How much noise floor and other things matter in oscilloscope usability

[bdunham7]

Well if we're having a noise contest, the new models are going to have to answer to the antiques.

In the DSO category, here's a Tek 2221A with a 150uV_rms signal:

And to take the cake in the CRO category, here is a Kikusui COS5100A with a 20uV_rms (-81dBm) signal at 1mV/div.  You can barely see it, but amazingly the trigger locks on steady.  I've never seen another full-bandwidth scope (100MHz in this case) that can do this.

[0culus]

Piece of cake for a 7A22 amplifier. 10 µV/div, adjustable bandpass from DC-1 MHz.

[2N3055]

How did you get so low noise waveforms? Did all you live in a faraday cage? 
I can't go so low with my 1104x-e as it pickup always random noises also with all things turned off in the room except the scope. It get also noises from the cooking gas sparks for starting the flame two rooms away from me (kitchen). I get the 34khz from the fluorescent lamp on the desk when powered on. The scope itself send a decent ammount of noise from the lcd panel (i think the lcd backlight buck is causing it), and you need to be very careful to the routing of the wire of the probe. Also if i connect the output of a battery powered signal generator, i can't reach those so low level of noise (still better than the fg fy6900, correctly grounded and with all-linear psu). In any case before the siglent i had an owon 7102... and hell, that thing was horrible in noise.

Good question..
Everything coaxial, 50Ohm, attenuators.. And relatively quiet lab. No WiFi or phones within 5 meters... And signal source was Picoscope 4262 internal low distortion AWG.

[2N3055]

Well if we're having a noise contest, the new models are going to have to answer to the antiques.

In the DSO category, here's a Tek 2221A with a 150uV_rms signal:

And to take the cake in the CRO category, here is a Kikusui COS5100A with a 20uV_rms (-81dBm) signal at 1mV/div.  You can barely see it, but amazingly the trigger locks on steady.  I've never seen another full-bandwidth scope (100MHz in this case) that can do this.

20uV RMS at full 100 MHz BW ? That is good!

Piece of cake for a 7A22 amplifier. 10 µV/div, adjustable bandpass from DC-1 MHz.

7A22 is special no doubt..

Picoscope 4262 has noise floor of 8uV if we talk 5 MHz BW max..
It can come close to 7A22.

But original discussion was for at least 20MHz BW...

[tautech]

Which initiates another question; what would the MSO5000 sampling drop to if one more channel was activated ?

Lower cost DSOs do that because they either have only one bank of acquisition memory with limited bandwidth shared between 4 digitizers, or because the digitizer is interleaved between the 4 channels.  The later is practically universal in lower cost DSOs.

Yet this is not the case for the 2 Siglent models that have been brought to this discussion whereas for the MSO5k AFAIK its 8GSa/s is shared by all 4 channels. OTOH SDS1104X-E and SDS2104X+ both have dual ADC's that give the user the opportunity to maintain high sampling rates when 2 channels are active by assigning them to each ADC that BTW each have their own memory support.

In the case of the screenshots previously posted with a single channel active the MSO5k is displaying 500MSa/s at the timebase selected whereas the SDS2104X+ displayed 400MSa/s.
Therefore activating any other channel should drop the MSO5k to 250MSa/s whereas if a second channel was assigned the the 2nd ADC on the SDS2104X+ its sampling rate and memory depth will remain unchanged.

As yet nobody has provided this info for MSO5k. 

[David Hess]

I duplicated your test as best as I could on my 2232.  Note that front end noise dominates digitizer noise at 2mV/div.  The third example shows peak detection with noise reduction applied.  I think the noise reduction algorithm is a noise gate.

[David Hess]

How did you get so low noise waveforms? Did all you live in a faraday cage? 
I can't go so low with my 1104x-e as it pickup always random noises also with all things turned off in the room except the scope. It get also noises from the cooking gas sparks for starting the flame two rooms away from me (kitchen). I get the 34khz from the fluorescent lamp on the desk when powered on. The scope itself send a decent ammount of noise from the lcd panel (i think the lcd backlight buck is causing it), and you need to be very careful to the routing of the wire of the probe. Also if i connect the output of a battery powered signal generator, i can't reach those so low level of noise (still better than the fg fy6900, correctly grounded and with all-linear psu). In any case before the siglent i had an owon 7102... and hell, that thing was horrible in noise.

Good question..
Everything coaxial, 50Ohm, attenuators.. And relatively quiet lab. No WiFi or phones within 5 meters... And signal source was Picoscope 4262 internal low distortion AWG.

