Calibration of an Oscilloscope-Based NVNA for Periodic Modulated Signals

This paper presents a new calibration procedure for oscilloscope-based nonlinear vector network analyzer (NVNA) for the acquisition of periodic modulated signals. This new calibration scheme is needed when the downconverted modulation products are themselves at too low of a frequency to be acquired by the NVNA. In such a case, the lowest RF frequency injected is then used as a reference signal instead of the fundamental beat frequency of the modulation. The standard procedures are used for the linear and power calibration of the NVNA at all of the harmonic and intermodulation products. However there exists a multiplicity of solutions for the harmonic and intermodulation phase reference (HIPR) given the beat fundamental frequency is a sub-harmonic of the reference frequency. A theory and methodology for the HIPR calibration and its subsequent validation are presented to resolve this phase ambiguity. The various HIPR calibrations are performed with a dual-channel calibrated arbitrary wave generator (AWG) which maintains a phase lock at the beat frequency without using a trigger. The calibration measurements are performed for 53 RF tones with a beat frequency of 5 MHz and reference frequency of 80 MHz to to subsequently perform real-time active loadpull (RTALP) measurements with gate and drain excitations at 95 and 100 MHz, respectively, for a device with nonlinearity order up to 7. The measured signals from the NVNA after calibration are experimentally verified to be in agreement with subsequent periodic modulated verification waveforms. The resulting calibrated oscilloscope-based NVNA system can then be used to acquire, beat period by beat period, a periodic modulated signal under either steady state or pulsed operation. An example of pulsed RTALP measurement for the acquisition of the DUT IV characteristics in $0.2 \mu \mathrm{s}$ is presented in the paper.