Minimum required sample rate for 1GHz bandwidth scope

When purchasing an oscilloscope to test new designs, the primary performance specification that most engineers consider first is the scope's real-time bandwidth. "Realtime" simply refers to the scope's ability to deliver the specified bandwidth on each and every acquisition cycle, or trigger event. In other words—real-time bandwidth is basically the same as single-shot bandwidth—not repetitive or equivalent-time bandwidth.

Closely related to a scope's real-time bandwidth- and also another key consideration—is the scope's maximum specified sample rate. Many engineers believe that higher sample rate scopes produce more accurate measurement results since more closely-spaced samples will provide increased timing resolution, and hence a more continuous waveform display. Although this is may be true for scopes that don't have the ability to perform digital waveform reconstruction/filtering, nearly all of today's digital storage oscilloscopes automatically perform sin(x)/x digital filtering to provide very high-density reconstructed samples based on the Nyquist sampling theorem.

Agilent Technologies' 1GHz bandwidth oscilloscopes in the InfiniiVision 3000 X-Series sample at 5 GSa/s in a half-channel interleaved mode, or 2.5 GSa/s when using all channels of the scope. This means that these scopes' sample rate-to-bandwidth ratio can be as low as 2.5:1. This may sound like a stretch to many, and would definitely put Nyquist to the test. So let's put the scope to a test by capturing some high frequency analog and highspeed digital signals that would stress any 1GHz bandwidth scope's measurement capabilities to its limits. We will then follow-up these empirical tests with the supporting theory behind it.

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Originally published by Agilent Technologies Inc. at www.agilent.com as "Minimum Required Sample Rate for a 1GHz Bandwidth Oscilloscope".