How much noise floor and other things matter in oscilloscope usability

[Kleinstein]

The high res mode averaging helps with the white noise, like much of the noise behind the amplifier. With a BW limit of 70 to 350 MHz much of the noise below this would not be reduced from 2 to 16 times averaging and 32 fold averaging only just starts to reduce the input noise.

In this respect it makes some sense that they did not provide less averaging. The lower averaging steps may still help with the otherwise more favorabel settings (maybe 100 mV/div), where input noise is no such a factor.

[gf]

The high res mode averaging helps with the white noise, like much of the noise behind the amplifier. With a BW limit of 70 to 350 MHz much of the noise below this would not be reduced from 2 to 16 times averaging and 32 fold averaging only just starts to reduce the input noise.

In this respect it makes some sense that they did not provide less averaging. The lower averaging steps may still help with the otherwise more favorabel settings (maybe 100 mV/div), where input noise is no such a factor.

You are right, and this should not be neglected: An implicit analog BW limit of 70 to 350 MHz is always present in the AFE, depending on the model.
Edit: Possibly one should really measure the noise floor SPD at different settings, like Performa01 did for the Siglent.

[mawyatt]

No arbitrator, just only those with actual "hands on" experience with the equipment/instrument under consideration should comment. This way one can get information related to actual experience with such equipment.

That way you only get mostly information from people who are still on their -what I call- buyer's 'high'. On top of that, not everyone's needs are the same. What is a great tool for one, totally sucks for someone else. It would be a lot more useful to asses equipment using standarised testing but that takes a lot of time. All in all having a mix of owners and people who (based on experience with a wide range of equipment) know what works for a certain use case and what doesn't is a good compromise.

Maybe on a "buyers high", or maybe on a buyers remorse, all depends on how well the the item behaves, and maybe long after the purchase which is good indicator. Not all purchases turn out as expected!!

Regarding the requesters "needs", that would be up to the requestor to indicate such and the responses should be from folks with actual "hands on" experience with the equipment & the perceived "needs".

In the end, it's up to the requestor to sift through the fanboys/girls and chicken little responses for a reliable equipment assessment. However, this would be orders of magnitude easier for the requestor than through the countless pages and posts we've all experienced.

"Been there done that" when we wanted to consider a modern MSO/DSO, and it took some time and effort to figure out where the posts were coming from, who's behind them, what experience was involved and so on, and I personally feel that others should not have to suffer thru all this

Best,

[normi]

The OP had  already made up his mind when he made the post, he just wanted some reassurance. If someone thinks something matters then it matters even if it does not matter to others. And that basically applies to everything in life.
Most posters had no experience with the scopes and are just debating ideas, and yes all the information on the scopes have been posted many times in other threads.

Unfortunately there are no clear cut choices when buying and there are numerous parameters to consider, so the debates will continue.

[David Hess]

You are right, and this should not be neglected: An implicit analog BW limit of 70 to 350 MHz is always present in the AFE, depending on the model.
Edit: Possibly one should really measure the noise floor SPD at different settings, like Performa01 did for the Siglent.

As I pointed out earlier, any bandwidth limit, including high resolution mode, lowers the spot noise (1) but does nothing for the noise density.  For lower noise density, better or more suitable devices are required.

(1) Above the flicker noise corner frequency,  RMS Spot noise = Noise Density * Sqrt(Noise Bandwidth)

[Fiorenzo]

So, here we have a photo of the ripple with and with out ERES 3bit.
It seem that ERES avarages the signal removing all the visible spikes that was possibile to see with the Siglent due its lower noise front end.

Some information about the signal Is lost.

[bdunham7]

So, here we have a photo of the ripple with and with out ERES 3bit.
It seem that ERES avarages the signal removing all the visible spikes that was possibile to see with the Siglent due its lower noise front end.

Try increasing the memory depth to get the sample rate up, or else speed up the timebase to 1ms/div.  ERES shouldn't be averaging out your noise, but it will reduce the bandwidth.

[rf-loop]

It is also "averaging" but not simplest "boxcar" filtering as is in HiRes. ERES have "bell shape" filtering.
Also it acts like LPF as @bdunham7 told. (same but bit different  happen with HiRes where it is used)
In @Fiorenzo last image LPF -3dB corner is at around 3.2MHz.


Before this and that wild stories or wondering about ERES, it is good to understand basic fundamentals about it (and HiRes)

Here (Teledyne LeCroy  and it is enough compatible for also Siglent)

@Fiorenzo
For what is reason you display these horrible photographs instead of direct scope TFT screenshots in png format.

