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Oscilloscope input noise comparison
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srce:

--- Quote from: maxwell3e10 on November 14, 2018, 03:17:17 am ---
--- Quote from: srce on November 13, 2018, 04:54:53 pm ---One other point to bear in mind is that the scope h/w does actually have >8GHz of b/w. Mine is just s/w limited to 2GHz.

--- End quote ---

This brings an interesting general point with the recent trend of software-upgradable scopes. When a scope is software limited in the maximum sampling rate and bandwidth, does it typically run the ADC at the maximum rate and then averages samples? Does it have a switchable analog front-end  filter?  It needs some type of anti-aliasing filter, both for noise purposes and to avoid signal aliasing.

--- End quote ---
For the S series the ADCs still run at 20GSa/s (with downsampling being selectable) - I think it's just a s/w low pass filter (similar to what a user can select for any lower frequency).


srce:
MSOX2024A - 200MHz - 2GSa/s - 1mV/div - 1MOhm

200MHz - 1.4mVpp - 174uV AC RMS

20MHz - 800uVpp - 119uV AC RMS

200MHz data

20MHz data


rf-loop:

--- Quote from: srce on November 14, 2018, 11:41:47 am ---MSOX2024A - 200MHz - 2GSa/s - 1mV/div - 1MOhm

200MHz - 1.4mVpp - 174uV AC RMS

20MHz - 800uVpp - 119uV AC RMS

200MHz data

20MHz data

--- End quote ---

My calculator give from your 200MHz data  168 Sdev (AC RMS) and 1.45mVpp from full data
But as we know Keysight (eta: some X models) use somehow decimated data for measurements when modern scopes use full acq mem data.

But then, also 1mV/div is not its most sensitive full resolution sensitivity. It is digitally magnified from 4mV/div what also is its most sensitive full resolution V/div. Perhaps it show near same noise with 4mV/div setting.
(it have only FullScale/64 resolution using 1mV/div aka 6 bit resolution)
Example if I set Siglent for 5mV/div and other ways comparable setting it have 134uV Sdev. But if use 1mV/div (also full resolution) it show 79uV Sdev
This is why I suspect that it (Keyshit) can not compare using 1mV/div to scope what have full resolution 1mV/div, least it is questionable.
srce:

--- Quote from: rf-loop on November 14, 2018, 12:44:25 pm ---My calculator give from your 200MHz data  168 Sdev (AC RMS) and 1.45mVpp from full data
But as we know Keysight use somehow decimated data for measurements when modern scopes use full acq mem data.

--- End quote ---
It depends on the scope, I think. The X uses what is on screen - the S can use all data.


--- Quote from: rf-loop on November 14, 2018, 12:44:25 pm --- Perhaps it show near same noise with 4mV/div setting.

--- End quote ---
Yes, it does.
maxwell3e10:
Here is a comparison of Keysight MSOX2024 and MSOX3104. They have very similar noise performance. Another peculiar thing is that MSOX3104 has virtually identical noise on the 1 MOhm and 50 Ohm setting. I guess it means that the scope doesn't have a different front end amplifier, just an internal 50 Ohm resistor switched across the input.
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