Experience tells us that the bandwidth of the oscilloscope should be at least five times higher than the fastest digital clock rate of the system under test. If the oscilloscope we choose satisfies this criterion, the oscilloscope can capture the five harmonics of the measured signal with the minimum signal attenuation. The five harmonics of the signal are very important in determining the overall shape of the digital signal. But if accurate measurements of the high speed edge are required, this simple formula does not take into account the actual high frequency components contained in the rapidly rising and falling edges.

Formula: fBW is greater than 5 x FCLK.

A more accurate way to determine the bandwidth of an oscilloscope is to rely on the highest frequency in the digital signal, not the maximum clock rate. The highest frequency of a digital signal depends on what the fastest edge velocity is in the design. Therefore, we must first determine the rise and fall time of the fastest signal in the design. This information can usually be obtained from the open specification of the device used in the design.

Step 1: determine the fastest edge speed.

Then you can use a simple formula to calculate the maximum "actual" frequency component of the signal. Dr. Howard w. Johnson wrote a book about high-speed digital design on the subject. In the book, he calls this frequency component the "inflection point" frequency (fknee). All quick edge spectrum contains an infinite number of frequency components, but one of the turning point, or "knee"), higher than the frequency of the frequency components to confirm the shape of the signal is irrelevant.

Step 2: calculate the fknee.

Fknee = 0.5/RT (10-90%)

Fknee = 0.4/RT (20-80%)

For the signal that the uptime characteristic is defined in terms of 10% to 90% threshold, the inflection frequency fknee is equal to 0.5 divided by the rise time of the signal. For the signal that the uptime feature is defined in terms of 20% to 80% of the threshold (which is commonly used in today's device specification), the fknee is equal to 0.4 divided by the rise time of the signal. But be careful not to confuse the signal uptime here with the rise time of the oscilloscope. What we are talking about here is the actual signal edge velocity.

The third step is to determine the amount of oscilloscope bandwidth required to measure the signal based on the exact degree of time and time required to measure the rise time. Table 1 shows the relationship between the bandwidth of oscilloscope and fknee, which is required by the various precision requirements, for the oscilloscope with the gaussian frequency or the maximum flat frequency. It is important to remember that most of the oscilloscopes with a bandwidth of 1 GHz and below are usually gaussian, and the bandwidth over 1 GHz is usually the maximum flat frequency response.

Table 1: calculate the bandwidth required for the oscilloscope based on the required precision and the type of oscilloscope frequency response.

Step 3: calculate oscilloscope bandwidth.

Here's a simple example:

The minimum bandwidth required is determined for the oscilloscope with the correct gaussian frequency response when measuring the rise time of 500ps (10-90%).

The maximum actual frequency component of the signal (fknee) is about 1 GHz if the signal's rise/fall time is about 500ps(by 10% to 90%).

Fknee = (0.5/500ps) = 1 GHz.

If 20% of the timing error is allowed in the measurement of the time of rise and time of descent, then the oscilloscope with the bandwidth of 1 GHz can meet the requirements of the digital measurement application. However, if the timing accuracy is required to be within 3%, then it is better to use an oscilloscope with bandwidth of 2GHz.

20% timing accuracy:

Oscilloscope bandwidth =1.0x1GHz=1.0GHz.

3% timing accuracy:

Oscilloscope bandwidth = 1.9x1ghz =1.9GHz.

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