[Kleinstein]

You are right, and this should not be neglected: An implicit analog BW limit of 70 to 350 MHz is always present in the AFE, depending on the model.
Edit: Possibly one should really measure the noise floor SPD at different settings, like Performa01 did for the Siglent.

As I pointed out earlier, any bandwidth limit, including high resolution mode, lowers the spot noise (1) but does nothing for the noise density.  For lower noise density, better or more suitable devices are required.

(1) Above the flicker noise corner frequency,  RMS Spot noise = Noise Density * Sqrt(Noise Bandwidth)

Skipping samples to reduce the data rate instead of averaging (or a similar filter with slightly different weights) adjacent samples will cause some aliasing and this way also the measured noise density. It depends of the noise of the input stage on how important the later stage and ADC noise is.
In case of the Rigol sope it looks like most of the noise comes from the input stage, even though the ADC part is way higher BW than actually needed.

Especially for the lower BW versions a more conventional input stage would be a much better choice and not necessary that expensive.

[David Hess]

You are right, and this should not be neglected: An implicit analog BW limit of 70 to 350 MHz is always present in the AFE, depending on the model.
Edit: Possibly one should really measure the noise floor SPD at different settings, like Performa01 did for the Siglent.

As I pointed out earlier, any bandwidth limit, including high resolution mode, lowers the spot noise (1) but does nothing for the noise density.  For lower noise density, better or more suitable devices are required.

(1) Above the flicker noise corner frequency,  RMS Spot noise = Noise Density * Sqrt(Noise Bandwidth)

Skipping samples to reduce the data rate instead of averaging (or a similar filter with slightly different weights) adjacent samples will cause some aliasing and this way also the measured noise density.

Another way to say that is aliasing folds the noise bandwidth over below the Nyquist frequency increasing the noise density so that the spot noise remains the same.  This does not apply to ADCs in DSOs from Tektronix and Keysight and other high end manufacturers which perform noise shaping, but that does nothing for the noise from earlier stages.

It depends of the noise of the input stage on how important the later stage and ADC noise is.
In case of the Rigol sope it looks like most of the noise comes from the input stage, even though the ADC part is way higher BW than actually needed.

In an integrated CMOS design, it would not surprise me if the noise of the preamplifier following the input buffer is as high or higher in some cases.  Companies like Tektronix and Keysight can rely on exotic processes to achieve much better noise performance for a given bandwidth than available with CMOS.  Otherwise JFETs provide the lowest noise but are limited to lower bandwidth.

I wonder what the fastest available JFET is these days.  My Tektronix notes list the 2N5397 at 260 MHz but that was as of 1982, and faster JFET front ends even in 1984 used hybrid construction.

On the other hand, a Toshiba 3SK293 dual gate MOSFET comes out to 1.5 GHz so if someone wants to home build a faster front end, or an active probe, the parts are available.

Especially for the lower BW versions a more conventional input stage would be a much better choice and not necessary that expensive.

I hope they considered it and decided that their intended market did not require lower noise so it would have been an unnecessary expense.  Otherwise it is just bad design.

The best solution with the existing Rigol ASIC would be a separate input buffer with lower bandwidth and lower noise followed by a preamplifier with enough gain to overcome the noise of the following stages, but doing this would make the lower bandwidth models cost more to produce than the higher bandwidth models?  Rigol is making these things incredibly cheap.

[nctnico]

The best solution with the existing Rigol ASIC would be a separate input buffer with lower bandwidth and lower noise followed by a preamplifier with enough gain to overcome the noise of the following stages, but doing this would make the lower bandwidth models cost more to produce than the higher bandwidth models?  Rigol is making these things incredibly cheap.

IMHO too cheap. This thread is one of many examples where people returned their Rigol MSO5000 scope and bought something else. If they can bypass the problem with a different frontend then it would save Rigol's bacon because it is a relatively easy fix. But at this point it is unknown where the majority of the noise is coming from (fronted or ADC).

[Martin72]

This thread is one of many examples where people returned their Rigol MSO5000 scope and bought something else.

Like me..
And we shouldn´t forget, the much more expensive 7000 series is not better in this case.
And since more than 3 yrs it is crystal clear, that the noise is the showstopper - But no reaction from rigol, no improved hardware fix, nothing.
Why..

[Kleinstein]

I wonder what the fastest available JFET is these days.  My Tektronix notes list the 2N5397 at 260 MHz but that was as of 1982, and faster JFET front ends even in 1984 used hybrid construction.

