GB2257335A

GB2257335A - Antenna switching circuit

Info

Publication number: GB2257335A
Application number: GB9213455A
Authority: GB
Inventors: Aimo Turunen
Original assignee: LK Products Oy
Current assignee: Pulse Finland Oy
Priority date: 1991-06-25
Filing date: 1992-06-24
Publication date: 1993-01-06
Anticipated expiration: 2012-06-24
Also published as: GB2257335B; FI913086A0; GB9213455D0

Abstract

Switching/filter circuit arrangement for a radio apparatus to realize simplex operation, comprises a transmission line (2) of approximately a quarter-wave length between the transmitter (1) of the radio apparatus and the first and second filters (3 and 5), and a switch (K1) between the common node of the transmitter (1) and the transmission line (2). A transmitted signal will pass from the transmitter (1) through the transmission line (2) and the first filter (3) to the antenna (4) when the switch is open, and a received signal will pass from the antenna (4) through the first filter (3) to a second filter (5) and from there further to the receiver (6), when the switching means is closed. <IMAGE>

Description

A Filter Circuit Arrangement The present invention relates to a filter circuit arrangement to realize simplex operation in a radio apparatus e.g. a cellular radiotelephone.

In simplex operation signals are transmitted only in one direction at a time. Thus it is necessary to be able to switch the radio equipment into a transmitting or receiving mode, as required. Simplex operation is usually realized by having a switch between the antenna and the transmitting and receiving filters of the radio apparatus so that the transmitted and received signals are separated. In such an embodiment both the received signals and the high-power transmitted signals are switched through the same switch. During transmission the transmitted signals pass from the transmitter to the transmitting filter, and from there through the switch to the antenna. Correspondingly, during reception the received signal passes from the antenna through the switching means to the receiving filter and from there to the receiver.

The switch effectively separates the received signals and the transmission signal from each other, but it has the disadvantage that the switch must have a high-power rating, since the high-power transmission signal is connected through it.

In GB 1545 64 there is disclosed a coupler for coupling signals between an antenna and a transceiver. In a receiving mode, signals pass from the antenna to the receiver of the transceiver, but in a transmitting mode, the receiver is effectively decoupled, so that a signal from the transmitter of the transceiver passes to the antenna, but not to the receiver.

According to the present invention, there is provided a filter circuit arrangement for a radio apparatus comprising a transmitter, a receiver and an antenna, the arrangement comprising transmitter filter means for passing transmission signals of a transmission frequency from the transmitter to the antenna; receiver filter means for passing received signals of the receiver frequency from the antenna to the receiver; and means for selectively coupling the transmitted signal from the transmitter to the antenna in a transmission mode, and the receiver signals to the receiver from the antenna in a receiving mode, characterized in that the transmitter filter means comprises a first filter for coupling to the antenna; the receiver filter means comprises the first filter and a second filter coupled thereto; the selective coupling means comprises means for coupling, in the transmission mode, the transmitter to the first filter, to supply the transmitted signal thereto and for decoupling the transmitter from the first filter in the receiving mode such that, in the receiving mode, the received signal is coupled from the antenna via the first and second filters to the receiver. This has the advantage that no high-power switching means is required.

The selective coupling means may comprise a transmission line coupled at a first end to the common junction of the first and second filters, and at a second end for coupling to the transmitter; and switching means coupled to the common junction of the second end and the transmitter, whereby, in the transmission mode, the switch is arranged to be open such that the transmission line presents a low impedance to the transmitted signal so that transmitted signal passes through the transmission line from the transmitter and to the antenna via the first filter, and, in the receiving mode, the switch is arranged to be closed such that the transmission line presents a high impedance to the received signal so that the received signal passes through the first and second filters to the receiver, but not through the transmission line to the transmitter.The transmission line and first and second filters are therefore used to effectively separate the received and transmitted signals. The switching means is now located so that no high-power signals pass through the switching means so that a low-power switch, for example a PIN diode or a relay can be used, so that switching speed increases and power consumption decreases.

