JPS6239902A - Band stop type filter circuit - Google Patents

Abstract

PURPOSE:To increase the Q without increasing the shape of a filter circuit by providing an active element coupled in common to plural resonance circuits, operated when the resonance circuits are in the resonance state and causing a negative resistor. CONSTITUTION:A transmission filter 12 is formed by series connection between 3-stage of band-pass filters 16 and 4-stage of band-stop filters 18, the band-pass filters 16 are designed for comparatively wide band and its attenuation region is parted greatly from a desired frequency. Then an undesired frequency signal is stopped by the band-stop filters 18. A resonator of the band-stop filters 18 is combined with an active element T. The resonance circuit R is resonated with the attenuation region of the band-stop filters 18, the active element T is operated to show a negative resistance. Thus, energy is supplied from the active element T to the resonance circuit T. That is, in adjusting the Qex between the resonance circuit R and the active element T equal to the specific Qo to the resonance circuit R, the apparent Q of the resonance circuit R becomes a very large value.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a band-elimination filter circuit, and particularly to a band-elimination filter circuit used in a duplexer or antenna duplexer of a car phone or other wireless equipment. This invention relates to shaped filter circuits.

(Prior Art) FIG. 5 is a schematic diagram of an antenna duplexer for a car telephone, which forms the background of the present invention and to which the present invention can be applied.

The antenna duplexer includes a transmission filter 1 whose one end is connected to a transmitter, and a reception filter 2 whose one end is connected to a receiver, and the other ends of the transmission filter 1 and the reception filter 2 are A common connection is made and connected to the antenna. 1 required for the transmission filter 1 of such an antenna duplexer
One condition is that Q be large due to the need to minimize loss in the filter. Further, the receiving filter 2 is also required to have a large Q value and low loss, since it is necessary to pass the weak signal received by the antenna with a good S/N ratio.

(Problems to be Solved by the Invention) In order to increase the Q of a filter, both conventional coaxial type filters and filters using dielectric resonators only consider dispersing energy concentration. In this case, the shape becomes large, which conflicts with the demand for miniaturization.

Therefore, the main object of the present invention is to provide a small band-stop filter circuit with a large Q value.

(Means for Solving the Problems) Simply put, the present invention is a band rejection filter that includes a plurality of filters, each of which has a desired signal frequency in its pass band and an undesired frequency in its attenuation band. a plurality of resonant circuits included in each of the filters and whose resonant frequency is in the attenuation range, and a plurality of resonant circuits that are commonly coupled to the plurality of resonant circuits and operate when the resonant circuit is in a resonant state. This is a band-elimination filter circuit that includes an active element that becomes a negative resistance.

(Operation) In the passband of each filter, the resonant circuit included therein is in a non-resonant state, and the active element does not operate. In the attenuation range of that filter, the corresponding resonant circuit is in a resonant state, and therefore the active element commonly connected to the resonant circuits of other filters operates and acts as a negative resistance. Therefore, the isometric resistance value of the resonant circuit becomes small, and the Q of the resonant circuit appears to be a very large value. If the center frequencies of the attenuation ranges of the respective filters are set apart, mutual signal leakage between the filters will not occur.

(Effects of the Invention) According to the present invention, with a simple configuration in which an active element is connected to a resonant circuit, the Q of the filter circuit can be increased without increasing the size of the filter circuit. Furthermore, since the active element is commonly coupled to each resonant circuit included in the plurality of filters, a band rejection filter circuit with a large Q can be obtained in a smaller size and at a lower cost.

The above objects, other objects, features and advantages of the present invention will become more apparent from the following detailed description of embodiments with reference to the drawings.

(Example) Hereinafter, the present invention will be described in detail by taking a filter circuit used in an antenna duplexer as an example.

However, it should be pointed out in advance that the present invention can be applied to other than such antenna duplexers and duplexers.

FIG. 4 is a schematic block diagram of an antenna duplexer to which an embodiment of the present invention is applied. The antenna duplexer 10 includes a transmission filter 12 and a reception filter 14.

Both the transmission filter 12 and the reception filter 14 are band pass filters (BPF) 16 (16').
and band-elimination filter (BEF) 18 (1B'
) are connected in series.

