandwiched between the first and second substrates31,32, and is formed with a plurality of rectangular patterns411spaced apart from each other. The patterns411are coplanar and are periodically arranged along the first direction (X). Each pattern411extends in the second direction (Y), crosses the conductive traces11along the second direction (Y), and has two halves that are symmetrical with respect to the centerline10. Each pattern411is coupled electrically to the conductive traces11through two coupling capacitances each formed between a corresponding one of the conductive traces11and a respective pattern411. Preferably, the via units are aligned with the centerline10. Each via unit interconnects electrically a corresponding one of the patterns411and the grounding layer2. In this embodiment, each via unit includes a via42formed in the second substrate32such that opposite ends of the via42contact electrically and respectively the corresponding one of the patterns411and the grounding layer2.

Each pattern411and the corresponding via unit (the via42) cooperate with the differential transmission line1, the grounding layer2and the dielectric unit3to constitute a unit cell5. Thus, the filtering device shown inFIG. 1has four unit cells5.

FIG. 3illustrates an equivalent lumped circuit of the unit cell5that serves as a differential signal transmission circuit. The differential signal transmission circuit includes an input terminal, an output terminal, a pair of mutually coupled first inductors61, a mutual capacitor62, a series connection of two first capacitors63, and a parallel connection of a second capacitor64and a second inductor65. Each first inductor61has opposite first and second ends611,612, and a node (n) disposed between the first and second ends611,612such that a corresponding first inductor61is divided into two halves. The first ends611of the first inductors61serve as the input terminal, and the second ends612of the first inductors61serving as the output terminal. The mutual capacitor62is coupled between the nodes (n) of the first inductors61. The series connection of the first capacitors63is coupled between the nodes (n) of the first inductors61. The parallel connection of the second capacitor64and the second inductor65is coupled between a common node (p) between the second capacitors63, and a reference node, such as ground.

For each unit cell5, the conductive trances11correspond respectively to the first inductors61each having an inductance (L1) in this embodiment. A mutual inductance (Lm) is formed between the mutually coupled conductive traces11. The mutual capacitor62is formed between the conductive trances11, and has a capacitance (Cm). The first substrate31of the dielectric unit3corresponds to the first capacitors63each of which has a capacitance (C1) formed between the pattern411and a corresponding conductive trace11. The second substrate32of the dielectric unit3corresponds to the second capacitor64that has a capacitance (C2) formed between the pattern411and the grounding layer2. The via unit, i.e., the via42, corresponds to the second inductor65that has an inductance (L2).

Due to odd and even symmetries, the differential signal transmission circuit ofFIG. 3can further be represented as two equivalent circuits shown inFIGS. 4 and 5. By odd-mode analyzing the equivalent circuit ofFIG. 4, a cutoff frequency (fc) of the differential signal transmitted by the filtering device is represented as follows:

By even-mode analyzing the equivalent circuit ofFIG. 5, a lower-side cutoff frequency (fL) and an upper-side cutoff frequency (fH) having a bandgap therebetween are represented as follows:

As discussed above, each unit cell5thus configured exhibits an effective negative permittivity (i.e., the unit cell5is a metamaterial) and a positive permeability in the bandgap, which indicates an evanescent mode that exists in the transmission line1when the unit cell5is operated at a frequency ranging from the lower-side cutoff frequency (fL) to the upper-side cutoff frequency (fH), thereby suppressing the common-mode noises coupled to the conductive traces11in the bandgap.

When the size of the filtering device is reduced for miniaturization purposes by reduction of the period (p) of the patterns411(seeFIG. 1), the capacitance (C1) formed between each pattern411and anyone of the conductive traces11, and the capacitance (C2) formed between each pattern411and the grounding layer2are decreased correspondingly, thereby resulting in an increase in the lower-side and upper-side cutoff frequencies (fL, fH). Hence, when the size of the filtering device is to be reduced while maintaining the lower-side and upper-side cutoff frequencies (fL, fH) at desired operating levels, a meandering structure for the conductive traces11, and a meandering structure for the via unit, as shown inFIGS. 6 and 7, can be used to increase the capacitance (C1) formed between each pattern411and any one of the conductive traces11, and the inductance (L2) of each via unit, respectively.

FIGS. 6 and 7illustrate the second preferred embodiment of a filtering device capable of suppressing common-mode noises upon transmission of a differential signal according to this invention, which is a modification of the first preferred embodiment. In this embodiment, the filtering device can be implemented in a four-layer PCB.

