Don’t assume you are getting consistent amplitude measurements across your oscilloscope’s frequency range. The passband flatness specification is the consistency of amplitude measurements throughout the frequency measurement range of an oscilloscope. An oscilloscope with exceptional flatness will have little change in measured amplitude of the same voltage level at all frequencies it is specified to measure.
Figure 2. The typical frequency response of an oscilloscope (red) deviates greatly from the ideal, theoretical frequency response (grey) within the operating frequency of the oscilloscope, which can result in inaccurate measurements.
You can check this by looking at the frequency response Bode plot of the oscilloscope, signal-to-noise ratio or SFDR at multiple frequencies and observing the differences in the numbers, or the flatness specification of an instrument. An instrument with good flatness allows you to make measurements at any frequencies and compare data with minimal correction for frequency, which makes correlation easier and faster.
An example of an oscilloscope with excellent passband flatness is the 1.5 GHz, 10-bit PXIe-5162 oscilloscope. See the Bandwidth and Transient Response section of the PXIe-5162 specification document.