There are some pretty fast FETs available for sat LNAs. As an example CE3512: they give a gain of 13 dB at 12 GHz.
Not sure if they are silicon or maybe some other material. The voltage is limited and they may not be suitable for a high impedance input stage, but fast JFETs are available.

The main ASIC in the Rigol scope seems to be OK and good for the price. The small front end ASIC seems to be more of a problem.
For the lower BW models even a front end like in the 1054 may be an improvement, and I doubt it would be much more expensive.
One may still need mode gain steps, as some of the Hitite ADCs are quite good and include some gain adjustment, that may not be available in the RIGOL ADC.

[David Hess]

I wonder what the fastest available JFET is these days.  My Tektronix notes list the 2N5397 at 260 MHz but that was as of 1982, and faster JFET front ends even in 1984 used hybrid construction.

There are some pretty fast FETs available for sat LNAs. As an example CE3512: they give a gain of 13 dB at 12 GHz.
Not sure if they are silicon or maybe some other material. The voltage is limited and they may not be suitable for a high impedance input stage, but fast JFETs are available.

I think I checked those out 20 years ago with that idea in mind.  They are gallium arsenide and among other deficiencies, have a gate leakage 100s to 1000s of times too large to be used for a 1 megohm input.  The highest performance active probes use them with an input divider to get a useful input voltage range, which explains why some active probes only have a moderate input resistance.  The same thing was also sometimes done with RF bipolar transistors in active probes.

[gf]

For the lower BW models even a front end like in the 1054 may be an improvement, and I doubt it would be much more expensive.

But BW were not upgradable then. Who buys a 8GSa/s scope with only 70MHz BW? I guess that many (most?) who buy the cheaper 70MHz model have the intention in mind to do a "free upgrade" to full 350+ MHz BW.

[David Hess]

For the lower BW models even a front end like in the 1054 may be an improvement, and I doubt it would be much more expensive.

But BW were not upgradable then. Who buys a 8GSa/s scope with only 70MHz BW? I guess that many (most?) who buy the cheaper 70MHz model have the intention in mind to do a "free upgrade" to full 350+ MHz BW.

I assume Rigol is grading their custom modules and placing the lower performance ones which would otherwise go to waste in their 5000 series, so 8 GS/s is essentially free.

[2N3055]

DS2000A had good 350MHz front end. That would be one to put into MSO5000, together with 50 Ohm path it doesn't have now.
That would make it a good scope, provided AFE is dominant source of noise.

But as it was pointed out, it is not clear where does noise come from exactly.  Looking at architectural diagram from Rigol, there is some buffering and amplification in ADC chip. Which is not a simple ADC chip, but like MegaZoom, it has some other functions inside too. Also some of the noise could come from ADC itself.. In diagram they claim some DSP capability on ADC chip itself. Maybe there is some digital correction for equalization and linearization? Are they loosing bits there?
We don't know..

I'm sure there must be a sweet spot in input settings where input is not attenuated and not heavily amplified. There must be a sweet spot where SINAD and corresponding ENOB is maximum. Measure that and ADC cannot be worse that that. It still can be a bit better if there are funny choices made in architecture, but cannot be worse. If those numbers are respectable for a 8 bit ADC, then you know actual front end before it is to blame..

[tv84]

This forum lacks Rigol insiders... 

One day we'll understand the delay on a major FW update for the MSO5000. Maybe they are trying their best to correct via software the HW shortcomings or the HW is a lost cause and may only serve as a "recycle bin" (as David Hess basically hypothesized) for their less performant ICs.

[Martin72]

Maybe they are trying their best to correct via software

Remember the thing, as there were no hi-res mode on both series, 5000 and 7000...That was already somehow suspicious.

[tv84]

Remember the thing, as there were no hi-res mode on both series, 5000 and 7000...That was already somehow suspicious.

I'm going out of my confort zone here but I suspect that is another consequence of the software uniformization between models... A feature not intended for this specific device got NOPed to make it work and, as such, little or just some psychological effect is visible.

[2N3055]

This forum lacks Rigol insiders... 

One day we'll understand the delay on a major FW update for the MSO5000. Maybe they are trying their best to correct via software the HW shortcomings or the HW is a lost cause and may only serve as a "recycle bin" (as David Hess basically hypothesized) for their less performant ICs.