Note that the connection order of the band pass filter 16 (16') and the band rejection filter 1B (18') may be reversed to that in FIG. 4.

Further, in this embodiment, both the transmitting filter 12 and the receiving filter 14 are configured by a combination of a band-pass filter and a band-elimination filter.
Any of these may be configured using only a band rejection filter.

FIG. 2 is a block diagram showing the idea behind this invention. Note that although the transmission filter 12 will be described as an example here, the reception filter 14 can be considered in the same way.

The transmission filter 12 includes three stages of bandpass filters 16 and 4.
The band-pass filter 16, which includes a series connection with the band-stop filter 16 of the stage, must be a filter with as little loss as possible in the transmission signal passband, and a resonator of a limited size must be used. Therefore, it is designed to have a relatively wide band, and its attenuation range is far away from the desired frequency.

Then, signals of undesired frequencies are removed by a band rejection filter 18.
is configured to prevent

The band rejection filter 18 has a transmission line of 3λ/4 and a transmission line of 3λ/4.
4 resonators R4, R5 coupled to this transmission line
, R6 and R7. An active element (for example, a high frequency transistor) T is combined with each stage of the resonator. The feature of this embodiment is that the active element T is combined with the resonator R in this way.

The active element T is the resonance region of the resonator of the band-stop filter 4,
That is, it operates as a negative resistance in the attenuation range of the band-elimination filter 4. Therefore, when the resonant circuit resonates, the active element T operates and energy is supplied from the active element T to the resonator, thereby canceling out the loss of the resonator.

FIG. 3 is an equivalent circuit diagram of the one-stage resonant circuit shown in FIG. 2 and its related parts. The resonant circuit R includes a resistor and an inductor connected in series, and an active element T is inductively coupled to the inductor. In the passband of the band-stop filter 18, the resonant circuit R is in a non-resonant state and therefore the active element T does not operate.

In the attenuation range of the bandpass element filter 18, the resonant circuit R resonates and the active element T operates. The active element T then exhibits negative resistance (-r) when it operates. Therefore, energy is supplied from this active element T to the resonant circuit R. That is, if the Qex between the resonant circuit R and the active element T is adjusted to be equal to the Qo specific to the resonant circuit R, the apparent Q of the two-resonant circuit R becomes a very large value.

Note that the transmission line of electrical length θ' connected in series to the active element T is for phase adjustment to bring the input and output into the same phase.

In addition, in the equivalent circuit of FIG. 3, the resonant circuit R and the active element T
is inductively coupled, but this may also be capacitively coupled.

In the example shown in FIG. 2, of the band-pass filter 16 and the band-elimination filter 18 that constitute the transmission filter 12, the active element T is combined only in the resonance circuit of the band-elimination filter 18. This is due to the following reasons. That is, in the case of a car phone, the power level of the transmitted signal is, for example, 40 dBm. On the other hand, the output power level of the active element T for normal small signals is 10 to 20 dBm. Therefore, even if the active element T is combined with the resonant circuit of the band pass filter 16, the active element T is saturated and does not operate in the operating frequency range of the band pass filter 16, and as a result, Q cannot be increased.

As described above, the reception filter 14 is also constituted by a series connection of a bandpass filter 16' and a band rejection filter 18'. The bandpass filter is designed to pass a frequency signal in the reception signal passband, and the bandstop filter is designed to block a frequency signal in the transmission signal passband.

The bandpass filter 16' is designed to have as little loss as possible in order to transmit a very weak received signal to the receiver with a good S/N ratio.

To that end, the bandpass filter 16' is designed to be relatively broadband, with its attenuation range being far away from the desired frequency. A band rejection filter 18' is configured to reject signals of undesired frequencies.

The specific configuration of the band-elimination filter 18' is the same as that of the band-elimination filter 18 of the transmission filter 12. However, by design, the band-stop filter 18' is designed so that its resonant circuit is in resonance only when a signal with a frequency in the passband of the transmission filter enters from the antenna, and the active element is therefore activated. . Therefore, the active element T
The noise generated by this signal will not be transmitted to the receiver.

