Waveform acquired from scope, what is the sample rate for post processing?

Question

I want to make a noise measurement with my scope, but I need it to be band-limited.

I am using a Tektronix TBS1102, which allows me to save to a USB flash drive an Excel/CSV file with the X and Y values displayed on the screen. I can't get the output directly from the scope's ADC, but rather at the output of the signal displayed "on the screen" which has already been downsampled.

After I export this file, I import it into MATLAB so I can apply a digital FIR filter to limit the bandwidth, and then I am able compute the RMS value of the bandlimited noise.

The problem is that, as you know, scopes decimate the samples to be able to print them on the screen, my scope exports the Y values in a file with 2500 samples regardless of the time interval chosen for the horizontal scale.

My question is: does the sample rate, and therefore, the Nyquist frequency of the "screen" signal which is exported to the file, simply the inverse of the time span divided by the number of samples? Let me make an example to explain it better:

If the horizontal time interval is 0.5s, and the total file length is 2500 samples. This would mean that the sample rate would equal 5KHz, which would allow a maximum measured frequency of 2.5KHz, is the frequency content of the signal I am measuring actually band-limited to 2.5KHz? it doesn't seems to me to be band limited, because this would mean that the signal reduces its amplitude as you zoom-out the horizontal scale, and this clearly doesn't happen.

So what is the solution here? Select a smaller time span so the sample rate produces a higher Nyquist frequency than the one I am trying to limit the noise?

I guess that it all boils down to: what is the sample frequency of the signal displayed on the screen and the highest frequency of the signal being measured?

The oscilloscope is 1Gs/s with a 100MHz bandwidth, it doesn't have an RS232 or USB port

Answer 1

You will always get a buffer of 2500 samples, but the sampling rate used depends on the timebase used.

The exact sampling rate values for timebases are listed in the manual.

Answer 2

It would appear the TBS1102 is not a suitable scope for what you want to do:

(from here)

2.5 kpoint record length means it will only ever record 2500 data points, which is either a limitation of the software the scope runs, or of the hardware--it may only have enough sufficiently-fast RAM to record 2500 data points in a single trigger.

Regardless of the reason, though, it doesn't look like there's any way to get this particular scope to give you any more points than that. If you have access to a higher-end or even just more recent scope, try one of those--models with Mpoints of memory are now common, and some expensive scopes even have over a Gpt.