Bandwidth Oscilloscope bandwidth is defined as the frequency at which a sine wave input signal is attenuated to 70.7% of the signal’s true amplitude, known as the –3 dB point, a term based on a logarithmic scale. Bandwidth determines an oscilloscope’s fundamental ability to measure a signal. As signal frequency increases, the capability of the oscilloscope to accurately display the signal decreases. Without adequate bandwidth, high-frequency signals will be displayed with lower amplitude than the true signal. Rise and fall times will appear slower than the true signal. A common rule of thumb, to achieve better than 3% measurement accuracy, is to use an oscilloscope with five times more bandwidth than the fastest signal component you need to measure.