What is the oscilloscope bandwidth limit?
In the channel button of the instrument, you press the CH1 button, the menu should have the option of bandwidth limiting.
It is mainly used to filter out high-frequency noise, turn on the bandwidth limit, then the oscilloscope bandwidth will be limited to 20MHz and no longer the nominal bandwidth. This is good for measuring small amplitude signals and signals with high interference.
The two types of oscilloscope frequency response each have their own advantages and disadvantages. Oscilloscopes with a maximum flat frequency response have less attenuation of in-band signals than oscilloscopes with a Gaussian frequency response, which means that the former can measure in-band signals more accurately. However, oscilloscopes with a Gaussian frequency response have less attenuation for out-of-band signals than oscilloscopes with a maximum flat response, which means that oscilloscopes with a Gaussian frequency response usually have faster rise times for the same bandwidth specification. However, sometimes a large attenuation of out-of-band signals can help to eliminate high frequency components that may cause aliasing according to the Nyquist criterion (fMAX < fS).
Whether you have an oscilloscope with a Gaussian frequency response, a maximum flat frequency response, or something in between, we consider the bandwidth of the oscilloscope to be the lowest frequency at which the input signal passes through the oscilloscope and is attenuated by 3 dB. The bandwidth and frequency response of an oscilloscope can be measured by sweeping with a sine wave signal generator. The attenuation of the signal at the oscilloscope's -3 dB frequency can be converted to an amplitude error of about -30%. Therefore, we do not have the luxury of making accurate measurements on signals whose major frequency components are close to the oscilloscope's bandwidth.
Closely related to an oscilloscope's 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 that is generally in the 0.4/fBW range, which varies with the steepness of the oscilloscope's frequency roll-off characteristics. However, it must be remembered that the oscilloscope rise time is not the fastest edge speed that can be accurately measured by the oscilloscope, but rather the fastest edge speed that can be obtained by the oscilloscope when the input signal has a theoretically infinite rise time (0 ps). Although in practice this theoretical parameter is impossible to measure because a pulse generator cannot output a pulse with an infinitely fast edge, we can measure the oscilloscope's rise time by inputting a pulse with an edge speed that is three to five times the oscilloscope's rise time specification.
Oscilloscope Bandwidth Required for Digital Applications
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 ≥ 5 x fclk 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.
