Oscilloscope bandwidth definition
All oscilloscopes exhibit a low-pass frequency response that rolls off at higher frequencies as shown in Figure 1. Most oscilloscopes with bandwidth parameters of 1 GHz and below typically exhibit a Gaussian response with a slow roll-off starting at about one-third of the -3 dB frequency.
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Oscilloscopes with bandwidth specifications exceeding 1GHz typically have a maximum flat frequency response, as shown in Figure 2. This frequency response usually manifests itself as a flatter in-band response and a steeper roll-off at a frequency of about -3dB.
Each of these two frequency responses of an oscilloscope has its own advantages and disadvantages. An oscilloscope with a maximum flat frequency response attenuates in-band signals less than an oscilloscope with a Gaussian frequency response, which means that the former can measure in-band signals more accurately. However, an oscilloscope with a Gaussian frequency response attenuates out-of-band signals less than an oscilloscope with a maximum flat frequency response. That is to say, under the same bandwidth specification, an oscilloscope with a Gaussian frequency response usually has a faster rise time. However, sometimes attenuation of out-of-band signals can go a long way toward eliminating high-frequency components that may cause aliasing according to the Nyquist criterion (fMAX < fS). For a more in-depth discussion of Nyquist sampling theory, please refer to Agilent Application Note 1587 (Agilent Application Note 1587).
Whether the oscilloscope you have has a Gaussian frequency response, a maximum flat frequency response, or something in between, we consider the lowest frequency at which the input signal is attenuated by 3dB after passing through the oscilloscope as the bandwidth of the oscilloscope. The bandwidth and frequency response of the oscilloscope can be measured by sweeping the sine wave signal generator. The attenuation of the signal at the -3dB frequency of the oscilloscope can be expressed as an amplitude error of approximately -30% after conversion. Therefore, we cannot expect to make accurate measurements of signals whose dominant frequency components are close to the oscilloscope's bandwidth.
Closely related to an oscilloscope's bandwidth specification is its rise time parameter. An oscilloscope with a Gaussian frequency response has a rise time of about 0.35/fBW based on the 10% to 90% standard. The rise time specification of an oscilloscope with the largest flat frequency response is generally in the 0.4/fBW range, which varies with the steepness of the frequency roll-off characteristic of the oscilloscope. But what we must remember is that the rise time of an oscilloscope is not the fastest edge speed that the oscilloscope can accurately measure, but the fastest edge speed that the oscilloscope can get when the input signal has a theoretically infinitely fast rise time (0ps) . Although it is actually impossible to measure this theoretical parameter because the pulse generator cannot output a pulse with an infinitely fast edge, we can measure the rise of the oscilloscope by inputting a pulse with an edge speed that is 3 to 5 times the oscilloscope's rise time specification. time.
