JPS61126809A - Filter circuit - Google Patents

Abstract

PURPOSE:To obtain an excellent characteristic over high frequencies by providing a coil to form a low pass filter thereby cancelling a capacitive additional impedance at board mounting and connecting the coil in series with a switching diode connected to each filter. CONSTITUTION:In receiving the signal of the pass band of the band pass filter, e.g., 100-170MHz, a positive voltage is applied to a band switching electric power supply terminal 5 to make diodes 15, 16, 19, 20 conductive and other diodes all nonconductive, a signal from a signal input terminal 1 is outputted from an output terminal 2 via the band pass filter 9 to obtain a desired signal. A high resistor (nearly 100kOMEGA) is connected normally in parallel with each diode so that a uniform reverse bias is applied to each diode when diodes are nonconductive. The operation is similar for the reception in selecting a signal via filters 8 and 10.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a filter circuit such as a tuner that divides a signal band such as a television signal into a plurality of signal bands and receives the signal band.

[Background of the invention]

For example, as an input filter for a high frequency circuit that divides a reception band signal into a plurality of bands and receives the signal, Japanese Patent Laid-Open No. 57-724
As shown in Japanese Patent No. 10, a circuit is known in which switching diodes are provided at the input and output of a plurality of filters, these are connected in parallel, and the diodes are switched to selectively pass a desired reception band.

This circuit can achieve high high frequency separation between filters,
It has the advantage of being able to easily obtain the desired filter characteristics, but when a single filter whose characteristics have been obtained is mounted on an input circuit board such as a tuner, impedance from other filters including switching diodes is added. There was a problem that the synchronization would shift and the loss would increase.

[Purpose of the invention]

An object of the present invention is to provide a filter circuit in which good characteristics can be obtained even when a single adjusted filter is mounted as is.

[Summary of the invention]

In the present invention, the capacitive additional impedance when mounted on the board is canceled by providing a coil to form a low-pass filter, and the tuning shift when the filter is mounted is canceled by connecting the coil in series with the switching diode connected to each filter. The structure is such that the influence on other bands of the coil is reduced, and good characteristics can be obtained over a high frequency range.

[Embodiments of the invention]

Hereinafter, one embodiment of the present invention will be described with reference to FIG. 1. FIG. 1 shows a filter circuit comprising a plurality of filters that switch reception bands using diodes according to the present invention. 1 is a signal input terminal.

2 is a signal output terminal; 3, 4.5 and 6 are respectively band switching power supply terminals; 7 is a bypass filter; 8, 9.
10 are band pass filters, 11 to 20 are switching diodes, 21 to 28 are resistors, and 29 to 55 are high frequency grounding capacitors.

36 to 43 are coils not adopted in the present invention.

This filter circuit is a filter circuit installed at the input section of the tuner for dividing and receiving television signals in the band of 50 to 900 MHz.The no-pass filter 7 has nine different circuit configurations as shown in FIG. 44 is a signal input terminal, 45 is a signal output terminal, 46 to 50 are capacitors, and 51.52 is a coil. Bandpass filters 8, 9, and 10 are the fifth
With the circuit configuration as shown in the figure, 55 is a signal input terminal, 54
are signal output terminals, 55-58 are capacitors, 59-62
is a coil. The passband of the bandpass filter 10 is 50 to 100M2. The pass band of the band pass filter 9 is 100 to 170 MH2, the pass band of the band pass filter 80 is 170 to 270 M1lz, and the pass band of the bypass filter is 270 to 900 M2.

For example, when trying to receive a signal in the pass band of a band pass filter of 100 to 170 MHz, a positive voltage is applied to the band switching power supply terminal 5 and the diode 15.
With 6.19.20 in a conductive state and all other diodes in a non-conductive state, the signal from the signal input terminal 1 is outputted from the output terminal 2 via the bandpass filter 9 to obtain a desired signal. Usually, a high resistance (about 100 KΩ) is connected in parallel to each diode so that a uniform reverse bias is applied to each diode when the diode becomes non-conductive, but this is omitted here. When signals are selected through filters 8 and 10, they are received in a similar manner. 27
When receiving a signal band of 0 to 9 ooMHz, apply a positive voltage to the power supply terminal 5, make the diode 11 conductive through the resistors 21 and 22, and make the diode 12 conductive through the resistors 24 and 25. , all other diodes are non-conducting.

Filters 7 to 10 each have a hybrid configuration of capacitors and coils for the purpose of downsizing the tuner or minimizing the number of adjustment points in the tuner, and are connected to diodes, resistors, capacitors, etc. shown in Fig. 1. Install it on the tuner board.

In FIG. 4, the loss characteristic of the filter circuit shown in FIG. 1 is shown by a solid line, and the loss characteristic when the coils 36 to 45 are not used is shown by a dotted line. When a filter whose characteristics have been obtained by itself is mounted on a tuner board, without the coils 36 to 43, capacitive impedance will be added due to stray capacitance of the board pattern, reverse capacitance of diodes in other receiving bands, etc. Therefore, the synchronization will shift and the loss will increase.

The impedance added is 25 to 3 between the signal line and ground on both the input and output sides during each filter operation.
It is about pF. In order to cancel this stray capacitance, for example, when trying to receive a signal in the passband of a bandpass filter of 100 to 170MnZ, the coil 38.
39 to configure a low-pass filter that passes the reception band. In the case of other reception bands, coils are similarly provided to configure low-pass filters. By adding a coil, it is no longer affected by shadow bullets due to mounting, so there is no deviation in tuning, and as shown by the solid line, the loss is improved by 0.3 to 0.5 dB compared to the case where no coil is added. Figure 5 shows the attenuation characteristics when receiving a signal band from 270 to 900 MIIZ, and the capacitive impedance is canceled out more in the solid line with the coil added than in the dotted line without the coil 42.45. As a result, the cutoff frequency becomes high, and the M loss becomes small over high frequencies. The trap near 1400MH2 is the reverse capacitance (approximately 1 p
F) and coils 56 to 41, etc., but since the coils are connected through a series connection of two diodes, the trap frequency is very high, and the high frequency Good characteristics have been obtained up to this point.

In addition, the inductance value of the coils is 56 to 41.
is about 20 nH; A#42.43 is about 5-10 nH, which is a size that allows pattern formation on the substrate, so the number of parts does not increase in this case. Furthermore, since the value of the coil can be selected according to each reception band, optimal design is possible.

Although the case where the filter is configured with four filters has been described here, it can also be applied when the filters are mounted and the impedance added is capacitive. Also, a coil was installed between the diode and the filter, but
It is clear that a similar effect can be obtained by replacing the diode and coil.

Furthermore, although we have described the case of mounting a hybrid filter, even when filters 7 to 10 are formed using coils and capacitors on a tuner board, the constants can be set as designed according to the present invention. There are also advantages.

〔Effect of the invention〕

According to the present invention, i! l! It becomes possible to attach a well-equipped filter to the tuner board, and good filter characteristics can be obtained over high frequencies. This makes it possible to configure the coil capacitor as a hybrid and attach the nine filters as components to the tuner, making it possible to make the tuner unnecessary and compact.

[Brief explanation of the drawing]

Claims (1)

[Claims] 1. It is equipped with at least one switching diode on the input side and the output side, and consists of N filters that divide one continuous frequency band into N frequency bands, and each filter is connected in parallel. 1. A filter circuit characterized in that at least one of each switching diode is constituted by a series connection circuit of a switching diode and an inductance element.