In wireless systems, power amplifier circuits (PAs) are used to amplify a RF input signal prior to providing an amplified RF output signal to a load. In delivering of RF power from the PA to a load coupled thereto, an impedance match between the PA, feedline circuit, and the load is important in order to facilitate maximum power transfer therebetween. Any portion of the signal provided to the load that is not transferred reflects back into an output port of the PA and results in the PA producing unwanted signal emissions and lower efficiency. For wireless appliances, the FCC imposes strict radiation emission standards. If a wireless device does not fall within these standards, then such a wireless device is not salable, since broadcasting of RF signals outside a designated frequency band for the wireless devices is known to cause interference to surrounding electrical devices.
Furthermore, if the amplified RF output signal reflects back from the load, then less than a maximum transfer of RF signal power occurs and this results in unnecessary energy consumption by the PA. In addition to reduced power efficiency, amplified signals reflected back into the PA can damage the PA as relatively high voltages build on the output terminals of the circuit. The reflected signal is, effectively, energy that is not transferred from the PA to the load. Rather, this energy can be absorbed by the PA circuit resulting in a rise in junction temperatures concomitant with the loss of net power amplifier efficiency.
A Voltage Standing Wave Ratio (VSWR) is an indicator that is used with RF antenna systems to measure the coupling efficiency between the PA output port and an antenna. Typically, most antennas are not directly connected to a PA. The antenna is usually located some distance from the transmitter and PA and uses a feedline to transfer power therebetween. If the feedline has no loss and is impedance matched to both the PA output impedance and the antenna input impedance then maximum RF signal power is delivered to the antenna. In this case the VSWR is 1:1 and the voltage and current are constant over the whole length of the feedline. Any deviation from this situation causes a “standing wave” of voltage and current to exist on the feedline therebetween. This standing wave results in wasted energy and thus leads to wireless system inefficiencies.
Various techniques for measuring of RF signal power transfer between a PA output port and a load coupled thereto are known to those of skill in the art. For example, voltage sensing is performed at a final power amplification stage of the PA. Typically, a peak voltage detection scheme is utilized and it is therefore directly affected by a VSWR mismatch error. Unfortunately, with this technique, no indication of a level of VSWR mismatch is provided because the rise in peak voltage can be attributed to an increase in output power from the PA or a change in the VSWR. In effect, the peak voltage detection scheme can be ambiguous since voltage is not indicative of power transferred.
Another technique for measuring of RF signal power transfer utilizes voltage sensing at a penultimate stage of the PA. Unfortunately, this technique implements a peak voltage detection scheme and is therefore directly affected by VSWR mismatch error. Due to buffering of a final amplification stage of the PA, errors in the VSWR measurement are reduced. A disadvantage however, is that the final amplification stage is typically outside a control loop of the PA. Thus, the RF output signal is typically susceptible to supply voltage and temperature changes in the final amplification stage of the PA. Additionally, with this technique, there is no indication of VSWR mismatch provided.
A third technique for measuring of RF signal power transfer, which is known to those of skill in the art, is to provide the RF output signal through an off-chip directional coupler for forming a coupled signal. The coupled signal is connected to a detector circuit, which is usually a diode. Unfortunately, this approach has a number of off-chip components and often the detectors used for such an approach do not have the temperature stability of on-chip detectors.
Measuring of the VSWR is advantageous in that an amount of amplified RF signal power that is not coupled to the load is known and thus corrections to vary the RF output signal power are performable as a result thereof. A need therefore exists for measuring of power coupling between a PA output port and a load in the presence of supply voltage variations, temperature variations, and VSWR mismatch. It is therefore an object of the invention to provide a method and system of measuring power coupling between a circuit for emitting an amplified RF signal and a load coupled thereto.