Because the oscilloscope front analog parts present a low-pass filtering characteristics of the input signals of different frequency components through capacity, high frequency component amplitude attenuation rate than low frequency component of the attenuation rate, so the amplitude frequency response curve is not linear transformation, but showed a trend of roll down (RollOff), especially for higher than oscilloscope - 3 db bandwidth of frequency spectrum component showed a sharp roll-offs characteristics. According to different rolling modes of amplitude Response curves, currently the mainstream high-performance oscilloscopes in the industry mainly provide two types of Response, namely Flat Response and Bessel Response.
Flattening response has two major advantages.The first is that the amplitude-frequency response of the signal before the -3db bandwidth is relatively flat and the attenuation is small, which can be measured very accurately.Second, when the bandwidth exceeds -3db, the frequency response curve drops sharply and the high-frequency components are effectively cut off (showing a "brick wall" effect). All the passed low-frequency components can be sampled by the back-end ADC with high fidelity, which can greatly reduce the sampling aliasing opportunity in the digital oscilloscope and reduce the waveform distortion degree. Although the flatten response oscilloscope has these outstanding advantages, it also has a very significant disadvantage: because of the lack of more high-frequency components, the pulse of the fifth harmonic superposition has more overimpulse and ringing than the pulse of the 21st harmonic superposition.The flat response cuts off a large number of high frequency harmonics, thus showing relatively large overshooting and ringing phenomenon, especially when the signal rises quickly and far beyond the scope of accurate measurement of oscilloscope, this negative effect is more prominent.
The Bessel amplitude-frequency response has a relatively slow attenuation rate to the high-frequency components exceeding -3db bandwidth, thus showing a better impulse effect with smaller overshoot and ringing.However, since the response to signal amplitude is not very flat in the -3db bandwidth, and a long tail will be dragged out of the -3db bandwidth, the subsequent adcs need a higher sampling rate to ensure that frequency aliasing does not occur.