Bandwidth

Errors in measurements occur when the frequency content (–3 dB point) of the signal being measured approaches or exceeds the bandwidth of the probe. The probe can be modeled as a low-pass filter for the signal. For example, if a 700 MHz probe is used to measure a 1 ns signal, the rise time error can be calculated using the following two equations. For this example, assume that the instrument bandwidth is great enough not to contribute any errors.

1. Calculate the rise time of the 700 MHz probe using the equation:

2. Calculate the rise time of the 1 ns signal as measured by the 700 MHz probe. Use the following equation where, t_r probe is the rise time of the probe and t_r signal is the rise time of the signal:


The measurement error between the actual signal and what was measured is 12%. To keep measurement errors less than 6%, use a probe with a bandwidth three or more times that of the signal.

3. Calculate the bandwidth of the 1 ns signal:


Use a probe with a bandwidth of 1.05 GHz (the rise time is 0.333 ns, equation 1).

4. Calculate the rise time of the 1-ns signal measured by the 1.05-GHz probe (equation 2).The measurement error is now less than 6%.