Agilent 5991-0662EN Bandwidth and Rise Time Requirements for Making Accurate Oscilloscope Measurements c2014 free download

File name 5991-0662EN Bandwidth and Rise Time Requirements for Making Accurate Oscilloscope Measurements c2014

Keysight Technologies Bandwidth and Rise Time Requirements for Making Accurate Oscilloscope Measurements Application Note Introduction How much oscilloscope bandwidth do you need and how fast does the rise time need to be to measure your signals accurately? Oscilloscope users ask this question regularly, but getting a good answer is typically dificult. The answer depends on the frequency response roll-off characteristics of the signal under test (SUT); it must be down about 15 db or more at the scope bandwidth. Most users don't know the roll-off characteristics of their signals, so this answer isn't likely to be useful. The roll-off characteristics will largely determine the amount of overshoot present in the step response of the SUT, so it may be more useful to have some "rules of thumb" for the bandwidth and rise time margin you need, based on the amount of overshoot. To that end, we ran simulations in ADS (Keysight Technologies, Inc. Advanced Design System) for two different cases: a step with no signiicant overshoot (a ifth-order Bessel response) and a step with about 10% overshoot (a fourth-order Butterworth response). The oscilloscope frequency response used for these simulations is the lat phase and magnitude response used in Keysight high performance oscilloscopes. These simulations were normalized to a 1-GHz bandwidth, but the results apply for any bandwidth. 03 | Keysight | Bandwidth and Rise Time Requirements for Making Accurate Oscilloscope Measurements - Application Note Signal under test with little or no overshoot in the step response Figure 1. 1-GHz scope step response and scope measuring a step with little overshoot In Figure 1, the red trace is the step response of a 1-GHz scope channel, the blue trace is the step response of a ifth-order Bessel ilter, and the magenta trace is the step response of the scope measuring the ifth-order Bessel ilter step. The bandwidth of the ifth-order Bessel ilter was adjusted as high as possible until the rise time of the scope measurement of the Bessel step (magenta) was within 3% of the Bessel step rise time (blue). Measurement results Scope rise time 434.7 pS Fifth-order Bessel rise time 678.0 pS Scope rise time measurement of ifth-order Bessel step 697.8 pS Error in scope measurement 2.911% Fifth-order Bessel rise time/scope rise time 1.560 The bandwidth of the Bessel ilter that limited the rise time error to 3% or less turned out to be 520 MHz. The frequency response of the Bessel ilter and the scope are shown in Figure 2. Note that the Bessel ilter is down -14.3 db at the scope bandwidth. 04 | Keysight | Bandwidth and Rise Time Requir