1. Field of the Invention
This invention relates to an inductor circuit board, a method of forming an inductor, and a bias-T circuit, and more particularly to an inductor circuit board having an inductor wired on a circuit board for a high-frequency transmission application, such as 40 Gb/s, a method of forming an inductor on a circuit board for a high-frequency transmission application, and a bias-T circuit that supplies a high-frequency signal by superposing a DC component thereon.
2. Description of the Related Art
Recently, with the development of multi-media technology, there is an increasing demand for constructing optical communication networks that transmit high-speed, large-volume information at low costs over long distances. To meet the demand, there have been developed optical communication systems whose transmission rate is in the order of 10 Gb/s, and further, for even higher-speed, larger-volume communications, optical communication systems whose transmission rate is in the order of 40 Gb/s are under development.
In the meanwhile, an electronic circuit called a bias-T is used in optical transmitter-receivers, measurement equipment, and so forth. The bias-T is comprised of an inductor (coil) and a capacitor, and supplies a high-frequency signal by superposing a DC component, e.g. a DC current or a DC voltage, on the high-frequency signal, without adversely affecting the high-frequency signal.
In the bias-T for use in optical communication at a transmission rate of 10 Gb/s or less (≦10 Gb/s), it is possible to use a small-sized surface-mount inductor (surface-mount type having a size of approximately 1.0 mm×0.5 mm) as a component of the bias-T. Insofar as the bias-T is for applications at a transmission rate of 10 Gb/s or less, there occurs no marked degradation in high-frequency characteristics even with the use of such an inductor.
However, in performing optical communication at a transmission rate in the order of 40 Gb/s, a broad band ranging from several hundreds of KHz to 40 GHz is used. This makes it impossible to directly use such a surface-mount inductor as described above in the transmission line, and it is necessary to make the inductor compatible with broadband.
“To make the inductor compatible with broadband” specifically means to expand a blocking band of the inductor to a high-frequency band so as to prevent a high-frequency signal from being spoiled in high-frequency characteristics due to flow of the high-frequency signal into the inductor when the signal is passed through the transmission line connected to the inductor.
In general, in making the inductor compatible with broadband, it is ideal to connect component inductors having different inductances in series. However, actually, it is impossible to use an inductor formed simply by connecting the component inductors in series (hereinafter referred to as “the series inductor”) since the characteristics thereof are degraded e.g. due to occurrence of a parasitic capacitance of the inductor. Hereinafter, the problems of the series inductor will be discussed.
FIG. 15 is a diagram showing the series inductor. The series inductor L10 is formed by connecting component inductors L11 to L13 having different inductances in series. Now, it is assumed that when the values of the inductances of the respective component inductors L11 to L13 are represented by L11a, L12a, and L13a, the values or magnitudes of the inductances satisfy the relationship of L11a<L12a<L13a. 
FIG. 16 is a diagram showing a circuit configuration in which the series inductor L10 is connected to a transmission line. A measurement circuit 50 is formed by connecting the series inductor L10 to the transmission line 5 having an impedance of 50Ω through which a high-frequency signal flows. Further, it is assumed that the high-frequency signal is passed from a port p1 toward a port p2.
FIG. 17 is a diagram showing the frequency characteristics of the component inductors. In this figure, the vertical axis represents dB, and the horizontal axis represents the frequency. The individual frequency characteristics of the respective component inductors L11 to L13 forming the series inductor L10 are collectively shown.
The self-resonance frequency of the component inductor L13 having the largest inductance is represented by fr3, the self-resonance frequency of the component inductor L12 having a medium inductance by fr2, and the self-resonance frequency of the component inductor L11 having the smallest inductance by fr1 (as shown in FIG. 17, as the inductance is smaller, the self-resonance frequency becomes larger).