In this embodiment, the conductive traces11′ are meandering so as to increase the capacitance (C1) formed between each pattern411and any one of the conductive traces11′ and to decrease the lower-side cutoff frequency (fL).

In this embodiment, the dielectric unit3′ further includes a third substrate33stacked with the first and second substrates31,32in the third direction (Z) such that the second substrate32is disposed between the first and third substrates31,33.

In this embodiment, each via unit42′ includes a first via421formed in the second substrate32, a second via423formed in the third substrate33, and a conductive line422sandwiched between the second and third substrates32,33. For each via unit42′, the first via422extends in the third direction (Z), and contacts electrically the corresponding pattern411. The second via423extends in the third direction (Z), is misaligned and spaced apart from the first via422, and contacts electrically the grounding layer2. The conductive line422is straight and interconnects electrically the first and second vias421,423. As a result, the inductance (L2) of each via unit42′ is increased, and the lower-side and upper-side cutoff frequencies (fL, fH) are reduced.

FIG. 8illustrates the measurement results S-parameter and frequency for the filtering device ofFIG. 6that has four unit cells5′.FIG. 9illustrates the S-parameter and frequency for the filtering device that has eight unit cells5′. For example, the configuration of the filtering device is as follows. The width of each of the conductive traces11′ is 0.1 mm. Three distances (s1, s2, s3) between the conductive traces11′ are 1.38 mm, 2.18 mm, 0.58 mm, respectively. The dielectric constant of the dielectric unit3′ is 7.8. The length (d) of each pattern411is 3.2 mm. The period (p) of the patterns411is 1.28 mm. The gap (g) between two adjacent ones of the patterns411is 0.18 mm. The diameter and length (L1) of each first via421are 75 μm and 0.468 mm, respectively. The diameter and length (L2) of each second via423are 75 μm and 0.312 mm, respectively. The width and length (L3) of each conductive line422are 0.1 mm and 1 mm, respectively. The filtering device ofFIG. 6has a bandgap ranging from 3.8 GHz to 7.1 GHz, whereas the filtering device with eight unit cells has a bandgap ranging from 3.8 GHz to 7.4 GHz, which is wider than that of the filtering device ofFIG. 6. The filtering device ofFIG. 6has a common-mode insertion loss, i.e., S-parameter, of about −10 dB on average, whereas the filtering device with eight unit cells has a common-mode insertion loss of about −20 dB on average. Hence, the greater the number of the unit cells5′ of the filtering device, the better will be common-mode noise suppression capability of the same.

FIG. 10illustrates the third preferred embodiment of a filtering device capable of suppressing common-mode noises upon transmission of a differential signal according to this invention, which is a modification of the second preferred embodiment. In this embodiment, the conductive line422′ of each via unit42″ is generally spiral in shape such that the inductance (L2) of each via unit42″ is further increased.

FIGS. 11 and 12illustrate the fourth preferred embodiment of a filtering device capable of suppressing common-mode noises upon transmission of a differential signal according to this invention. The fourth preferred embodiment is a modification of the second preferred embodiment. In this embodiment, the filtering device can be implemented in a five-layer PCB, and has only one unit cell5″.

In this embodiment, the dielectric unit3″ further includes a fourth substrate34stacked on the first substrate31.

In this embodiment, each of the conductive traces11″ has first and second segments111,112overlaid on the dielectric unit3″, and a third segment113and first and second vias114,115embedded in the dielectric unit3″. For each conductive trace11″, the first and second segments111,112are coplanar and are overlaid on the fourth substrate34. The third segment113is spaced apart from the first and second segments111,112in the third direction (Z), is sandwiched between the first and fourth substrates31,34, and is generally spiral in shape. The first via114is formed in the fourth substrate34, extends in the third direction (Z), and interconnects electrically the first and third segments111,113. The second via115is formed in the fourth substrate34, extends in the third direction (Z), and interconnects electrically the second and third segments112,113. As a result, the capacitance (C1) formed between the pattern411and any one of the conductive traces11″ is increased, which results in a decrease in the lower-side cutoff frequency (fL) correspondingly, and the inductance (L1) of each conductive trace11″ is increased so that the differential signal transmitted by the filtering device can substantially remain intact.

In sum, due to the presence of the conductive structure4, the filtering device of the present invention can eliminate the aforesaid drawbacks associated with the prior art. In addition, due to the presence of the conductive traces11′,11″ having meandering and spiral structures, and the via units42,42′, the filtering device of this invention can be miniaturized while maintaining the desired bandgap.