Filter Apparatus

1. Field of the Invention

The present invention relates generally to a filter apparatus, and in particular to a filter apparatus suitable for coupling an up-converter and a down-converter in a tuner or a CATV (cable television) converter of double super heterodyne type.

2. Description of the Related Art

In recent years, CATV converters of the up-down type have been sold. In such CATV converters, a low-pass filter and a band-pass filter are connected between an up-converter and a down-converter.

FIG. 1 is a block diagram of a prior art CATV converter, for example, published unexamined patent applications No. Sho 58-31622, No. Sho 59-81925). A signal applied to an input terminal A is led through an input filter 1. Output from the input filter 1 is converted into a first intermediate frequency by a mixing circuit 2. The converted signal is led through a low-pass filter 4 and band-pass filter 5, and is amplified by a first intermediate frequency amplifier 6. Output from the first intermediate frequency amplifier 6 is converted into a second intermediate frequency, which is lower than the first intermediate frequency, by a mixing circuit 7. The converted signal is led through a second intermediate frequency amplifier 9 and is applied to an output terminal B.

The input filter 1, the mixing circuit 2 and the first local oscillator 3 form an up-convert C. The first intermediate frequency amplifier 6, the mixing circuit 7, the second local oscillator 8 and the second intermediate frequency amplifier 9 form a down-convert D. The low-pass filter 4 and the band-pass filter 5 are provided between the up-convert C and the down-convert D.

FIG. 2A is a plan view of the low-pass filter 4 and the band-pass filter 5, FIG. 2B is a circuit diagram of the low-pass and band-pass filters 4, 5 and FIG. 2C is a sectional view of the low-pass filter 4 and the band-pass filter 5. The band-pass filter 5 is provided on the substrate 10. The low-pass filter 4 consists of capacitors 12, 14 and a coreless-coil 13. The coreless-coil 13 is enclosed in a shield plate 15. Output of the low-pass filter 4 is connected to a connecting lead 16, which is further connected to an input terminal of the band-pass filter 5 through a conductive layer 17. The low-pass filter 4 has a characteristic as shown in FIG.3A and the band-pass filter 5 has a characteristic as shown in FIG. 3B. A combination characteristic of the low-pass filter 4 and the band-pass filter 5 is shown in FIG. 3C.

In a conventional filter apparatus, there is a problem that spurious interference can take place through interference between a differential component of local oscilation frequencies of the first and second local oscilators 3, 8 and higher harmonic frequencies. The low-pass filter 4 is provided for removing the local oscilation frequency. The band-pass filter 5 is provided for preventing cross modulation etc. by removing signals of unnecessary channels.

In such a conventional filter apparatus, the low-pass filter 4 must be enclosed in a shield plate 15, and winding and soldering operations of the coreless-coil 13 and the capacitors 12, 14 must be carried out in a manner so as to be isolated above the substrate 20. Therefore, complex steps are necessary for production. Further, greater area is necessary, because the low-pass filter 4 and the band-pass filter 5 are separately provided.

If both filters 4, 5 are provided on one substrate or no shield plate 15 is provided, a higher harmonic frequency signal to be inputted to the low-pass filter 4 is led directly to the output terminal of the band-pass filter 5. Therefore, the performance of the filter apparatus is degraded.

It is a purpose of the present invention to provide a filter apparatus which can save space and which can be produced is a few simple steps.

A filter apparatus in accordance with the present invention comprises;

a substrate of insulating material,

a low-pass filter having at least one capacitor provided on a first surface of the substrate and at least one coil formed by a strip of a specified pattern of conductive layer provided on the first surface of the substrate,

a band-pass filter is provided on a second surface and having an input terminal and an output terminal provided on the first surface of the substrate,

a first grounding conductive layer provided on substantially the whole part of the first surface of the substrate excluding small areas for passing an input terminal and an output terminal for the band-pass filter,

a second grounding conductive layer provided on the second surface of the substrate excluding small areas for passing an input and output terminal for the band-pass filter,

through-holes with a connection conductor therein provided between the low-pass filter and the terminals of the band-pass filter thereby connecting the first grounding conductive layer and the second grounding conductive layer.

While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.

FIG. 1 is the system block diagram of a prior art converter.

FIG. 2A is a plan view of a low-pass filter of the prior art.

