GB2326799A

GB2326799A - Radio communications transceiver and radio frequency signal router therefor

Info

Publication number: GB2326799A
Application number: GB9713614A
Authority: GB
Inventors: Shimon Krausz; Yossie Bercovich
Original assignee: Motorola Israel Ltd
Current assignee: Motorola Solutions Israel Ltd
Priority date: 1997-06-28
Filing date: 1997-06-28
Publication date: 1998-12-30
Anticipated expiration: 2017-06-28
Also published as: GB2326799B; GB9713614D0

Description

RADIO COMMUNICATIONS TRANSCEIVER AND RADIO FREQUENCY SIGNAL ROUTER mEREFOR Field of the Invention This invention relates to a switch. The invention is applicable to, but not limited to, an antenna switch for use in a communications device for switching the coupling of an antenna between a transmitter section and a receiver of the communications device.

Background of the Invention A transmit / receive (T/R) antenna switch of radio communication equipment is typically used to alternate (switch) the coupling of an antenna between a transmitter and a receiver of a radio device. The antenna is coupled to the transmitter during a transmit operation and to the receiver during a receive operation. The antenna switch typically includes various types of switching elements such as diodes that are biased for providing either a high impedance path to preventing the passage of signals or a low impedance path to facilitate the passage of signals.

The common antenna switch arrangement includes two diodes as shown in FIG. 1. The antenna switch circuit 10 includes a transmit path and a receive path. The transmit path includes a power amplifier 11 operably coupled via a fifty ohm transmission line 12 to a first diode 13, which in turn is connected to a harmonic filter 17. The harmonic filter is connected to the antenna 18 and rejects signals that are outside of the desired frequency range of the communications unit, from either being emitted from the transmit section, or entering into the receive section. The dc biasing to diodes 13 and 20 are provided by voltage applied at Von. Inductors 16 and 22 are radio frequency (RF) chokes isolating the bising network from the radio frequency signal paths.

In receive mode a zero voltage is applied at Von so that diodes 13 and 20 are zero biased. A signal input to antenna 18, passes through harmonic filter 17 and is prevented from entering the transmit section by first diode 13, which is zero biased and matched to a high impedance by inductor 14 and capacitor 15. This means that the transmit path does not load the receiver and cause loss of sensitivity or desensitivity. The signal passes down a quarter wave-length transmission line 19 to a receive section. The second diode 20 is also reverse biased so that it presents a high impedance and allows the signal to flow unhindered into the receiver.

When the radio device (not shown) is in a transmit mode, a high dc voltage level is applied at Von which forward biases the diodes 13 and 20. The first diode 13 presents a low impedance to the transmit path allowing the transmitted power to reach the antenna 18 after passing the harmonic filter 17. The forward biased second diode 20 is tuned to an RF short circuit by capacitor 21. This short circuit is transformed by the quarter wave transmission line 19, to a high impedance at the transmitter path and thereby isolates the receiver from signals exiting the power amplifier 11.

There are several disadvantages to such a prior art antenna switch arrangement. In particular, diode 13 attenuates the transmitted signal so that the power amplifier module must output more RF power to ensure the required output power at the antenna 18. This adds to the current consumption of the voltage source powering the device. If this is a battery, for example, then the lifetime between recharging is shortened. Diode 13 is also a potential hot spot. For medium and high power applications considerable RF power flows through this diode and a large amount of heat is dissipated on it. Heat sinking this diode is not simple and requires difficult cooling techniques to ensure that it doesn't overheat. Series configured diodes such as diode 13 for medium and high power applications are also expensive. In order to provide the necessary low impedance during transmit cycle, diode 13 must also be biased with a high current, adding greatly to the current budget of the voltage source, V"on".

This invention seeks to provide a switching arrangement that mitigates at least some of the above mentioned disadvantages.

Summarv of the Invention According to a first aspect of the present invention there is provided a radio frequency signal router for coupling an antenna to a receiver section and a transmitter section of a radio. The radio frequency signal router includes a first signal path for coupling in use the antenna to the transmitter section; and a second signal path for coupling in use the antenna to the receiver section. The first and second respective signal paths are transmission line paths such that a first impedance of the first transmission line signal path viewed from the transmitter section to the antenna is less than a second impedance of the second transmission line viewed from the transmitter section to the receiver section in a transmit. A third impedance of the second transmission line signal path viewed from the antenna to the receiver section is less than a fourth impedance of the first transmission line viewed from the antenna to the transmitter section in a receive mode of operation.

Preferably, the first and third impedances are matched impedances for coupling the transmitter to the antenna in the transmit mode of operation and for coupling the antenna to the receiver in the receive mode of operation.

In the preferred embodiment of the invention, the second transmission line is a quarter length transmission line of a receive frequency to provide matching of the third impedance between the antenna and the receiver. Additionally, the radio frequency signal router is preferably an antenna switch and the first transmission line has opposing impedance values when viewed from the antenna to the transmitter section and when viewed from the transmitter section to the antenna.

