All oscilloscopes have a low-pass frequency response that rolls-off at higher frequencies. The lowest frequency at which your signal is attenuated by 3 dB is considered the oscilloscope’s bandwidth. At this frequency, your signal has approximately –30% amplitude error. This means, if you want to measure a 1 GHz signal, you can’t use a 1 GHz oscilloscope. You can’t expect to make accurate measurements on signals that have significant frequencies near your oscilloscope’s bandwidth. You need more oscilloscope bandwidth than the fastest frequency in your signal.

As a rule of thumb, your oscilloscope’s bandwidth should be at least five times higher than the fastest digital clock rate in your system under test. If your oscilloscope meets this criterion, it will capture up to the fifth harmonic of your signal with minimum signal attenuation. The fifth harmonic is an important component because helps determine the overall shape of your digital signals.

While the “5x” rule of thumb is a great guide for many applications, keep in mind that you will have to make other calculations if you want to make accurate measurements on high-speed edges. The 5x rule of thumb does not account for the highest-frequency components embedded in fast rising and falling edges. To find out the bandwidth needed for measuring high-speed edges, determine the rise and fall times of your fastest signals. You’ll likely have this information published in your application specifications. Using the rise and fall times, calculate the fastest frequency component in your signal. You can then find the required oscilloscope bandwidth based on the accuracy you need. All the equations you need for these steps, including modified equations for your required accuracy and the type of response (Gaussian or Maximally-flat) your oscilloscope exhibits, are described in this application note, Evaluating Oscilloscope Bandwidths for Your Applications.

Bandwidth needed for Analog Applications

If you are primarily testing analog signals, your oscilloscope’s bandwidth should be at least three times higher than the maximum signal frequency. You need to use the oscilloscope in the portion of the frequency band where it is still relatively flat with minimal attenuation to make accurate measurements on analog signals. At approximately one-third the oscilloscope’s bandwidth, a well-designed oscilloscope should have virtually no attenuation (0 dB).

However, not all scopes exhibit this type of response, so be careful when using the “3x” rule. An oscilloscope’s bandwidth specification, which is the 3-dB attenuation frequency, says nothing about the attenuation or amplification at other frequencies. So be sure to check the flatness of your oscilloscope’s response before you make your purchase. You can usually find this information in the datasheet. If it isn’t there, or you want to double-check the oscilloscope’s flatness, run a swept frequency response test.

Get more Bandwidth for Free

Higher oscilloscope bandwidth allows you to more accurately measure signals with higher frequency content. And when you are considering an oscilloscope for today’s applications, keep in mind your future measurement needs, which usually require additional bandwidth. To make oscilloscope bandwidth accessible, Keysight is running a limited-time promotion that puts more bandwidth in the instrument you need. Running August 1, 2019 – March 31^{st}, 2020, you can purchase a 4-channel InfiniiVision 3000T X-, 4000 X-, or 6000 X-Series mixed-signal oscilloscope (MSO) and receive a free bandwidth upgrade to the next-higher-bandwidth model.

Summary

For digital applications, you should select an oscilloscope that has a bandwidth at least five times higher than the fastest clock rate in your design. But if you need to make accurate edge-speed measurements on your signals, you will need to determine the maximum practical frequency present in your signal.

For analog applications, select a scope that has a bandwidth that is at least three times higher than the highest analog frequency of your designs. But this rule-of-thumb recommendation only applies to scopes that have a relatively flat response in their lower frequency band. This is something you won’t need to worry about with Keysight oscilloscopes.

A little extra bandwidth can go a long way in both measurement accuracy and ensuring your investment stays relevant for future test requirements. Make the most of your budget now while the “Supercharge Your Oscilloscope Bandwidth” promotion is available.

## Melissa Garza

Benchtop

## Supercharge Your Oscilloscope Bandwidth

2019-08-16 | 6 min read

Oscilloscope Bandwidth and Its ImportanceAll oscilloscopes have a low-pass frequency response that rolls-off at higher frequencies. The lowest frequency at which your signal is attenuated by 3 dB is considered the oscilloscope’s bandwidth. At this frequency, your signal has approximately –30% amplitude error. This means, if you want to measure a 1 GHz signal, you can’t use a 1 GHz oscilloscope. You can’t expect to make accurate measurements on signals that have significant frequencies near your oscilloscope’s bandwidth. You need more oscilloscope bandwidth than the fastest frequency in your signal.

For a limited time, get more bandwidth for the same priceBandwidth needed for Digital ApplicationsAs a rule of thumb, your oscilloscope’s bandwidth should be at least five times higher than the fastest digital clock rate in your system under test. If your oscilloscope meets this criterion, it will capture up to the fifth harmonic of your signal with minimum signal attenuation. The fifth harmonic is an important component because helps determine the overall shape of your digital signals.

While the “5x” rule of thumb is a great guide for many applications, keep in mind that you will have to make other calculations if you want to make accurate measurements on high-speed edges. The 5x rule of thumb does not account for the highest-frequency components embedded in fast rising and falling edges. To find out the bandwidth needed for measuring high-speed edges, determine the rise and fall times of your fastest signals. You’ll likely have this information published in your application specifications. Using the rise and fall times, calculate the fastest frequency component in your signal. You can then find the required oscilloscope bandwidth based on the accuracy you need. All the equations you need for these steps, including modified equations for your required accuracy and the type of response (Gaussian or Maximally-flat) your oscilloscope exhibits, are described in this application note, Evaluating Oscilloscope Bandwidths for Your Applications.

Bandwidth needed for Analog ApplicationsIf you are primarily testing analog signals, your oscilloscope’s bandwidth should be at least three times higher than the maximum signal frequency. You need to use the oscilloscope in the portion of the frequency band where it is still relatively flat with minimal attenuation to make accurate measurements on analog signals. At approximately one-third the oscilloscope’s bandwidth, a well-designed oscilloscope should have virtually no attenuation (0 dB).

However, not all scopes exhibit this type of response, so be careful when using the “3x” rule. An oscilloscope’s bandwidth specification, which is the 3-dB attenuation frequency, says nothing about the attenuation or amplification at other frequencies. So be sure to check the flatness of your oscilloscope’s response before you make your purchase. You can usually find this information in the datasheet. If it isn’t there, or you want to double-check the oscilloscope’s flatness, run a swept frequency response test.

Get more Bandwidth for FreeHigher oscilloscope bandwidth allows you to more accurately measure signals with higher frequency content. And when you are considering an oscilloscope for today’s applications, keep in mind your future measurement needs, which usually require additional bandwidth. To make oscilloscope bandwidth accessible, Keysight is running a limited-time promotion that puts more bandwidth in the instrument you need. Running August 1, 2019 – March 31

^{st}, 2020, you can purchase a 4-channel InfiniiVision 3000T X-, 4000 X-, or 6000 X-Series mixed-signal oscilloscope (MSO) and receive a free bandwidth upgrade to the next-higher-bandwidth model.SummaryFor digital applications, you should select an oscilloscope that has a bandwidth at least five times higher than the fastest clock rate in your design. But if you need to make accurate edge-speed measurements on your signals, you will need to determine the maximum practical frequency present in your signal.

For analog applications, select a scope that has a bandwidth that is at least three times higher than the highest analog frequency of your designs. But this rule-of-thumb recommendation only applies to scopes that have a relatively flat response in their lower frequency band. This is something you won’t need to worry about with Keysight oscilloscopes.

A little extra bandwidth can go a long way in both measurement accuracy and ensuring your investment stays relevant for future test requirements. Make the most of your budget now while the “Supercharge Your Oscilloscope Bandwidth” promotion is available.

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