What is the bandwidth of an oscilloscope?
Oscilloscope bandwidth refers to the input of a signal with the same amplitude and changing frequency, when the oscilloscope reading is attenuated by 3dB from the true value, the frequency at this time is the oscilloscope bandwidth. That is to say, the input signal in the oscilloscope bandwidth at the test value for the true value -3dB, bandwidth is not the highest frequency oscilloscope can display. In general, the oscilloscope bandwidth should be 3~5 times of the highest frequency of the measured signal.
Closely related to the oscilloscope bandwidth specification is its rise time parameter. Oscilloscopes with Gaussian frequency response have a rise time of about 0.35/fBW, measured on a scale of 10% to 90%, and oscilloscopes with maximum flat frequency response have a rise time specification in the range of 0.4/fBW, which varies with the steepness of the oscilloscope's frequency roll-off characteristics. If a timing error of 20% is allowed when making measurements of rise time and fall time parameters, then an oscilloscope with a bandwidth of 1 GHz will meet the requirements of this digital measurement application. However, if the timing accuracy is required to be within 3 per cent, then an oscilloscope with a bandwidth of 2 GHz is preferable.
How to choose oscilloscope bandwidth
Experience tells us that the bandwidth of an oscilloscope should be at least 5 times higher than the fastest digital clock rate of the system under test. If we choose an oscilloscope that meets this criterion, then the oscilloscope will be able to capture the 5th harmonic of the signal under test with minimal signal attenuation. The 5th harmonic of the signal is important in determining the overall shape of the digital signal. However, this simple formula does not take into account the actual high-frequency components contained in the fast rising and falling edges if accurate measurements of high-speed edges are required.
Formula: fBW ≥ 5xfclk
A more accurate way of determining the bandwidth of an oscilloscope is based on the highest frequency present in the digital signal, rather than the maximum clock rate. The highest frequency of the digital signal depends on what the fastest edge speed in the design is. Therefore, we first need to determine the rise and fall times of the fastest signals in the design. This information can usually be obtained from the published specifications of the devices used in the design.