Further, in the reception filter 14 as well, the active element T is combined only with the resonance circuit of the band rejection filter 18'. This is different from the case of the transmission filter 12 for the following reason. In other words, what is particularly required of the RF stage of a receiver for a car phone is the circuit noise figure (NF).
) is a characteristic.

On the other hand, if an active element is combined with the bandpass filter 16' in the reception filter 14, the active element T will operate when the reception signal enters from the antenna. Therefore, the noise generated in the active element T is added to the received signal, resulting in a poor S/N ratio of the received signal. That is, in the reception filter 14, the bandpass filter 16' and the active element T
If these are combined, the NF of the circuit will deteriorate.

As is clear from the above explanation, in the antenna duplexer,
The band rejection filter functions to block signals in the receiving frequency range in the transmitting filter, and to block signals in the transmitting frequency range in the receiving filter. Therefore, when each of these band-elimination filters is operated in combination with an active element T, the transmission signal ratio at the time of transmission.

The input saturation of the active element due to power does not occur, and the noise of the active element is not transmitted to the receiver together with the received signal during reception. This makes it possible to realize an antenna duplexer with low loss and good NF.

FIG. 1 is an equivalent circuit diagram showing the configuration of an embodiment of the present invention. This invention utilizes the concept shown in FIGS. 2 and 3 above.

In this embodiment, the band-elimination filter circuit includes two band-elimination filters 18a and 18 of different systems, respectively.
'b is included. The band rejection filters 18a and 18b are
3-stage resonant circuits R1a-R3a and R1b, respectively.
- Contains R3b. The center frequencies of the attenuation ranges of the band-elimination filters 18a and 18b are set to fa and fb, respectively.

One common active element TI is coupled to the resonant circuit R1a included in the band-elimination filter 18a and the resonant circuit R1b included in the band-elimination filter 18b. Similarly, active elements T
2, and an active element T3 is commonly coupled to the resonant circuits R3a and R3b, respectively.

The center frequencies fa and fb of the attenuation ranges of the respective band-elimination filters 18a and 18b are sufficiently apart and are included in the frequency band of the common active elements T1 to T3.

The signal resonated in one band-elimination filter 18a flows into the corresponding resonant circuits R1b-R3b of the other band-elimination filter 18b through common active elements T1-T3.

However, these resonant circuits R1b-R3b are non-resonant at the center frequency fa of the band rejection filter 18a. Therefore, the signal from one band-elimination filter 18a does not appear on the other band-elimination filter 18b.

The same holds true for the signal flow from the band-elimination filter 18b to 188.

In this way, active elements can be commonly coupled to different series of resonant circuits. The actions of such active elements T1 to T3 are as previously described in detail with reference to FIG. 3.

Note that the number of filters and the number of stages of resonant circuits are arbitrary, and one active element may be commonly used for three or more resonant circuits.

According to this invention, as shown in FIGS. 2 and 3, 1
Compared to a system in which one active element is coupled to two resonant circuits, this structure can not only be made smaller but also more cost-effective.

[Brief explanation of the drawing]

FIG. 1 is an equivalent circuit diagram showing the configuration of an embodiment of the present invention. FIG. 2 is a block diagram showing the basic idea behind this invention. FIG. 3 is an equivalent circuit diagram showing the relationship between the one-stage resonant circuit shown in FIG. 2 and the active elements. FIG. 4 is a schematic block diagram of an antenna duplexer to which an embodiment of the present invention is applied. FIG. 5 is a schematic block diagram showing an example of an antenna duplexer which is the background of the present invention. In the figure, 12 is a transmitting filter, 14 is a receiving filter, 18a and 18b are band rejection filters, Rla-R
3a and Rlb-R3b are resonators, and T1 to T3 are active elements. Patent applicant Murata Manufacturing Co., Ltd. Representative Patent attorney Yama 1) Yoshito (and 1 other person)

Claims (1)

[Claims] A band rejection filter circuit including a plurality of filters in which respective desired signal frequencies are in their passbands and undesired frequencies are in their attenuation bands, each of the filters including: , a plurality of resonant circuits whose resonant frequencies are in the attenuation range of the corresponding filter, and an active element that is commonly coupled to the plurality of resonant circuits and operates to provide negative resistance when the resonant circuits are in a resonant state. A band rejection filter circuit comprising: