Select an oscilloscope with sufficient bandwidth to accurately capture the highest frequency components of your signal.
As shown in Figure 1, all oscilloscopes exhibit a low-pass frequency response that rolls off at higher frequencies. Most oscilloscopes with bandwidth specifications of 1 GHz and below typically exhibit a Gaussian frequency response, which resembles the characteristics of a low-pass filter. The bandwidth of an oscilloscope is the lowest frequency at which the signal is attenuated by 3 dB. The attenuation of a signal at the -3 dB frequency is approximately -30% amplitude error. In other words, if a 1 Vp-p, 100 MHz sine wave is input into an oscilloscope with a 100 MHz bandwidth, the peak-to-peak voltage measured by the oscilloscope will be within 700 mVp-p (-3 dB = 20 Log (0.707/1.0)). Therefore, if the signal frequency is very close to the oscilloscope bandwidth, it will be difficult to make accurate measurements.
How do I determine the bandwidth required for a specific measurement application?
For full analog signal measurements, you need to select an oscilloscope with a bandwidth at least three times higher than the highest frequency of the sine wave you are measuring. The signal frequency is at least 1/3 of the oscilloscope bandwidth specification when attenuation is minimal. As the main application area of oscilloscopes today, how much bandwidth do digital applications need? As a rule of thumb, we recommend that you select an oscilloscope with a bandwidth at least five times higher than the fastest clock rate of the system being measured. For example, if the highest clock rate is 100 MHz, you should select an oscilloscope with a bandwidth of 500 MHz or higher. If this condition is met, your oscilloscope will be able to capture the 5th harmonic with minimal signal attenuation. The 5th harmonic is very important in determining the overall shape of the digital signal.
Figure 1: All oscilloscopes exhibit a low-pass frequency response that rolls off as frequency increases.
Figure 2: A 100 MHz clock signal captured by a 100 MHz bandwidth oscilloscope.
Figure 3: A 100 MHz clock signal captured by a 500 MHz bandwidth oscilloscope.
Figure 2 shows an example of a 100 MHz digital clock signal being captured and displayed using a 100 MHz bandwidth oscilloscope. In this example, the oscilloscope attenuates the high frequency signal portion and can only display the 100 MHz sine wave fundamental frequency of the signal. The oscilloscope bandwidth is insufficient to capture this signal. Figure 3 shows the same 100 MHz clock signal captured and displayed by a 500 MHz bandwidth oscilloscope. Using a higher bandwidth oscilloscope, we can see and measure more details of this digital signal.
Note that the rule of thumb of five times the signal frequency (oscilloscope bandwidth relative to clock rate) does not take into account signals with slower clock rates and faster edge speeds. These signals may contain frequency components far beyond the fifth harmonic.
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