Technical Field
The present invention relates to switches for changing operation between transmitter and receiver paths on a single antenna.
Description of the Related Art
Radio frontends that act as both a transmitter and receiver use a transmit/receive switch to change a path connected to the antenna between a transmit and a receive path. The objective of such a switch is to enable the radio to select between two states of operation.
Referring now to FIG. 1, a prior art transmit/receive switch is shown. A power amplifier (PA) 102 is connected to an antenna 108 along a transmit path and a low noise amplifier (LNA) 104 is connected to the same antenna 108 along a receive path. In each path, a quarter-wavelength transmission line 106 is in series with the respective amplifier and a switch 110 to ground is placed in parallel. Turning on one of the switches 106 creates, along with the respective transmission line 106, an apparent electrical open to the radio frequency (RF) signals on the path. This allows one branch to be turned off while the other is active.
Since the switches 110 and transmission lines 106 reside at the very front of the transceiver, their performance is important for the overall system. For example the loss of the switch 110 and the associated transmission line 106 directly reduce the efficiency and compression point during transmit mode and the noise figure during receive mode. The transmitter efficiency is significantly crucial in large phased array transceivers because as the efficiency drops, the required DC power required to produce a desired output power increases proportionally. This means that the switch insertion loss in the transmit mode is important to the overall power consumption and temperature of a large transceiver system. As a result it is highly beneficial to have a switch with low insertion loss in transmit mode.
One end of the two transmission lines 106 connects together and to the antenna 108, while the input to each side is connected to a respective switch 110. In this scheme, the presence of the transmission lines 106 is needed for the operation of the switches 106 in “off” mode. During off mode of a branch, the combination of the quarter wavelength transmission line and the switch are responsible for creating a high input impedance. However, a problem with this method is that, during the “on” state, the RF signal has to pass the through the transmission line that connects the module to the antenna. This extra line introduces undesired loss in the signal path, particularly at higher frequencies (e.g., at the tens of gigahertz). In other words, the transmission line is a means to turn off the branch, but is harmful when the branch is on.