Some high-bandwidth oscilloscopes have upgradeable bandwidth, so, if 2-GHz bandwidth is sufficient for today, you can initially use an entry level 2-GHz scope and then upgrade all the way up to 13-GHz if you need additional bandwidth.
Analog Applications
Years ago, most oscilloscope vendors recommended that your scope's bandwidth should be at least three times higher than the maximum signal frequency. Although this 3X multiplying factor would not apply to digital applications based on clock rates, it still applies to analog applications, such as modulated RF. To understand where this 3-to-1 multiplying factor comes from, let's look at an actual frequency response of a 1GHz bandwidth scope.
Figure 7 shows a swept response test (20 MHz to 2 GHz) on a 1-GHz bandwidth oscilloscope. As you can see, at exactly 1 GHz, the input is attenuated by about 1.7 dB, which is well within the -3-dB limitation that defines a scope's bandwidth. However, to make accurate measurements on analog signals, you need to use the scope in the portion of the frequency band where it is still relatively flat with minimal attenuation. At approximately one-third the scope's 1-GHz bandwidth, this particular scope exhibits virtually no attenuation (0 dB). Recognize, however, that not all scopes exhibit this type of response.
The swept frequency response test shown in Figure 8 was performed on a 1.5-GHz bandwidth scope from another scope vendor. This is an example of a very non-flat frequency response. The characteristics of this response are neither Gaussian nor maximally-flat. It appears to be very peaked, which can result in severe waveform distortion for both analog and digital signals.
Unfortunately, a scope's bandwidth specification, which is the 3-dB attenuation frequency, says nothing about the attenuation or amplification at other frequencies. Even at one-fifth this scope's bandwidth, signals are attenuated by approximately 1 dB (10 percent). So, in this case, following the 3X rule of thumb would not be wise. When you are selecting a scope, it is a good idea to choose a reputable scope vendor and pay close attention to the relative flatness of the scope's frequency response.
For digital applications, consider selecting a scope with a bandwidth that is at least five times higher than the fastest clock rate in your design. However, 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 signal frequency in your designs. But this rule-of-thumb recommendation only applies to scopes that have a relatively flat response in their lower frequency band.
And when you are considering a scope for today's applications, don't forget about tomorrow. If your budget is flexible, buying a little extra margin today may save you money in the future.
References:
- For a deeper understanding of Nyquist's sampling theory, refer to Agilent Application Note 1587, "Evaluating Oscilloscope Sample Rates vs. Sampling Fidelity."
- High-Speed Digital Design, A Handbook of Black Magic, Howard Johnson, Martin Graham, 1993, Prentice Hall PTD, Prentice-Hall, Inc, Upper Saddle River, New Jersey 07458
- For more information about selecting the right oscilloscope bandwidth, view a video demo.
About the author
Johnnie Hancock is a Signal Integrity Applications Engineer within Agilent Technologies Electronic Products Group. He began his career with Hewlett-Packard in 1979 as an embedded hardware designer, and holds a patent for digital oscilloscope amplifier calibration. Johnnie is currently responsible for worldwide application support activities that promote Agilent's digitizing oscilloscopes and he regularly speaks at technical conferences worldwide. Johnnie graduated from the University of South Florida with a degree in electrical engineering. In his spare time, he enjoys restoring his 112-year-old Victorian home located in Colorado Springs. Johnnie can be reached at [email protected].
