System Bandwidth at the Probe Tip

System Bandwidth at the Probe Tip

System bandwidth is the bandwidth of the instrument when a probe is used. This bandwidth greatly impacts measurements because the probe becomes part of the circuit being measured. The bandwidth of the combined instrument and probe system must be sufficient to accurately reproduce the input signal. Otherwise, time-interval measurements are inaccurate. For example, the rise time that is measured depends on

• the actual rise time of the input signal
• the rise time of the instrument/probe system

The following equation calculates the measured rise time:

where

t_r probe is the rise time of the probe and t_r signal is the rise time of the signal. If the instrument and probe have a combined rise time of 1 ns, and the signal also has a 1 ns rise time, the measured rise time is

The answer is in error by 41 percent. If the instrument and probe have a combined rise time of 330 ps, and the signal has a 1 ns rise time, the measured rise time is

Now the error is only 5 percent. For an error of less than 5 percent, the combined system rise time of the instrument and probe should be less than 1/3 the rise time of the measured signal. For an error of less than 1 percent, the combined system rise time should be less than 1/7 of the rise time of the measure signal.