1. Technical Field
The present invention concerns a method of manufacturing bandpass filters to be used in GHz bands, particularly, some hundreds of MHz to ten and some GHz bands. (Hereinafter “bandpass filter” is abbreviated to “BPF”.)
2. Prior Art
In these days radio wave in the frequency regions of some hundreds MHz to ten and some GHz has been preferred. For example, 800 MHz (0.8 GHz) band or 1.5 GHz band for mobile telephone, 1.9 GHz band for PHS (Personal Handyphone System), 5.8 GHz band for ETC (Electronic Toll Collection System), 2.4 GHz band or 5.2 GHz band for wireless PAN (Personal Area Network) and 5.8 GHz band for DSRC (Dedicated Short Range Communication).
Because the radio waves in these frequency regions are all used or may be possibly used for operation of automobiles, it has been intended to receive the radio waves with one antenna and treat by digital processing. For such occasions and for the cases where the waves of the respective frequency regions are solely used, for the purpose of cutting noise caused by harmonics or reflected waves before treating the date, it is necessary to use a bandpass filter which passes only the signal of a determined band width in respective bands and cuts the other signal.
On the other hand, trial has been made to carry out ultra wideband transmission by using short (nanometers or less) pulse without using carrier waves. As the frequency region for this communication FCC (Federal Communication Committee) of the United States decided to assign 3.1–10.6 GHz. If a BPF which enables bandpassing of this ultra wideband region, particularly, that of small and low loss is provided, it will be useful for the devices realizing the above mentioned ultra wideband communication.
One of the inventors invented various types of electromagnetic wave-shielding materials prepared by dispersing powder of soft magnetic substances in a matrix of elastomeric or plastic substances, which are already in practical use. He also invented a low-pass (high-cut) filter using this electromagnetic wave-shielding material and disclosed (Japanese Patent Disclosure 2002-171104). The filter is of chip-type and characterized in that it has the structure of one signal line of a conductive material and at least one GND line are disposed in parallel direction on one surface or both the surfaces of a square plate of dielectric substance and that, as the dielectric substance, an electromagnetic wave-absorbing material prepared by dispersing a soft magnetic powder in a synthetic resin matrix is used. The product of this invention exhibits insertion loss of −5 dB for high frequency wave of 1 GHz or higher.
Also, the other of the inventors utilized the knowledge mentioned above to invent a BPF for GHz bands, which is used in the frequency region of some hundreds of MHz to ten and some GHz, and already proposed (Japanese Patent Disclosure 2004-222086). The BPF for GHz bands has a structure that, in principal, an input signal line and an output signal line made of conductive strips are disposed to run in series direction with a small gap between them on one side of a sheet, which is prepared by dispersing soft magnetic metal powder in a polymer matrix, that the opposite ends of the above signal lines are connected with a capacitance means, and that a GND line is disposed on the reverse side of the sheet.
A concrete example of the above-mentioned structure is shown in FIG. 1. The BPF for GHz bands is made by disposing an input signal line (2) and an output signal line (3) made of conductive strips so that both the signal lines may run in the series direction with a small gap between them on the sheet (1), which is prepared by dispersing soft magnetic metal powder in a sheet-formed polymer matrix, connecting the opposite ends of both the signal lines with a capacitance means, and disposing a GND line (4) on the reverse side of the sheet (1). The BPF is characterized in that the capacitance means is formed by laminating an internal line (6), which is also made of a conductive strip, with intermediation of an insulating film (5) in such a manner that the internal line bridges the input signal line and the output signal line so that electrostatic capacitance may be formed both between the input signal line and the internal line and between the internal line and the output signal line. The passing-band of this BPF can be varied by choosing the respective electrostatic capacitance, the impedance determined by the lengths, widths, thicknesses and forms of the input signal line (2) and the output signal line (3), as well as the conditions of particle form of the soft magnetic metal powder, filling rate in the matrix of the powder, and the form and thickness of the sheet.
The BPF for GHz bands mentioned above has frequency characteristics of permeability as shown in FIG. 2. This type of BPF is advantageous because of the simple structure, and hence, it can be manufactured in small sizes. However, automation of manufacturing is not easy and it is difficult to meet the demand for cost saving. Further, the BPF has the problem of low percentage of passing the standard due to the fact that, if the values of the electrostatic capacity and the impedance of the BPF are not accurate, desired frequency characteristics cannot be obtained.