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Why do oscilloscopes have bandwidth limits?

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Anding:
“… because an oscilloscope can be considered as a low-pass filter and this is the -3db cutoff frequency”, yes but why should it be the case that an oscilloscope is a low pass filter?

“… because it’s good for the market if manufacturers can segment buyers according to need”, yes but that’s a way of taking advantage of bandwidth limits - not a reason for them

If my scope can sample at, say, 8GSa/S and I am happy to sample my waveforms with 8 sample points each, why do I not have a scope capable of measuring (some aspects of) a 1GHz digital signal regardless of the scope “bandwidth”?

ataradov:
The fundamental reason is that designing a high bandwidth front end is not easy and there are physical limitations. There are further limits set by marketing, of course, but usually they are not too far away from the physical limits, especially for higher end models in the same range.

And of course it is possible to design 1 GHz front end, but that would put the whole device into much higher price bracket.

Anding:
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?

ataradov:
125 MHz square wave will look like a sine wave with 350 MHz bandwidth. The rest of it would be filtered out. Remember the bandwidth of the scope is stated for sines, and if you look at the spectrum of the square wave, only the first harmonic would fit.

And if you want to preserve the phase to that extent, you would need a way better equipment. This is not going to happen on consumer-level gear.

jonpaul:
Not a simple question!

SR, Bandwidth, risetime, aberration are all different but interrelated specifications.
Digital scope vertical BW is a spec for the amplifiers.
Read about  the theory of ADC, SR, BW.

Sample rate is not directly related, and there is a limit to the BW for a particular max SR, the Nyquist rate. BW < 1/2 SR.
The filter shape for antialiasing is the determinant.

See the Tektronix Measurement and circuits concepts book on vertical amplifiers.

Finally the best analog scopes are  TEK


The most unreliable specs are on the Chinese knockoffs.

We use Yokogawa DL and DLM series digital scopes, and TEK 2465/7/B, 7000 analog.

Jon



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