The invention will now be described, by way of example only, with reference to the enclosed figures, in which: Figure 1 shows a block diagram for a filter circuit arrangement according to the invention for simplex operation; and Figure 2 shows a circuit diagram of the filter circuit arrangement of Figure 1; and Figure 3 shows the frequency response of the first filter in the transmission mode; and Figure 4 shows the frequency of the first and second filters in the receiving mode.

As shown in Figure 1 the filter circuit arrangement of the invention comprises a first filter 3 which is coupled to the antenna 4 of a cellular radio telephone.

The first filter 3 is also coupled to the transmitter 1 of the telephone via a transmission line 2. Coupled to the junction of the transmission line 2 and the first filter 3 is a second filter 5 which is also coupled to the receiver 6 of the telephone. Thus, a received signal, received by the antenna 4, can be coupled through the first and second filters 3,5 to the receiver 6, and, conversely, transmitted signals from the transmitter 1, can be coupled through the transmission line 2 and first filter 3 to the antenna 4.

A switch K1 is coupled to the common junction of the transmission line 2 and transmitter 1.

The structure and operation of the transmitter, receiver and antenna of a cellular radio are well known to a person skilled in the art and will, therefore, not be described here as they are not relevant to the instant invention.

The switch is open when the transmitter 1 is transmitting a signal i.e. the circuit is in a transmission mode. In this case, the transmission line 2, which has a length corresponding to approximately a quarter-wavelength appears as a low impedance line coupled to the first filter 3 so that the transmitted signal passes via the transmission line 2 and first filter 3 to the antenna 4. A small amount of the transmitted signal will be coupled to the receiver 6 through a coupling aperture 12 located between the first and second filters 3,5 but as the attenuation is so high, it will not destroy the front end of the receiver 6. The first filter 3 is a low-pass filter which passes signals at the transmission frequency to the antenna 4 and attenuates received signals which are at a higher receiving frequency.

When signals are received i.e. in a receiving mode, the switch K1 is closed and the transmission line 2 is therefore shorted at the end closer to the transmitter 1, and therefore it appears as a high impedance to the received signal. Thus the transmission line 2 will not pass the received signals from the antenna 4 in the direction towards the switch K1 and so the transmitter 1 is electrically decoupled. In the receiving mode the filters 3 and 5 act as a band-pass filter and pass signals at the receiving frequency and attenuate other frequencies.

From Figure 2 it can be seen that the first filter 3 comprises resonators 7 and 8. In the transmitting mode the resonators 7 and 8 are in resonance at the receiving frequency, such that the first filter 3 absorbs signals at the receiving frequency and attenuate them. On the other hand, transmitted signals at a lower frequency are passed to the antenna 4.

When the switch K1 is closed, the transmission line 2 is short circuited at the end closest to the transmitter 1. Thus, as described above, the transmission line 2 appears as a high impedance to the received signal and thus the received signal coming from the antenna 4 will not be substantially coupled through the transmission line 2 and thus not to the switch K1 or the transmitter 1. Therefore the transmitter 1 is decoupled. In this case, the received signal passes through the first filter 3 comprising resonators 7 and 8 and is coupled through the aperture 12 to the receiving filter 5 comprising resonators 9, 10, 11. In receiving mode the resonators 7, 8, 9, 10, 11 act in parallel resonance at the receiving frequency fRX, and they pass signals at the receiving frequency fRX and attenuate other frequencies, i.e. they act as a band-pass filter.

Figure 3 shows the frequency response of the first filter 3, in the transmitting mode showing the attenuation of the filter 3 as a function of the frequency. The graph shows the resonance zeros introduced by the resonators 7 and 8, attenuating the signals at the receiving frequency with a center frequency fRX It will pass signals at the transmission frequency, below the cut-off frequency fTX - Figure 4 shows the pass-band with the center frequency fRX provided by the first and second filters 3 and 5 in parallel resonance. Other frequencies are attenuated as signals at the receiving frequency pass to the receiver 6.