Advantageously, appropriate design of the first transmission line utilises the non-resistive nature of the output impedance when looking into the transmitter section in receive mode. A transmission line of suitable length will transform this non-resistive effect into a high impedance value, thereby isolating the transmitter section from the antenna during receive mode. This removes the need for an isolating device such as a diode, consequently reducing the power consumption of the high power output stages of the transmitter, removing the need to consider heat-dissipation arrangements for the isolating device and improving the receiver sensitivity.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the drawing.

Brief Description of the Drawings FIG. 1 shows a prior art design of an antenna switch; and FIG. 2 shows an antenna switch according to a preferred embodiment of the invention.

Detailed Description of the Drawings Referring now to FIG. 2, an antenna switching arrangement 30, of say a radio communication device, is shown, according to a preferred embodiment of the invention.

The antenna switching arrangement 30, includes a transmit radio frequency power amplifier 31, operably coupled via a first transmission line 32 and an harmonic filter 33 to an antenna 34. The antenna 34 is also coupled to a receiver section, via a second transmission line 35, preferably of quarter wave length of the receive frequency. A radio frequency isolating diode 36 is connected to one end of the transmission line 35, biased via an inductor 39. The impedance at the end of the transmission line 35 is dictated by the transmission line 35, inductance 37 and capacitor 38.

In a transmit mode of operation, a high dc voltage level is supplied to the diode 36 through the inductor 39 for forward biasing the diode 36.

The first transmission line 32, presents a low impedance to the transmit path allowing the transmitted radio frequency (RF) signal to reach the antenna 34 after passing through the harmonic filter 33. The harmonic filter rejects any spurious emissions from outside the desired transmit band of operation. The diode 36 shorted through the capacitor 38, presents a short circuit at the receiver end of the quarter wave transmission line 35. The short circuit at the junction where the receiver is connected to the one end of the quarter wave transmission line 35 is transformed to an open circuit (high impedance), for example 20 dB, back at point 'A' at the transmitter path thereby isolating the receiver during transmit mode.

To increase the isolation of the receiving path an additional quarter wave transmission line can be coupled in series to the quarter wave transmission line 35. Each additional quarter wave transmission line will increase the isolation between the transmitter path to the receiver path with an additional 20 dB.

In the receive mode, no dc voltage is applied to the diode 36 and the RF power amplifier 31 is turned off. A received RF signal from the antenna 34 passes through the harmonic filter 33 and the quarter wave transmission line 35 and reaches the receiver. The quarter wave transmission line 35 is merely a 50 ohm impedance line to the receiver, the diode 36, as well as inductor 37 and capacitor 38, represents a high impedance to the signal and thereby does not impede the received RF signal passing to the receiver section. The length of the first transmission line 32 is designed so that the transmit path at point "A" presents a high impedance to the incoming received RF signal.

Several techniques of matching impedances with the use of transmission line are known in the art. One of those techniques is used to determined the length of the first transmission line 32. For example, the output impedance of the power amplifier 31 is measured in the "OFF" position. This impedance is mapped onto a Smith Chart and may fall on the inductive part of the Smith Chart. The length of the transmission line 32 is then calculated so as to transform this impedance to a high impedance at point "A" which is represented on the Smith Chart as a high impedance.

The transmission line 32 is a 50 ohm line (in a 50 ohm system) so that in transmit mode the power amplifier transmits into a matched load. With the use of the present invention in high power radios such as 50W and 100W radio transmitters, where transmit path loss and thermal considerations are paramount, no cooling techniques are needed in order to provided heat dissipation to the antenna switch.

The switching arrangement of the present invention can be implemented in most narrow band transceivers thereby mitigating at least some of the aforementioned disadvantages with switching techniques in radio transceivers.

Claims

1. A radio frequency signal router for coupling an antenna to a receiver section and a transmitter section of a radio comprising: a first transmission line path for coupling in use the antenna to the transmitter section; and a second transmission line path for coupling in use the antenna to the receiver section, wherein a first impedance of the first transmission line path viewed from the transmitter section to the antenna is less than a second impedance of the second transmission line path viewed from the transmitter section to the receiver section in a transmit mode of operation and a third impedance of the second transmission line path viewed from the antenna to the receiver section is less than a fourth impedance of the first transmission line viewed from the antenna to the transmitter section in a receive mode of operation.

2. A radio frequency signal router as claimed in claim 1, wherein the first and third impedances are matched impedances for coupling the transmitter section to the antenna in the transmit mode of operation and for coupling the antenna to the receiver section in the receive mode of operation.

3. A radio frequency signal router as claimed in claim 2, wherein the second transmission line path is a quarter wavelength transmission line of a receive frequency to provide matching of the third impedance between the antenna and the receiver.

4. A radio frequency signal router as claimed in claim 3 wherein the second transmission line path is connected to at least one further quarter wavelength transmission line for further isolating the receive section during a transmit mode of operation.

5. A radio frequency signal router as claimed in any one of claims 1 to 4 wherein the radio frequency signal router is an antenna switch and the first transmission line has opposing impedance values when viewed from the antenna to the transmitter section and viewed from the transmitter section to the antenna.

6. A radio communications transceiver comprising a transmitter operably coupled to a receiver via a radio frequency signal router of claims 1 to 5.

7. A radio frequency signal router substantially as hereinbefore described with reference to FIG. 2 of the drawings.