Oscilloscope input noise comparison

[nctnico]

is it me or is the 20 MHz trace for the keysight showing a digital filter?

Well spotted and I agree that it looks like a simple digital filter.  y[n]= (y[n-1] * (a-1) + x[n]) / a.

[EEVblog]

New Rigol 5000 & 7000, 20MHz and full bandwidth

https://www.dropbox.com/s/hxzjzidrofd9eli/Rigol5000-Noise.zip?dl=0

[BravoV]

To OP, suggesting to update the 1st post with all accumulated result, rather than piling up at new post that is hard to keep up. Just add change list as a note.

[maxwell3e10]

Dave, thanks for posting. I looked at the files, they are taken with 200 MHz sampling rate, so the noise is pretty high due to aliasing. Could you take data at the maximum sampling rate of the scope, as the other ones are.

[rf-loop]

Dave, thanks for posting. I looked at the files, they are taken with 200 MHz sampling rate, so the noise is pretty high due to aliasing. Could you take data at the maximum sampling rate of the scope, as the other ones are.

Is it possible that your suspected aliasing rise this noise to this enormous level. Also there is 20MHz front end BW on.
(!) 163.9 uV stdev (AC-rms) and over 1.6mVp-p. (Dave's 5000-1-20.csv)


Is it made using 4mV/div?  (most low true full resolution)

Vertical resolution in file is  135.4uV

Rigol: Note[3]: 1 mV/div and 2 mV/div are a magnification of 4 mV/div setting. For vertical accuracy calculations, use full scale of 32 mV for 1 mV/div and 2 mV/div sensitivity setting.

[srce]

is it me or is the 20 MHz trace for the keysight showing a digital filter?

Well spotted and I agree that it looks like a simple digital filter.  y[n]= (y[n-1] * (a-1) + x[n]) / a.

You can choose between 20MHz or 200MHz h/w filter or s/w with any bandwidth. That was indeed with s/w filter.

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.

[2N3055]

Keysight MSOX3104T (1GHz 5/GSa/sec) and Picoscope 3406D (200MHz 1/GSa/sec)

50 Ohm terminator on input.

https://drive.google.com/open?id=1kZEO_hKSXGY_-ScaOQO3ydRLsjV2okpD

Regards,
Sinisa

[maxwell3e10]

It looks like the noise on 50 Ohm setting and 1 MOhm setting is identical for Keysight MSOX3104T. Just want to make sure its not a mix-up of files.

[maxwell3e10]

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.

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.

[2N3055]

I checked, Std Dev on those files shows lower RMS noise on 50 Ohm.  Cca. 326uV RMS vs 343uV for 1GHz bandwidth (50 vs 1M) and 112uV vs 176uV for 20MHz bandlimited. I believe they should be right.

[srce]

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.

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.

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]

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

200MHz - 1.4mVpp - 174uV AC RMS

20MHz - 800uVpp - 119uV AC RMS

200MHz data

20MHz data

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]

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.

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

Perhaps it show near same noise with 4mV/div setting.

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.

[TheSteve]

On the Keysight 3104 the 50 ohm path is a different input on the front end ASIC.

[2N3055]

On the Keysight 3104 the 50 ohm path is a different input on the front end ASIC.

As I stated above, there is visble change in RMS noise level when changing from 50 to 1M , but it is not great...

I will be traveling for few days. When I get back I will repeat measurements.
I would like to see somebody's else 3104T noise figures...
Maybe my 3104 is noisy.

[TheSteve]

I can post mine but it will need a big star next to it as it's a MSOX3024t modified to 1+ GHz.

[ci11]

Teledyne LeCroy HDO4032 12-bit 350MHz, no averaging, no ERES.

DC50 Ohm

 20MHz                   62,90 µVrms   312,44 µVpp
 350MHz               124,37 µVrms   847,66 µVpp

DC1M Ohm

 20MHz                   83,95 µVrms   313,00 µVpp
 350MHz               165,33 µVrms   899,47 µVpp

DC50 Ohm data:
 
 20MHz BwL DC50 Ohm
 350MHz DC50 Ohm

DC1M Ohm data:

 20MHz BwL DC1MOhm
 350MHz DC1MOhm

A couple of interesting points from the FFTs: the 20MHz BwL seems very gentle but still effective as can be seen in the numbers, and for a 350MHz scope, it appears to have BW well beyond 1GHz. Perhaps the folks at LeCroy do not believe in brickwall filters.

[rf-loop]

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.

Of course these graphs and tables and curves are nice and useful.
Btv, is it possible to keep all graphs vertical scale and position in scale same for more quick and easy compare without thinking. 

But then also, what we do IRL. We look oscilloscope screens and it also matters what we really see (and measure).

Like these:
first @srce previously published  MSOX2024A - 200MHz - 2GSa/s - 1mV/div - 1MOhm
then same settings with SDS1204X-E. same BW, 1M input, 5us/div  but samplerate 1GSa/s (and memory of course due to samplerate and time scale "equal")

Not very easy imagine what is visible on the screen if only look V/(Hz^1/2) graphs.

[maxwell3e10]

Here is a comparison of the LeCroy HDO scope with the R&S RTM scope. On the LeCroy scope the noise is identical for 1 MOhm and 50 Ohm paths.

[ci11]

@maxwell3e10

Thanks for the comparison - would you mind adding the RTM 20MHz BW plots to the LeCroy HDO4032 vs.RTM 3004 chart?

[JPortici]

what's that peak? at.... 625MHz? only with 1Meg?

[ci11]

what's that peak? at.... 625MHz? only with 1Meg?

Sorry - cannot tell you. Same peak at 625MHz shows up in both 50R and 1M FFTs. Probably something LeCroy put in to deter overusing the BW actually paid for, but keep the roll-off gentle to reduce outband noise.

[JPortici]

My first guess is that it's correlated with the sampling clock (2.5GS/s -> 4 interleaved ADCs at 625MS/s)
that wouldn't be so strange but geez.. i would have expected better.
Shall i power up my ancient boat anchor and try to get some data, just for kicks ?