The frequencies of the transmitted signals and the received signals can also be exchanged, so that the transmitting frequency is higher than the receiving frequency. Then also the filters are arranged so that the first filter is a high-pass filter and the first and second filters constitute a band-pass filter, passing signals at the receiving frequency.

The filters can be realized e.g. as helix filters or as dielectric filters. The first filter 3 as well as the second filter 5 comprise resonators, having a mutual inductive or capacitive coupling or a combination of these. The coupling can be realized in any known way, for example with the aid of apertures or links.

Preferably the transmission line 2 is realized by a coaxial cable or an Inductor/Capacitor (LC) circuit and it is possible to integrate it in the filters, or on a printed circuit board on which the filters 3, 5 are secured.

The switch K1 could be a small-signal diode, e.g. a PIN diode, which is fast and has a low-power consumption.

It could also be a relay. The length of the transmission line must be slightly different from a quarter-wavelength, since it will also operate as a load of the transmitting filter.

It will be evident to a person skilled in the art from the foregoing description, that modification are possible within the scope of the present invention.

Claims

1. A filter circuit arrangement for a radio apparatus comprising a transmitter, a receiver and an antenna, the arrangement comprising: transmitter filter means for passing transmission signals of a transmission frequency from the transmitter to the antenna; receiver filter means for passing received signals of the receiver frequency from the antenna to the receiver; and means for selectively coupling the transmitted signal from the transmitter to the antenna in a transmission mode, and the receiver signals to the receiver from the antenna in a receiving mode, characterized in that: the transmitter filter means comprises a first filter for coupling to the antenna; the receiver filter means comprises the first filter and a second filter coupled thereto; the selective coupling means comprises means for coupling, in the transmission mode, the transmitter to the first filter, to supply the transmitted signal thereto, and for decoupling the transmitter from the first filter in the receiving mode such that, in the receiving mode, the received signal is coupled from the antenna via the first and second filters to the receiver.

2. An arrangement according to claim 1 wherein the selective coupling means comprises: a transmission line coupled at a first end to the common junction of the first and second filters, and at a second end for coupling to the transmitter; and switching means coupled to the common junction of the second end and the transmitter, whereby, in the transmission mode, the switch is arranged to be open such that the transmission line presents a low impedance to the transmitted signal so that transmitted signal passes through the transmission line from the transmitter and to the antenna via the first filter, and, in the receiving mode, the switch is closed such that the transmission line presents a high impedance to the received signal so that the received signal passes through the first and second filters to the receiver, but not through the transmission line to the transmitter.

3. An arrangement according to claim 1 or 2 wherein the first and second filters each comprise at least one resonator such that, in the transmission mode, the first filter attenuates signals at the receiving frequency while passing signals at the transmitting frequency, and in the receiving mode, the resonators of the first and second filters act to resonate in parallel to pass the received signal at the receiving frequency to the receiver and to attenuate other signals.

4. An arrangement according to any of claims 2 to 3 wherein in the transmission mode the transmission line has a length of approximately quarter of a wavelength.

5. An arrangement according to claim 3 or claim 4 wherein the transmission comprises a conductive strip present on an insulating substrate.

6. An arrangement according to claim 3 or claim 4 wherein the transmission line comprises a coaxial substrate.

7. An arrangement according to any of claims 2 to 6 characterized in that the first filter and/or the second filter are dielectric filters.

8. An arrangement according to any of claims 2 to 5, characterized in that the first filter and/or the second filter are helix filters.

9. An arrangement according to any of claims 2 to 5, characterized in that the switching means is a diode.

10. An arrangement according to any of claims 2 to 5, characterized in that the switching means is a relay.

11. An arrangement as herein before described with reference to the accompanying drawings.