PULSED MEASUREMENTS Techniques for Pulsed S - Parameter Measurements

Many devices, particularly power devices, are not designed to operate continuously or with CW signals. This is especially true when devices are being tested on-wafer, where the thermal resistance is greatly increased [1]. In these cases, S-parameter measurements are best performed in a pulsed test environment. The details of pulsed measurement are greatly dependent on the pulse properties being studied. At one extreme is the realm of high pulse repetition frequencies (PRFs) and fairly narrow pulses, as is common in radar applications. At the other extreme is the communications arena where PRFs are quite low and pulse widths fairly wide (e.g., GSM [2]). These two extremes exemplify two techniques, termed bandwidth limited and triggered, that are discussed in this note. To a certain degree, the two approaches overlap in terms of allowed parameters, so most situations can be covered by one if not both of them. The objective of this article is to provide an understanding of general S-parameter measurements performed with a vector network analyzer (VNA), over a range of pulsed conditions for both RF and microwave/mm-wave measurement applications. Pulse profiling of the detailed transient response is not covered here although it is briefly discussed elsewhere with regard to triggered measurements [3]. Certain rise/fall time behaviors can be studied using time domain mode [4] but that will not be discussed either. The Spectra of Pulsed RF Signals As the reader may be aware, a pulsed RF signal will have a spectrum composed of a series of spectral lines with an envelope described by a sinc function [5]. The spacing of the lines is set by the PRF while the envelope shape is fixed by the pulse width (assuming rise and fall times are small relative to the width). The relationship is shown in Figure 1. The ‘size’ of this spectrum (occupied frequency range) with respect to the IF bandwidth (IFBW) of the network analyzer determines the measurement mode. In the case of low PRF and wide pulses, the entire spectrum can fit within an IFBW. In this case, the measurement proceeds normally without a significant reduction in dynamic range. Some additional smoothing/averaging may be needed to reduce effects of the outlying Pulsed measurement is an essential tool for measuring the performance of power amplifiers under low duty cycle conditions, including on-wafer test applications and high peak-to-average modulation formats 1/T0