Well, as many will agree, you can "improve" some things with software. But mostly it is more of "hiding" it with software, or extracting partial data by ignoring the rest.
There is no software algorithm that can "improve" noise on wideband signal and retain all of it's characteristics.

I would happily make measurements, but I don't own Rigol scopes in question and I'm NOT going to buy one just out of curiosity. Not that curious or rich.

As for chip binning I  wouldn't call it "recycle bin" exactly. Binnig for bandwidth is standard practice for high value chips.
Of course, unless you use chips "rejected" for high noise...

[mawyatt]

I wonder what the fastest available JFET is these days.  My Tektronix notes list the 2N5397 at 260 MHz but that was as of 1982, and faster JFET front ends even in 1984 used hybrid construction.

There are some pretty fast FETs available for sat LNAs. As an example CE3512: they give a gain of 13 dB at 12 GHz.
Not sure if they are silicon or maybe some other material. The voltage is limited and they may not be suitable for a high impedance input stage, but fast JFETs are available.

I think I checked those out 20 years ago with that idea in mind.  They are gallium arsenide and among other deficiencies, have a gate leakage 100s to 1000s of times too large to be used for a 1 megohm input.  The highest performance active probes use them with an input divider to get a useful input voltage range, which explains why some active probes only have a moderate input resistance.  The same thing was also sometimes done with RF bipolar transistors in active probes.

GaN devices might be an alternative today, haven't looked into them for anything like a scope front end tho. Believe Keysight uses a InP front end on their higher end scopes, the Keysight InP process we designed some test circuits with some time ago were 600GHz, today likely >1THz. Northrup Grumman developed a 1THz communication system over a decade ago with InP, so there's definitely some very fast 3/5 compound processes available. CMOS is also very fast today, but the breakdown*speed product isn't as good as InP or GaN, and of course there's SiGe.

Best,

[David Hess]

GaN devices might be an alternative today, haven't looked into them for anything like a scope front end tho. Believe Keysight uses a InP front end on their higher end scopes, the Keysight InP process we designed some test circuits with some time ago were 600GHz, today likely >1THz. Northrup Grumman developed a 1THz communication system over a decade ago with InP, so there's definitely some very fast 3/5 compound processes available. CMOS is also very fast today, but the breakdown*speed product isn't as good as InP or GaN, and of course there's SiGe.

With the exception of active probes, the existing high speed MMIC (monolithic microwave integrated circuit) processes are used at 50 ohms.  In general none of the advanced processes are used to make improved small signal discretes, at least not ones mere mortals have access to, which would replace the small signal silicon JFETs and depletion mode MOSFETs which are currently on the endangered list.  All of the action is with high power switching and RF.

I noticed yesterday that NXP is discontinuing practically all of their small signal JFETs and MOSFETs leaving one chopper JFET, one RF JFET, and two RF dual-gate MOSFETs.

I found a faster JFET, but it is an old one that is still nominally available.  Calogic, On, and InterFET have the J308–J310 which could be good to 500 MHz.

[mawyatt]

GaN devices might be an alternative today, haven't looked into them for anything like a scope front end tho. Believe Keysight uses a InP front end on their higher end scopes, the Keysight InP process we designed some test circuits with some time ago were 600GHz, today likely >1THz. Northrup Grumman developed a 1THz communication system over a decade ago with InP, so there's definitely some very fast 3/5 compound processes available. CMOS is also very fast today, but the breakdown*speed product isn't as good as InP or GaN, and of course there's SiGe.

With the exception of active probes, the existing high speed MMIC (monolithic microwave integrated circuit) processes are used at 50 ohms.  In general none of the advanced processes are used to make improved small signal discretes, at least not ones mere mortals have access to, which would replace the small signal silicon JFETs and depletion mode MOSFETs which are currently on the endangered list.  All of the action is with high power switching and RF.

I noticed yesterday that NXP is discontinuing practically all of their small signal JFETs and MOSFETs leaving one chopper JFET, one RF JFET, and two RF dual-gate MOSFETs.

I found a faster JFET, but it is an old one that is still nominally available.  Calogic, On, and InterFET have the J308–J310 which could be good to 500 MHz.

Believe you can get discrete GaN devices, these would be depletion mode types and maybe a "drop in" replacement for a JFET.

Best,

[David Hess]

I found a faster JFET, but it is an old one that is still nominally available.  Calogic, On, and InterFET have the J308–J310 which could be good to 500 MHz.

Believe you can get discrete GaN devices, these would be depletion mode types and maybe a "drop in" replacement for a JFET.

I was only able to find power devices.  Do you have a link?