Highly Accurate Techniques to Measure SNR and Phase of Non-Linear RF Amplifiers

Abstract

Techniques for accurate measurements of the signal-to-noise ratio (SNR) and insertion phase of non-linear amplifiers will be discussed. These techniques remove the effect of linear and non-linear measurement equipment distortions in software. The vector demodulator (VDM) is the largest source of these linear and non-linear distortions. Data distortion also originates because of electromagnetic interference from the wall voltage. A description of the measurement equipment required for accurate SNR and phase measurements will be given. A non-standard piece of equipment required in this technique is an electronic phase shifter. The use of this electronic phase shifter allows the linear and non-linear VDM distortions to be characterized by sweeping the VDM through several different operating points. Details of the software algorithms for measuring SNR and phase will be discussed. The non-linear data analysis algorithm estimates SNR values in two steps: [11 Estimating the distortion levels by making a Fourier Transform of pulse-averaged I/Q values and [2] calculating an SNR value from amplitude and phase noise fluctuations derived from the measured I/Q values by using a small-signal expansion of the noise fluctuations about the non-linear VDM operating points. Phase measurements are also corrupted due to the presence of VDM distortions. We will introduce phase measurement techques that minimize the effects of these distortions. The presence of line voltage interference is detectable from the Fourier analysis of collected data. Experiments have indicated a detectable level of contamination at frequencies of 60 Hz, 120 Hz, and 180 Hz.