The validation requires disconnecting the cables from the oscilloscope’s channels 3 and 4 and connecting each to a Keysight N9010A spectrum analyzer. The results are shown in Table 1. Pout of the main channel shows minimal difference between measurement approaches, indicating that the power calculations based on the measured voltage waveforms are accurate. However, power levels in the adjacent channels differ by up to 1.7 dB, impacting ACPR (see Table 1). These discrepancies are attributed to the oscilloscope’s reduced dynamic range compared to the spectrum analyzer. According to the oscilloscope’s data sheet,27 the vertical resolution is 8 bits with averaging off and 12 bits with averaging on (used only for phase calculations), resulting in a theoretical dynamic range of approximately 48 dB and 72 dB, respectively. In practice, the oscilloscope’s dynamic range is also influenced by the vertical scale selected for the measurement; a lower V/div setting reduces the maximum voltage that can be measured without clipping.

Table 1


Also, the directional couplers that separate the incident and reflected waves have a coupling factor of approximately 35 dB, consequently attenuating the measured waves by 35 dB. This makes it more challenging to accurately measure lower power levels, such as those in the adjacent channels. This technique could benefit from using directional couplers with a lower coupling factor to overcome the disadvantage of an oscilloscope’s reduced dynamic range compared with equipment like a spectrum analyzer. Another option to improve dynamic range is to implement a statistical averaging technique.28

CONCLUSION

Load-pull measurements are performed with modulated signals using an oscilloscope, removing the need for complex equipment like an LSNA, NVNA or VNA. The proposed approach streamlines the calibration process by eliminating the need for prior tuner characterization, waveform corrections and a comb generator for phase reference. It provides accurate load-pull contours for output power, gain, drain efficiency, PAE and ACPR. Additionally, this work includes all the necessary details for setting up the measurement system and assembling the test bench. Experimental validation with an ATF38143 GaAs FET demonstrates the effectiveness of this method in optimizing the efficiency-linearity tradeoff in power amplifier design.

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