I did the same but of course my 2232 has no 50 ohm input.  My signal source was a Tektronix FG502 11 MHz function generator set for minimum output with 50 dB of attention attached to its output.

If you look carefully, my examples show a discontinuity in the sine function output from common mode noise which I should have removed by using a higher output level and more attenuation.

[normi]

@Fiorenzo
You face the common issue most new scope buyers face; you want a scope that will do as much as possible for the amount of money you spend, unfortunately that is not the case. The MSO5000 will do things the Siglent can't and visa versa, although the Siglent is 40-50% more expensive. So lets look at the difference, noise floor Vs Sample rate.

Sample Rate:
It is horizontal resolution, it allows you to see finer details horizontally (time). It is an invisible feature, meaning that if your sample rate is low and causing an issue most cases you won't know. Aliasing is a visible issue if you are experienced enough to see it, but other critical flaws will be hidden. (You won't know that you don't know)

When most needed:
If there is a non repetitive and infrequent glitch in a signal that would not be sampled by the scope because the spacing between the samples is too wide (AKA low sample rate).

Impact:
You could go weeks trying to figure out a problem and will not realize that your circuit's failure is caused by the glitch that your slow scope is unable to find, this can be very costly in time.

Work around:
1. Obtain a high sample rate scope, all are very expensive (except the MSO5000).

Noise floor, small voltage scales:
 It is the vertical resolution (volts). It is very visible, which is why you came to this forum, so unlike sample rate you will be fully aware of what you can't see.

When most needed:
Looking at very low voltage signals.

Impact:
If your system is susceptible to very small noise signals or you work with tiny signals then you want as low noise as possible. Since you can clearly observe your scopes noise level you won't spend as much time searching as you would be aware that you can't see signals cleanly below a certain level. So not likely to have a dramatic impact as the unseen glitch, because you can quickly proceed to a work around if you want to investigate low noise.

Workaround:
1. Use averaging to clean up noisy signal
2. use an amplifier - some a very cheap and can be modified for Oscilloscopes or Spectrum analyzers, or can be built. Professional ones are very expensive but are usually used for differential measurements and are needed by even the scopes with low noise floor to see much smaller signals.
3. Obtain a scope with lower noise, in some cases these lower noise scope can be cheaper than the MSO5000

Outside of the fact that the RIGOL MSO5000 has a much lower price than the Siglent SDS - Plus, you could still buy a cheaper Siglent scope which would have the same noise performance as the SDS -Plus, however it is impossible to find another non Rigol 8G/S scope for under a $1000, or under $2000. This is the reason why the scope is attractive to many buyers.

[gf]

To challenge Rigol, here are 1.5mV_pp on a real toy scope
Relatively clean trace despite no averaging (not supported), but admittedly just 2MHz BW.
Frontend buffer seems to be just an off-the-shelf JFET opamp like LF356 or LF357, or similar.

[nctnico]

Noise floor, small voltage scales:
 It is the vertical resolution (volts). It is very visible, which is why you came to this forum, so unlike sample rate you will be fully aware of what you can't see.

When most needed:
Looking at very low voltage signals.

Buuzzzzz wrong! As I wrote before: noise floor simply scales along with the V/div settings. In that perspective using low level signals is not representative for regular scope usage. The Rigol MSO5000 also sucks for higher level signals because the noise will still drown details of the signal. Averaging won't help because that also obscures the details that you want to catch.

And having a higher samplerate gives you nothing if it is far beyond the bandwidth of your scope; it only wastes valuable memory. It just becomes a ridiculous number like having an 8000kW engine in a go-kart.

[gf]

And having a higher samplerate gives you nothing if it is far beyond the bandwidth of your scope

It helps insofar, as you can you turn on HiRes acquisition, and still end up with a relatively high decimated sample rate.
E.g. 8-tap HiRes boxcar averaging @ 8GSa/s gives a decimated sample rate of still 1GSa/s, and noise is reduced by a factor of ~2.8.
If the primary sample rate were only 1GSa/s, then the decimated rate were already as low as 125MSa/s.
Edit: And enabling more than 1 channel reduces the sample rater further.

[David Hess]

As I wrote before: noise floor simply scales along with the V/div settings. In that perspective using low level signals is not representative for regular scope usage. The Rigol MSO5000 also sucks for higher level signals because the noise will still drown details of the signal. Averaging won't help because that also obscures the details that you want to catch.

