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Why do oscilloscopes have bandwidth limits?

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BillyO:
I can explain in one sentence as follows:

In real life we have things like stray capacitance, inductance and resistance coupled with power limitations.

TimFox:
Quoting from above: "Yes, but huge is not infinite. You can't use an analog cro to prove what happens at infinite bw."

David Hess:
Linear circuits have gain-bandwidth product limits.  Bandwidth can be increases by sacrificing gain, but then more stages are required and this quickly reaches diminishing returns.

I say linear circuits because *sampling* oscilloscopes avoid this limitation by sampling before amplification, and the result is massive bandwidth determined only by sampling strobe width.

kcbrown:

--- Quote from: David Hess on November 04, 2022, 12:33:04 am ---Linear circuits have gain-bandwidth product limits.  Bandwidth can be increases by sacrificing gain, but then more stages are required and this quickly reaches diminishing returns.

I say linear circuits because *sampling* oscilloscopes avoid this limitation by sampling before amplification, and the result is massive bandwidth determined only by sampling strobe width.

--- End quote ---

How does the A to D converter play into the bandwidth characteristics?  Surely it has some effect, no?

LaurentR:

--- Quote from: Anding on November 02, 2022, 06:36:08 am ---From experience how far can you use an oscilloscope beyond it's bandwidth limit?   Specifically, I'd like to compare the phase difference between in and out DDR2 data busses between an FPGA and a DRAM chip.  This is important because the input signals will need to be captured inside the FPGA using a clock with a phase delay to the output clock.  The slowest speed of the DRAM is 125MHz, which is starting to imply a 1GHz 'scope for many thousands of dollars.  Just for one measurement!  Will a 350MHz (soft upgraded) scope be any help to me at all?

--- End quote ---


If you have an FPGA, you may find that the easiest way to measure what you want to measure (e.g. tDQSCK seen from the FPGA I/Os) is probably to do it from inside the FPGA by using on-board circuitry. I am not sure what you're trying to do, but to a first degree, you should be able to make your circuit work without having to measure anything on the board with an oscilloscope.

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