Loading Effect Influence in Bandwidth Measurements for High Input Impedance Oscilloscopes

The influence of the loading effect in a bandwidth calibration setup for high input impedance oscilloscopes is described in this paper. First, a comparison between specified RC model input impedances versus actual measured values up to 300 MHz is done. Later in this paper, in order to avoid systematic errors in bandwidth measurements, different generator voltage corrections are considered depending on the measurement setup adopted. Also an improved measurement technique using a power meter is introduced in this paper. Finally, three oscilloscopes having nominal bandwidths of 60 MHz, 100 MHz and 200 MHz are measured using proposed correction methods against a typical bandwidth measurement where no voltage correction is applied at all. Results show that there is a bandwidth underestimation in the latter case, which varies in function of the oscilloscope under test.