FIG. 2B is the circuit diagram of a low-pass filter and a band-pass filter of the prior art.

FIG. 2C is a sectional view of the low-pass and band-pass filters of the prior art.

FIG. 3A is a graph showing the characteristic of the low-pass filter of the prior art.

FIG. 3B is a graph showing the characteristic of the band-pass filter of the prior art.

FIG. 3C is a graph showing a combination characteristic of a low-pass filter and a band-pass filter.

FIG. 4A is a conductive layer pattern on a front surface of a substrate, according to an embodiment of the present invention.

FIG. 4B is a conductive layer pattern on a rear surface of the substrate of FIG. 4A.

FIG. 4C is a exploded view of a filter apparatus of the embodiment of an present invention.

FIG. 5 is a circuit diagram of the filter apparatus of the embodiment of an present invention.

FIG. 6A is the graph showing the characteristic of the low-pass filter of the embodiment of the present invention.

FIG. 6B is the graph showing the characteristic of the band-pass filter of the embodiment of an present invention.

FIG. 6C is a graph showing a combination characteristic of the low-pass filter and the band-pass filter.

The structure of a preferred embodiment of a filter apparatus in accordance with the present invention is described in detail with reference to FIGS. 4 and 5.

FIG. 4A shows a pattern of conductive layer on a front surface of a substrate. FIG. 4B shows a pattern of conductive layer on a reverse surface of the substrate. FIG. 4C shows a exploded view of the filter apparatus. FIG. 5 shows a circuit diagram of a low-pass filter and a band-pass filter. In this embodiment, a dielectric filter of coaxial resonance type is used as band-pass filter.

As shown in FIG. 4A and in FIG. 4C an input terminal 24 and an output terminal 25 are provided on a first surface of the substrate 20. A conductive layer 27 for grounding is formed on substantially the entire first surface of the substrate 20 exclusing areas for the input terminal 24 and output terminal 25 of the band-pass filter 29. This conductive layer 27 operates as shield plate. Connecting a ground terminal 29c of the band-pass filter 29 to this conductive layer 27 improves the passband response of the band-pass filter 29.

FIG. 4B shows a layer pattern of the back side of the substrate 20. The input terminal 24 and output terminal 25 appear on one side of the back side of the substrate 20. From the input terminal 24 and output terminal 25, strips of conductive layers 30, 32 for input and output are extended. The conductive layers 30, 32 for input and output, respectively, are formed as strip-type a transmission line because the shielding conductive layer 27 is provided on the first surface of the substrate 20 as shown in FIG. 4A. In FIG. 4B, on the reverse surface of the substrate 20, the low-pass filter is provided. The low-pass filter consists of an input terminal 21, a coil 28 formed by printed layer and capacitors 22, 23. On one side of the reverse surface, of substrate 20 a conductive layer 27 for grounding is formed excluding areas for passing input terminal 24 and output terminal 25. A row of through-holes 26 with connecting conductors therein are provided to surround the low-pass filter and for connecting both the conductive layers 27 on the first surface and the reverse surface, and a further row of through-holes 26 are provided between the input and output terminals 24, 25 so as to shield them each other. The pitch of the through holes is preferably 2 mm in this embodiment where a thickness of the substrate is 0.8 mm. The through-holes shield the leakage of the signal between the low-pass filter and the band-pass filter.

In this embodiment, the low-pass filter 4 has a characteristic as shown in FIG. 6A and the band-pass filter 5 has a characteristic as shown in FIG. 6B. A combination characteristic of the low-pass filter and the band-pass filter is as shown in FIG. 6C.

According to the present invention, the filter apparatus in accordance with the present invention, attenuation outside the bandwidth becomes 40 dB or over, as a result of introducing the grounding conductive layer 27 and the through holes 26. Therefore, interference by the local oscilation frequency can be substantially prevented. Further, the production steps become simplified and the filter apparatus becomes compact, as a result of forming the printed conductive layer on one surface of the substrate and forming the band-pass filter on the other surface.

Although the invention has been described in its preferred embodiment with a certain degree of particularity, it is understood that the present disclosure of the preferred embodiment can be changed in the details of construction and other combinations and arrangements of parts may be resorted to without departing from the spirit and the scope of the invention as hereinafter claimed.