In most cases, the noise of the high impedance buffer at the input will dominate at the highest V/div sensitivities where attenuation immediately after the high impedance buffer is lowest.  For this not to be the case, their must be a gain stage following the attenuators which has higher noise.  This is not impossible, and could even be likely if the following gain stage is integrated CMOS instead of bipolar.

For the Rigol MSO5000 we could learn something from measuring the noise at all V/div settings.  If the high impedance input buffer noise is greater than the following stages, then there should be a jump to higher noise at about 0.1 V/div where the input attenuator is switched in, and the low impedance attenuators are switched out.

[Performa01]

While we would like the noise at low frequencies (<100 kHz) to be lower, yet it is certainly not high at 1 MHz and above. Very comparable with old analog scopes.

Here's a demonstration, what an SDS2000X Plus can show with an emulated ripple with asynchronous spikes riding on it if optimal probing (without additional noise pickup) is applied. 2.5 mVpp 1 MHz ramp with 300 µV 6.000001 MHz 10 ns wide spikes riding on it.

DSO Sensitivity is 500 µV/div, 10 bits mode, 100 MHz bandwidth. No averaging of course, in order to keep the spikes clearly visible.

SDS2354X Plus_Ramp_2.5mV_1M_Pulse_300uV_6000001Hz

[Fungus]

And having a higher samplerate gives you nothing if it is far beyond the bandwidth of your scope

It helps insofar, as you can you turn on HiRes acquisition, and still end up with a relatively high decimated sample rate.

The Rigol doesn't do HiRes mode though.

(unless they've added it and missed the memo)

[gf]

The Rigol doesn't do HiRes mode though.

I don't remember that I had seen a HiRes screenshot from the MSO5000 in this thread.

But the MSO5000 User Guide claims that it does:

High Resolution

This mode uses an over-sample technique to average the neighboring points of the sample waveform. This reduces the random noise on the input signal, generates a much smoother waveform on the screen and improves the vertical resolution. This is generally used when the sample rate of the digitalconverter is greater than the storage rate of the acquisition memory.

Note:
* The"Average"and "High Res"modes use different averaging methods. The former uses "Multi-sampleAverage"and the latter uses "Single-sampleAverage".
* In "High Res"mode,the signal bandwidth does not exceed 1/32 of the sampling rate.
* In "High Res"mode,the highest waveform refresh rate mode is not supported.

Edit: It does not tell the actual decimation factor or the number of averaged neighbor samples, though. A boxcar filter with 16 taps had a -3dB cut-off of ~fs/35, with 8 taps it were ~fs/17, and in order to get the documented fs/32, the closest number of required taps were 15. But this is pure speculation now and I think the "truth of the actual implementation" can only be determined experimentally by an owner.

Btw: One disadvantage of a boxcar averaging filter (and thus disadvantage of HiRes, if based on boxcar averaging) is that its sinc frequency response starts rolling off already beyond DC, i.e. the passband has no pronounced "flat top". If this matters for a particular use case, the filter cut-off should be rather chosen several times higher than the highest frequency of interest.

[Fungus]

The Rigol doesn't do HiRes mode though.

I don't remember that I had seen a HiRes screenshot from the MSO5000 in this thread.

But the MSO5000 User Guide claims that it does:

High ResolutionThis mode uses an over-sample technique to average the neighboring points of the sample waveform. This reduces the random noise on the input signal, generates a much smoother waveform on the screen and improves the vertical resolution. This is generally used when the sample rate of the digitalconverter is greater than the storage rate of the acquisition memory.

Note:
* The"Average"and "High Res"modes use different averaging methods. The former uses "Multi-sampleAverage"and the latter uses "Single-sampleAverage".
* In "High Res"mode,the signal bandwidth does not exceed 1/32 of the sampling rate.
* In "High Res"mode,the highest waveform refresh rate mode is not supported.

STOP THE THREAD!

I just downloaded the latest manual from Rigol and it says they've now added "High Res" mode.

HiRes will make a huge difference to the noise level by leveraging that massive 8Ghz sample rate.



Fiorino, we need another test with "HiRes" mode enabled. 

[2N3055]

STOP THE THREAD!

I just downloaded the latest manual from Rigol and it says they've now added "High Res" mode.

HiRes will make a huge difference to the noise level by leveraging that massive 8Ghz sample rate.



Fiorino, we need another test with "HiRes" mode enabled.

Yes, please, stop... 

He already did it. It didn't help, it simply low pass filtered out signal of interest. It hid it.

