Calibration of Wideband Digital Real-Time Oscilloscopes

We describe a frequency selection algorithm for calibrating the impulse-response (vector frequency response) and timebase characteristics of digital real-time oscilloscopes (DRTOs). The objective of the algorithm is to allow the DRTO to be calibrated as closely as possible to a chosen frequency grid while optimizing oversampling, timebase fidelity, and minimizing the uncertainty contribution due to instrument impairments. Measurement examples are given for cases where the measurement frequency can be selected by the user and where this frequency is predetermined. The use of the Allan variance in the frequency-selection process is described, and an algorithm and an example for interchannel delay measurement are presented. We also discuss the merits of representing the calibration results either as a convolutional correction or as an impulse response.