JPS61280103A - Band stop filter circuit - Google Patents

Abstract

PURPOSE:To obtain a band stop filter circuit with small size and large Q by providing at least one resonance circuit whose resonance frequency is at the attenuation band and an active element coupled with the resonance circuit and causing a negative resistance while being operated when the resonance circuit is resonated. CONSTITUTION:A resonance circuit R includes resistors and inductors connected in series, the active element T is coupled inductively to the inductor, the resonance circuit R is in non-resonance state at the pass band of the band stop filter 18 and the active element T is inoperative. The resonance circuit R is resonated at the attenuation band of the band stop filter 18 and the active element T is activated. When the active element T is operated, a negative resistance (-r) is caused to apply energy from the active element T to the resonance circuit R. That is, in adjusting the Qex between the resonance circuit R and the active element T so as to be equal to the specific Qo of the resonance circuit R, the Q of the resonance circuit R becomes apparently 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. 4 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 transmitting filter 1 whose one end is connected to a transmitter, and a receiving filter 2 whose one end is connected to a receiver, and the other ends of the transmitting filter 1 and the receiving 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 circuit in which the frequency of a desired signal is in its pass band and the undesired frequency is in its attenuation band, A band-stop filter circuit comprising at least one resonant circuit whose resonant frequency is in the attenuation range, and an active element that is coupled to the resonant circuit and operates to provide negative resistance when the resonant circuit is in a resonant state. .

(Function) In the passband of the band-elimination filter circuit, the resonant circuit is in a non-resonant state and the active element does not operate.

In the damping region, the resonant circuit is in resonance and therefore
The active element connected to it operates and acts as a negative resistance. Therefore, the resistance value of the resonant circuit becomes small, and the Q of the resonant circuit appears to be a very large value.

(Effects of the Invention) According to the present invention, with a simple configuration in which an active element is connected to a resonator, the Q of the filter circuit can be increased without increasing the size of the filter circuit.

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. 3 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 configured by series connection of a bandpass filter (BPF) 16 (16') and a band-elimination filter (BEF) 1B (1B').

Note that the connection order of the band pass filter 16 (16') and the band rejection filter 18 (18') may be reversed to that in FIG. 3.

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. 1 is a block diagram showing one embodiment of the present 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 must be a filter with as little loss as possible in the transmission signal passband, and must use a resonator of a limited size. 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

Band-rejection filter 18 includes a stripline of λ/4 and resonators R5, R6 and R7 coupled to the stripline every λ/4. 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 an erodible 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. 2 is an equivalent circuit diagram of the single-stage resonant circuit shown in FIG. 1 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, the Qe between the resonant circuit R and the active element T is determined by the Q unique to the resonant circuit R. If it is adjusted to be equal to , the Q of the apparent main resonant circuit R becomes a very large value.

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

Further, in the second equivalent circuit, the resonant circuit R and the active element T are inductively coupled, but this may be capacitively coupled.

In this embodiment, 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 active element T for normal small signals
The output power level of is 10-20 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, the band-stop filter 18' is designed such that its resonant circuit resonates and the active element operates only when a signal having a frequency in the passband of the transmission filter enters from the antenna. Therefore, the active element T
The noise generated by the receiver 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 reasons. 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, if each of these band rejection filters is operated in combination with an active element T, the input saturation of the active element due to the transmission signal output will not occur during transmission (and the noise of the active element will be transmitted to the receiver together with the received signal during reception). Therefore, if this invention is put to practical use, an antenna duplexer with low loss and good NF can be realized.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention. FIG. 2 is an equivalent circuit diagram showing the relationship between the one-stage resonant circuit shown in FIG. 1 and the active elements. FIG. 3 is a schematic block diagram of an antenna duplexer to which an embodiment of the present invention is applied. FIG. 4 is a schematic block diagram showing an example of an antenna duplexer which is the background of the present invention. In the figure, 2 is a transmitting filter, 14 is a receiving filter, 18.18' is a band rejection filter, R is a resonator,
T indicates an active element. Patent applicant Murata Manufacturing Co., Ltd. Agent Patent attorney Yama 1) Yoshito (and 1 other person) 11111!1 Figure 2

Claims (1)

[Claims] A band-stop filter circuit in which the frequency of a desired signal is in its passband and the undesired frequency is in its attenuation range, the circuit comprising at least one resonance whose resonant frequency is in the attenuation range. A band-stop filter circuit comprising: a circuit; and an active element coupled to the resonant circuit and operating to provide negative resistance when the resonant circuit is in a resonant state.