First thing, HiRes on MSO5000 has been around for some time now, and it's not big help...
They had to downsample 32 times to get something and then not much. HiRes makes it 250Msps/s single channel, down to 62.5Msps/s 3 or 4 channels enabled. 32 MHz max bandwidth at max sample rate, and if you grab longer sequences even lower.


You keep ignoring one fact (probably deliberately by now, because to you it's not about facts but about winning the argument even with lies and misdirection, apparently) that it is not about small signals per se but about details in signal that can be quite large.

High noise makes ENOB (effective number of bits) even less than already not very big 8 bits. So you see thick big sinewave on MSO5000, and a sinewave with a small squarewave superimposed on top on other scope that had low noise and had retained it's resolution better. Hi res scope would do even better, and that is why every manufacturer now is trying to go 10 or 12 or even more bits. It is not irrelevant.

MSO5000 is not useless. It's a scope that you can do work with. There are some usage scenarios when it's high noise won't stop you from doing your job. For some people that might be good enough. Mostly because if you don't know you have a problem, you cannot worry about it..

But, putting head in a sand doesn't make it better or equally good as some other equipment that actually have better specifications and can show signals MSO5000 cannot. Other instruments are literally much better for this kind of measurements. If you only need scope to decode 4 decodes at the same time, yeah, then MSO5000 will be better because other scopes mentioned don't have 4 decode channels. But mediocre analog performance didn't go away and suddenly became perfect. No, it is still mediocre but you don't care. Which is OK if you are FULLY AWARE of all pros and cons.

But fanboying like yours tries to make it like MSO5000 is equally good in EVERY parameter as other scopes. It is NOT.
If you're doing analog, there are better choices.

[Fungus]

But fanboying like yours tries to make it like MSO5000 is equally good in EVERY parameter as other scopes. It is NOT.

I never said that.

The title of this thread is "How much...?", you're the one burying your head and refusing to answer that question by fanboying the Siglents.

[tv84]

Here's a demonstration, what an SDS2000X Plus can show with an emulated ripple with asynchronous spikes riding on it if optimal probing (without additional noise pickup) is applied. 2.5 mVpp 1 MHz ramp with 300 µV 6.000001 MHz 10 ns wide spikes riding on it.

Nice example (pic worth a thousand words). How did you generate such signal?

[Martin72]

But the MSO5000 User Guide claims that it does:

Hi-Res mode was implemented by the second firmware update, AFAIK.
I´ve did some measurements, in one of them you could see that hi-res was active because of the reduced bandwith (squarewave measurement), the pics are somewhere in the rigol 5000 thread..

[Fungus]

Hi-Res mode was implemented by the second firmware update, AFAIK.

Yep, you're right.

So...

And, yes, Fiorenzo did post a screenshot of his test signal in Hires mode, my bad:


The manual says "In "High Res" mode, the signal bandwidth does not exceed 1/32 of the sampling rate" so it sounds like they do 32x oversampling with no user control. Not ideal.

You'd probably see the spikes on that signal if you zoom in a bit. They'd be much more visible in "peak" mode, too. I wish I had one here to fiddle with.

[Fungus]

Here's a demonstration, what an SDS2000X Plus can show with an emulated ripple with asynchronous spikes riding on it if optimal probing (without additional noise pickup) is applied. 2.5 mVpp 1 MHz ramp with 300 µV 6.000001 MHz 10 ns wide spikes riding on it.

Nice example (pic worth a thousand words). How did you generate such signal?

Any half decent signal generator will do it, including the one built into the Rigol. Just generate the two waves described above and modulate them. The trick is is in the "optimal probing".

Is there anybody here with a Rigol who can do generate the same test signal and optimize all the display settings to see how good it can be?

[2N3055]

Hi-Res mode was implemented by the second firmware update, AFAIK.

Yep, you're right.

So...

And, yes, Fiorenzo did post a screenshot of his test signal in Hires mode, my bad:

The manual says "In "High Res" mode, the signal bandwidth does not exceed 1/32 of the sampling rate" so it sounds like they do 32x oversampling with no user control. Not ideal.

You'd probably see the spikes on that signal if you zoom in a bit. They'd be much more visible in "peak" mode, too. I wish I had one here to fiddle with.

No you wouldn't see anything because it was filtered out.
And in Peak Detect mode you would see 10 mm thick solid wall of noise.. You don't seem to understand exactly how these acquisition modes work.