Here, a conceptual description will be given of an ideal signal flow to be obtained when a high-frequency signal is passed from the port p1 to the port p2 of the measurement circuit 50. Since a frequency signal, which is included in a frequency range A between frequencies a1 and a2 with the self-resonance frequency fr1 in its center, is blocked by the component inductor L11 (signal within the frequency range A is inhibited from flowing through the component inductor L11), the frequency signal flows from a port p3 in the X direction without flowing in the direction of the series inductor L10 (Y direction).
Further, in this case, if the component inductor L11 alone is connected to the transmission line 5, a frequency signal having frequencies smaller than the frequency a1 flows toward the component inductor L11. However, since the component inductor L12 is provided at the next-stage, a frequency signal, which is included in a frequency range B between frequencies b1 and b2 with the self-resonance frequency fr2 in its center, is blocked by the component inductor L12 (signal within the frequency range B is inhibited from flowing through the component inductor L12). As a consequence, a frequency signal in a frequency range between the frequencies b1 to a1 also flows in the X direction without flowing in the Y direction.
Similarly, since the component inductor L13 is connected, a frequency signal, which is included in a frequency range C between frequencies c1 and c2 with the self-resonance frequency fr3 in its center, is blocked by the component inductor L13 (signal within the frequency range C is inhibited from flowing through the component inductor L13). As a consequence, a frequency signal in a frequency range between the frequencies c1 to a2 flows in the X direction without flowing in the Y direction.
As described above, by forming the series inductor by connecting component inductors having different inductances in series, blocking bands of the respective component inductors are arranged in an overlapping manner such that no passbands of the inductors appear at any intermediate portions of the entire frequency range. Therefore, ideally, it is possible to make the inductor compatible with broadband.
FIG. 18 is a diagram showing ideal frequency characteristics of the measurement circuit 50. The vertical axis represents dB, and the horizontal axis represents the frequency. In this figure, F1 indicates the ideal frequency characteristics (dotted line) of a signal flowing in the X direction from the port p1 to the port p2 of the transmission line 5, and F2 indicates the ideal frequency characteristics (solid line) of the series inductor L10.
In the ideal frequency characteristics shown in FIG. 18, since the characteristics of a broadband signal within the frequency range between the frequencies c1 and a2 are flat, it is understood that the signal flows through the transmission line 5 in the X direction without being degraded in its characteristics.
However, the above state is an ideal one, and in the actual high-frequency circuit, parasitic capacitances of the component inductors themselves and earth capacitances cannot be ignored. This makes it impossible for a mere series inductor to be compatible with broadband.
FIG. 19 is a diagram showing an equivalent circuit of an inductor. The inductor 100 (corresponding to one component inductor of the series inductor L10) not only has an inductance inherent thereto but also includes a capacitor (parasitic capacitance or line capacitance) formed by wound electric wires, a winding resistance, and so forth.
The equivalent circuit 100a of the inductor 100 can be defined as a circuit in which an inductor L0 and a resistance R0 are connected in series, and a part formed by series connection of the inductor L0 and the resistance R0 and a capacitor Cr are connected in parallel. Further, when lead wires of the inductor 100 are mounted on a printed circuit board, earth capacitances appear at respective locations of pads (copper foils for soldering, for use in mounting the component on the printed circuit board), and therefore the equivalent circuit 100a looks as if it has capacitors C1 and C2 connected between the lead wires and ground GND.
The parasitic capacitance of the capacitor Cr has a very small value, and hence it raises no problem when the inductor 100 is used with low frequencies. However, when the inductor 100 is used as a high-frequency circuit, the parasitic capacitance is not negligible, but causes variations in the impedance of the inductor and the self-resonance frequency.
FIG. 20 is a diagram showing actual frequency characteristics of the measurement circuit 50. The vertical axis represents dB, and the horizontal axis represents the frequency. In this figure, F1a indicates the actual frequency characteristics (dotted line) of a signal flowing in the X direction from the port p1 to the port p2 of the transmission line 5, and F2a indicates the actual frequency characteristics (solid line) of the series inductor L10.
