Oscilloscope Things - Oscilloscope Bandwidth

 

    Bandwidth determines the basic ability of an oscilloscope to measure signals. As the signal frequency increases, the oscilloscope's ability to accurately display the signal decreases. This indicator indicates the frequency range that the oscilloscope can accurately measure.

 

    The bandwidth of an oscilloscope is the frequency at which a sinusoidal input signal is attenuated to the signal's true amplitude, known as the -3dB point. This term is based on a logarithmic scale, as shown in the figure.

 

    The oscilloscope bandwidth is the frequency at which the sinusoidal input signal is attenuated to 70.7% of the actual amplitude of the signal, known as the -3dB point.

 

    Without adequate bandwidth, the oscilloscope will not be able to resolve high frequency changes. Amplitudes will be distorted, edges will disappear, and details will be lost. Without adequate bandwidth, all the power and bells and whistles of an oscilloscope mean nothing.

 

    To determine the oscilloscope bandwidth required to accurately characterize the signal amplitude in a particular application, the “rule of five” should be applied.

 

                           Oscilloscope bandwidth ≥ (highest frequency component of the signal × 5)

 

    An oscilloscope selected using the rule of five will provide an error of less than ±2% in the measurement, which is generally adequate for current applications. However, as signal speeds increase, this rule of thumb may not be true. Keep in mind that the higher the bandwidth, the higher the signal reproduction that can be achieved, as shown in the figure.

 

    The higher the bandwidth, the more accurate the signal reproduction. As shown in the figure, these are the signals captured at three bandwidth levels: 250MHz, 1GHz, and 4GHz.

 

    Some oscilloscopes offer a way to enhance bandwidth through digital signal processing. You can use a DSP arbitrary equalization filter to improve the oscilloscope channel response. This filter expands the bandwidth, flattens the oscilloscope channel frequency response, improves phase linearity, and achieves better matching between channels. It also reduces the rise time and improves the time domain step response.

 

Summarize

 

    This chapter introduces the bandwidth of the oscilloscope.