Typically, the performance of a cellular antenna is far from ideal. A significant portion of a transmitter's power output is commonly reflected back from the antenna due to poor antenna impedance matching. When a single transmission frequency is transmitted via an antenna, often the antenna and the transmission line to the antenna are impedance matched to maximize the output transmit power by eliminating reflected power back from the antenna. When a transmitter is designed to transmit over a plurality of frequency channels an antenna tuner can be used to improve the antenna matching. In order to use an antenna tuning circuit, various techniques have been used to attempt to determine a proper impedance matching required to match the antenna impedance with a transmission line.
A traditional solution for determining an impedance adjustment for a transmission line that helps maximize the transmitter power output from the antenna by minimizing reflected power back from the antenna, is to measure the standing wave ratio (SWR) on the transmission line between the output of the transceiver's power amplifier and the antenna. Essentially, the SWR is a measurement of the forward and reflected power seen on the transmission line between the power amplifier and the antenna. The SWR value can be calculated from a measurement of the forward and reflected power on the transmission line.
Referring to FIG. 1, a prior art solution for helping calculate an SWR on a transmission line is shown. An RF transceiver 10 outputs a signal to be transmitted, which is input into a power amplifier 12. The power amplifier 12 amplifies the transmission signal and provides it to the transmission line 14, which ultimately provides the transmission signal to the antenna 16 for transmission. On the transmission line 14 a directional coupler 18 is traditionally used to couple with the RF signal on the transmission line 14 so as to provide a reflected power output 20 and a forward power output 22. The reflected power output 20 and forward power output 22 signals or measurements provide an indication of how much power is going forward to the antenna and how much is being reflected back. The impedance matching (antenna matching) between the antenna 16 and the output of the power amp 12 is not considered ideal if there is reflected power back from the antenna 16 on the transmission line 14. In order to help match the antenna impedance with the power amp's output impedance and the transmission line impedance, the reflected power out signal 20 and forward power out signal 22 may be utilized by other circuitry (not specifically shown) to provide a tuner control signal 24 to an antenna tuner 26. The antenna tuner may be used to adjust or change the impedance seen by the antenna 16 and the output of the power amp 12 so that there is a close matching of the impedances and thus antenna matching. When antenna matching occurs, a maximized amount of power provided by the power amp 12 will be transmitted via the antenna 16 with a minimal amount of reflected power back from the antenna 16.
A drawback of a prior art directional coupler is that it is a relatively large device and cannot be incorporated into an integrated circuit. A directional coupler is therefore more costly than an integrated circuit in that it must be manufactured and mounted on a printed circuit board. Thus, the directional coupler 18 takes up additional space within, for example a mobile telephone, where limited space is available. In addition, a directional coupler is costly to implement because it requires additional manufacturing steps and external component connections to and from various components associated with a transceiver device.
U.S. Pat. No. 4,380,767 depicts an SWR circuit 44 that uses a transformer 26 to couple with the transmission line leading to the antenna in order to provide a reflective power and forward power output. The transformer 26, like a directional coupler, is a discreet component that is rather large and cannot be integrated into a silicon chip.
What is needed is a device that can be integrated into a silicon chip and can measure the forward power and reflected power of an antenna so as to provide data that can ultimately be used by an antenna tuner circuit to help match the impedance of the antenna with that of the power amplifier at the RF frequency to be transmitted.