And I agree. We ALL would like if you could just maybe borrow one and play with it for a while. You would soon understand what we all are talking about.

I can't (or want to) speak for other people, but for me it is not about bashing Rigol. I have DG1062Z and it is very nice little AWG. I had DS1054Z too, and loved the little thing. Best thing after sliced bread at the time.
When Rigol announced new DS5000/7000 series, I was waiting at the door to buy DS7000.  Looked sooo good on paper. After it was released, I  tried it and month after I gave twice as much money for Keysight 3000T.
R&S RTM3000 was too expensive and had it's own share of problems. LeCroy was my first choice but I could not get good deal. Keysight was simply much better at business. They actually took effort to find something for me.
Big plus for Keysight then. Nowadays, it seems they would be same as LeCroy, and not care about small fish..

If SDS2000/5000/6000 from Siglent were available then, those would be my first choice, even compared to Keysight 3000T for many things..
Today, they would definitely be my first choice in their respective classes.
If Rigol releases MSO7000 MarkII with as low noise as Siglent, then I would be willing to reconsider.. Until then, that is my opinion.

[2N3055]

Here's a demonstration, what an SDS2000X Plus can show with an emulated ripple with asynchronous spikes riding on it if optimal probing (without additional noise pickup) is applied. 2.5 mVpp 1 MHz ramp with 300 µV 6.000001 MHz 10 ns wide spikes riding on it.

Nice example (pic worth a thousand words). How did you generate such signal?

Any half decent signal generator will do it, including the one built into the Rigol. Just generate the two waves described above and modulate them. The trick is is in the "optimal probing".

Is there anybody here with a Rigol who can do generate the same test signal and optimize all the display settings to see how good it can be?

No, that signal was created by combining two signals, either with digital combiner (Siglent AWGs have one) or by simple resistive combiner, that will serve as attenuator at the same time.

I use two two pass trough terminators  to 1k resistors from siggen, other side connected together, and grounded with 100 Ohm resistor. I have that one because we had some previous discussion about two tone testing, and someone did it with that one so I made it exactly like that so we can compare results...

[mawyatt]

Here's a demonstration, what an SDS2000X Plus can show with an emulated ripple with asynchronous spikes riding on it if optimal probing (without additional noise pickup) is applied. 2.5 mVpp 1 MHz ramp with 300 µV 6.000001 MHz 10 ns wide spikes riding on it.

Nice example (pic worth a thousand words). How did you generate such signal?

Any half decent signal generator will do it, including the one built into the Rigol. Just generate the two waves described above and modulate them. The trick is is in the "optimal probing".

Is there anybody here with a Rigol who can do generate the same test signal and optimize all the display settings to see how good it can be?

No, that signal was created by combining two signals, either with digital combiner (Siglent AWGs have one) or by simple resistive combiner, that will serve as attenuator at the same time.

I use two two pass trough terminators  to 1k resistors from siggen, other side connected together, and grounded with 100 Ohm resistor. I have that one because we had some previous discussion about two tone testing, and someone did it with that one so I made it exactly like that so we can compare results...

That might have been me since I was interested in the scopes DR performance. The 1K series R is to isolate the two AWG outputs from each other so they don't "see" the other signal as much with the ~26dB reverse isolation looking back from the shunt 100 ohm resistor. Was concerned about how the AWG output behaves in the presence of another signal and how this might affect the AWG output amp linearity. With this high an isolation the AWG output amplifier effects should be minimal and the resultant two tone IDM representing the scopes performance. BTW using the digital combining isn't as good, since the signal is created and then passed thru the AWG amplifier chain to the output, thus exposing the signal to the chains linearity effects. However may be OK since we are only looking at 65~75dB IMD with these scopes, but with Performa01 Picoscope results of ~100dB some of the AWG amplifier chain effects may contribute if using the AWG digital combining for the two tones.

Thanks to you, and a few others for pointing out the various features/limitations of the DSOs under consideration by means of actual "hands on" experience & usage rather than delusional speculation. Also don't think anyone is bashing Rigol, just pointing out features/limitations, seems not the case for Siglent which always seems to get bashed by some, especially when compared to the Rigol.

Personally feel bad for the OP and those that seek informative answers about these instruments, they shouldn't have to read and sift thru multiple threads and many hundreds of posts to get informative information.

Anyway, maybe a "Special Informative Instrument/Equipment Section" could be created where only those with actual "hands on" experience could comment, this would be highly beneficial to those seeking information to help decide on a purchase.



Best,