JPH053923B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a filter circuit, and particularly to a filter circuit used, for example, in a duplexer or antenna duplexer of a car telephone or other wireless equipment.

(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 transmitting filter 1 of such an antenna duplexer
One condition is that Q be large due to the need to minimize loss in the filter. Furthermore, since the reception filter 2 needs to pass the weak signal received by the antenna with a good S/N ratio, it is required to be a filter with a large Q value and low loss.

In order to increase the Q of a filter, both conventional coaxial type filters and filters using dielectric resonators only consider dispersing energy concentration. If this is the case, the shape will become large, which conflicts with the demand for miniaturization.

On the other hand, for example, IEEE MTT-S Int.
“AN ACTIVE” from Microwava Symp.Dig.1979
MICROWAVE FILTER WITH DIELECTRIC
A paper entitled ``Resonator'' proposes a technique for increasing the apparent Q by using a dielectric resonator and coupling an active element exhibiting negative resistance to the dielectric resonator.

Therefore, it is conceivable to use such a technique as a filter for an antenna duplexer.

(Problems to be Solved by the Invention) However, when the receiving filter 2 of the antenna duplexer shown in FIG. Therefore, the input of the active element becomes saturated, and the Q of the receiving filter 2 is reduced by the active element.
The effect of increasing the value will not be fully exerted.

Therefore, the main object of the invention is to provide a filter circuit in which the active elements do not saturate.

(Means for Solving the Problems) Simply put, the present invention is a filter circuit including a band rejection filter in which the frequency of a desired signal is in its pass band and the undesired frequency is in its attenuation band. The band rejection filter includes at least one resonant circuit whose resonant frequency is in an attenuation range, and an active element coupled to the resonant circuit and operating when the resonant circuit is in a resonant state to provide negative resistance.
A filter circuit characterized in that a band-pass filter having a pass band including a pass band of the band-stop filter and an attenuation band including an attenuation band of the band-stop filter is provided before the band-stop filter. .

(Operation) In the passband of the band rejection filter, the resonant circuit is in a non-resonant state and the active element does not operate. In the attenuation region, since the resonant circuit is in a resonant state, the active element coupled to it operates and acts as a negative resistance. Therefore, the resistance value of the resonant circuit becomes smaller,
The apparent Q of the resonant circuit becomes a very large value.

On the other hand, since the band-pass filter is placed before the band-elimination filter, the signal level in the attenuation range of the band-elimination filter becomes small, and the input to the active element does not become saturated.

(Effects of the Invention) According to the present invention, since the input saturation of the active element is suppressed, the Q of the filter circuit is reduced by the active element.
can be effectively increased.

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.

(Embodiments) Hereinafter, embodiments of the present invention will be described 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 transmitting filter 12 and a receiving filter 14. Both the transmitting filter 12 and the receiving filter 14 are constructed by serially connecting band pass filters (BPF) 16, 16' and band reject filters (BEF) 18, 18'.

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 a series connection of three stages of bandpass filters 16 and four stages of band rejection filters 18, and the bandpass filter 16 needs to be a filter with as little loss as possible in the transmission signal passband, and Since a resonator of limited size must be used, it is designed to be relatively broadband, and its attenuation range is far away from the desired frequency. A band rejection filter 18 is configured to block signals of undesired frequencies. That is, the passband of the bandpass filter 16 includes the passband of the bandstop filter 18, and the attenuation band of the bandpass filter 16 includes the attenuation band of the bandstop filter 18. The passband of the band rejection filter 18 includes the frequency range of the transmitted signal, and the attenuation range includes the frequency range of the received signal.

Band-stop 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 active element T operates as a negative resistance in the resonance region of the resonator of the band-elimination filter 4, that is, in the attenuation region 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 band element filter 18, the resonant circuit R resonates and the active element T operates. Then, the active element T has a negative resistance (-
r). 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, then
Apparently, the Q of the resonant circuit R becomes a very large value.

Note that the stripline of electrical length θ' connected in series with the active element is for phase adjustment to bring the input and output into 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, a band pass filter 16 and a band rejection filter 18 that constitute the transmission filter 12 are used.
Of these, only the resonant circuit of the band rejection filter 18 is combined with the active element T. This is due to the following reasons. In other words, for 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,
This is because 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 receiving filter 14 is also constructed by connecting a band pass filter 16' and a band rejection filter 18' in series. The bandpass filter is designed to pass frequency signals in the reception signal passband, and the bandstop filter is designed to block frequency signals in the transmission signal passband. That is, the passband of bandpass filter 16' includes the passband of bandstop filter 18', and the attenuation range of bandpass filter 16' includes the attenuation band of bandstop filter 18'. The passband of the band rejection filter 18' includes the frequency range of the received signal, and the attenuation range includes the frequency range of the transmitted signal.

The band pass 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, its attenuation range being far away from the desired frequency. A band rejection filter 18' is configured to block signals of undesired frequencies.

The specific configuration of the band rejection filter 18' is the same as that of the band rejection filter 18 of the transmission filter 12. However, due to the design, the band rejection filter 1
8' is designed so that its resonant circuit resonates and the active element operates only when a signal having a frequency in the passband of the transmitting filter enters from the antenna. Therefore, the noise generated by the active element T will not be transmitted to the receiver.

Further, in the receiving filter 14 as well, the active element T is combined only with the resonant circuit of the band rejection filter 18'. This is the transmission filter 1
Unlike case 2, this is due to the following reasons. That is, what is particularly required in the RF stage of a receiver for a car phone is the noise figure (NF) characteristics of the circuit. 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, and the S/N ratio of the received signal deteriorates. In other words, when the bandpass filter 16' and the active element T are combined in the receiving filter 14, the NF of the circuit becomes poor.

As is clear from the above explanation, in the antenna duplexer, the band rejection filter is configured such that the transmitting filter blocks signals in the receiving frequency range, and the receiving filter blocks signals in the transmitting frequency range. each working. Therefore, when each of these band rejection filters is operated in combination with an active element T, the input saturation of the active element will not occur due to the output of the transmitted signal during transmission, and the noise of the active element will not be received together with the received signal during reception. It cannot be conveyed to the vessel. By this,
If this invention is applied, 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, 12 is a transmitting filter, 14 is a receiving filter, 18 and 18' are band rejection filters,
R represents a resonator, and T represents an active element.

Claims (1)

[Scope of Claims] 1. A circuit including a band-elimination filter in which the frequency of a desired signal is in its passband and an undesired frequency is in its attenuation range, the band-elimination filter having a resonant frequency in its attenuation range. a filter circuit comprising: at least one resonant circuit located in a region of the resonant circuit; and an active element coupled to the resonant circuit that operates to provide negative resistance when the resonant circuit is in a resonant state; A filter circuit comprising a bandpass filter having a passband including a passband of the filter and an attenuation band including an attenuation band of the bandstop filter.