In the actual frequency characteristics shown in FIG. 20, instantaneous passbands in the Y direction appear at the frequencies f1 and f2 within the frequency range between the frequencies c1 and a2, which generate two dips in the frequency characteristics F1a. More specifically, a signal flowing through the transmission line 5 in the X direction flows in the Y direction as well from the port p3 into the series inductor L10 at the frequencies f1 and f2, which degrades the frequency characteristics. As described above, the simple series inductor formed by connecting component inductors having different inductances in series has not been applicable to high-speed optical communications at transmission rates in the order of 40 Gb/s or more.
Conventionally, a technique for forming a bias-T by using a conical coil has been proposed as the prior art of high-frequency circuits, e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2004-193886 (Paragraph numbers [0014] to [0019], and FIG. 1).
As the prior art of making inductors compatible with broadband, the use of an inductor called a conical coil which has a high self-resonance frequency is becoming popular.
FIG. 21 is a diagram showing the outline of the conical coil. The conical coil 110 is a conductor having a conical shape, which is formed by winding a conductor wire 111 covered with an insulating film, around an outer peripheral surface of a frustoconical core 112 made of a magnetic material, such that the winding diameter of the conductor wire progressively decreases from one end to the other end of the coil (from the right end to the left end, as viewed in FIG. 21). Further, the opposite ends of the conductor wire 111 have the insulating film peeled off to expose copper wire 111a, for use as terminals.
FIG. 22 is a diagram showing an equivalent circuit of the conical coil 110. The equivalent circuit 110a of the conical coil 110 is comprised of component inductors L1 to Ln which have different inductances and are connected in series. In this case, the component inductors L1 to Ln of the conical coil 110 are sequentially arranged in series in the increasing order of inductance, as viewed from the tip side of the frustoconical shape.
Compared with the above-described series inductor, the conical coil 110 configured as above is characterized in that it can secure broadband characteristics of approximately several hundreds of KHz to several tens of GHz, and since the tip thereof has a small diameter, the value of inductance thereof is small and the parasitic capacitance thereof is held small, whereby it is possible to maintain its characteristics up to a high frequency of several tens of GHz.
It should be noted that the conical coil 110 has its highest frequency characteristics determined by the component inductor L1, and the frequency characteristics in a higher to a lower frequency ranges are sequentially determined by the component inductor L1 to the component inductor Ln, respectively.
More specifically, the conical coil 110 is configured such that the high frequency characteristics are determined by the value of inductance of the component inductor L1, which is the first and smallest-diameter coil on the tip side of the conical coil 110 (the high frequency characteristics can be secured by the component inductor L1 since the component inductor L1 has a small diameter and hence has a small inductance value), and the frequency characteristics of the conical coil 110 from a higher to a lower frequency ranges are sequentially determined by the inductance values of the component inductors the diameter of which increases from the component inductor L1 to the component inductor Ln.
However, the conical coil 110 configured as above is difficult to mount on a circuit board, and is not easy to handle, either. FIG. 23 is a diagram showing how the conical coil 110 is bonded. The tip of the conical coil 110 is bonded (press-fitted) on a circuit board by heat or ultrasonic waves.
In general, the conical coil 110 is compact in size, i.e. approximately several mm long in the longitudinal direction. Further, the winding of the conical coil 110 is as thin as a small diameter of approximately several tens of μm, and has an unstable shape. Therefore, the conical coil 110 is generally mounted within an IC package. Further, it is necessary to connect the conical coil 110 by accurate bonding manually performed by a skilled worker. Therefore, the conical coil 110 can be used in limited areas or locations of devices, and is very difficult to handle.
Further, a lead wire is allowed to be extended from the tip of the conical coil 110 only by several hundreds of μm, and if it is further extended, the high frequency characteristics are degraded. Moreover, the characteristics of the conical coil 110 depend on the mounting angle thereof, and hence there is a problem that a large variation in the characteristics is caused when the conical coil 110 is mounted.