Using Digitally-Modulated Signals to Measure the Gain Compression and Phase Distortion of a Radio Frequency Amplifier

Radio frequency amplifier linearity test data obtained from a network analyzer cw power sweep does not always match the amplifier's response to a digitally modulated carrier (which is the signal present in a communications system). This paper describes a DSP-based RF test technique which extracts the gain compression and phase distortion characteristics of an amplifier from measurements performed with a digitally-modulated test stimulus. The technique utilizes the magnitude variations inherent in phase-shift keyed (PSK) modulation to sweep the input power to an amplifier, instead of using a power sweep from a network analyzer. By downconverting the modulated signal and digitizing in the time domain, the magnitude and phase of the modulation may be measured at the input and output of the amplifier. Using the modulation instead of a cw power sweep to drive the amplifier into compression might produce better correlation between test results and actual operation, at least for PSK modulation schemes. We have demonstrated this new technique experimentally on a commercial power amplifier, where we compare network analyzer power sweep data to DSP-based results using BPSK signals at two different bit rates. For this amplifier at 1 kbps the DSP-based measurement of saturation agrees with the network analyzer measurement and demonstrates the validity of the new technique. At 100 kbps, the amplifier saturation behavior during BPSK operation differs from that measured on the network analyzer in two repects: the nonlinearity of the amplifier decreases at the higher bit rate, and the saturation shows significant time-domain hysteresis.