Method to maintain RF power amplifier linearity in varying load VSWR conditions without isolator
The peak-to-average power ratio at the output of an rf power amplifier is detected and compared with, for instance, an ideal ratio. The result of this comparison is used to control the amplifier, particularly to reduce distortion in the output signal.
1. A method of controlling a radio frequency amplifier comprising:
sensing an input signal to the amplifier;
determining an input ratio of peak-to-average values of the sensed input signal;
sensing an output signal from a radio frequency amplifier;
determining an output ratio representing output peak-to-average values of the sensed signal;
comparing the output ratio with the input ratio; and
adjusting the output signal from the amplifier based on the results of the comparison.
2. The method defined by claim 1 , wherein each of the sensing steps comprises sampling an envelope of one of the input or output signal, and then computing from the samples the peak-to-average ratio.
3. The method defined by claim 1 , wherein each of the sensing and determining steps comprise:
detecting an envelope of the sensed signal;
determining an average value for the envelope;
determining a maximum value for the envelope;
converting the average value and maximum value to a digital signal; and
dividing the digital signal representing the peak by the digital signal representing the average to determine the peak-to-average ratio.
4. The method defined by claim 1 , wherein the sensed input and output signals are converted to digital signals which are then coupled to a digital signal processor (DSP), the DSP providing a control signal to a modulator, and where the DSP adjusts a modulated signal to control the input signal to the amplifier.
5. The method defined by claim 1 , wherein the sensed input and output signals are converted to digital signals which are coupled to a computational means for computing a control signal used to adjust the output signal from the amplifier.
6. A radio frequency amplifier including:
a first ratio determining means for determining a first ratio of the peak-to-average output signal from the amplifier;
a second ratio determining means for determining a second ratio of the peak-to-average input signal to the amplifier; and
control means for controlling the output signal from the amplifier based on the first and second ratios.
7. The apparatus of claim 6 , wherein the first ratio determining means includes:
a peak detector means coupled to receive the output signal from the amplifier;
an averaging means coupled to the peak detector means; and
an analog-to-digital converter coupled to the averaging means.
8. The apparatus of claim 7 , including an envelope detector coupled between the output of the amplifier and the peak detector means and averaging means.
9. The apparatus defined by claim 6 , wherein each of the first and second ratio determining means includes:
a peak detector means coupled to receive one of the input or output signals of the amplifier;
an averaging means coupled to the peak detector means; and
an analog-to-digital converter coupled to the averaging means.