Oscilloscope input noise comparison

[nctnico]

Very interesting. I really wonder why they made this design decision. In my opinion it looks like this contradics the whole purpose of a high resolution mode.

Maybe it's related to their main business background - automotive. High resolution is not only for looking at very low voltages. Would be useful with x100 probe etc.

The problem is that the input attenuator will attenuate the signals before they hit the amplifier so you'll get the noise also at lower sensitivities. A high resolution ADC doesn't make sense with that much noise. You also have to read the datasheet for the Picoscopes very carefully. On some models the high resolution mode is nothing more than oversampling an 8 bit ADC while other models do have an ADC with more bits.

[MrW0lf]

The problem is that the input attenuator will attenuate the signals before they hit the amplifier so you'll get the noise also at lower sensitivities. A high resolution ADC doesn't make sense with that much noise.

Cannot agree, it does makes very much sense, just as stated, when poking LV & HV:

Reference, x100 setting, 100V/div, input shorted with 50Ω:

PicoScope 2205, 195.3kSa/s
8bit AC RMS = 1100mV
12bit (software) AC RMS = 80mV

PicoScope 2408B, 125MSa/s
8bit AC RMS = 2400mV
12bit (software)  AC RMS = 250mV

Analog Discovery 2, 320kSa/s
14bit decimate AC RMS = 990mV
14bit average (hardware*) AC RMS = 250mV

*ADC is sampling at full speed (100MSa/s) in background

Pico TA041 probe at x100 setting, 100V/div, 10x H/V zoom windows, observing mains:

PicoScope 2205:



8bit trace fairly stepped, 12bit (software) trace deforms shape due to bw hit. Instrument not very suitable for the task.

PicoScope 2408B:



Much higher sample rate & analog bw, 8bit trace drowned in noise. 12bit (software) processing works because of much larger recordset hiding bw hit. However response becomes a bit sluggish because PC has to crunch the numbers.

Analog Discovery 2:



Here can see why 14bit hw is superior despite very poor AC RMS noise performance (see reference). Darker green trace is decimated one. Noise is visible but UI response is very good because of native 14bit and no software processing. When turn on hardware averaging (bright green) trace clears up w/o noticeable performance hit.

So can conclude that native high bit scopes are very useful, even if their frontend is not low noise.

[Performa01]

Please someone correct me if I am wrong, but is the noise of the 5444 about five to ten times higher than the noise from the cheap Chinese scopes from the first page?
This makes little sense to me, since it has modes up to 16 bit resolution.
 

Well, these tests have been with just 8 bits resolution.

We need to be cautious to not compare apples with oranges. I have used the best of the “cheap Chinese scopes” and tried hard to resemble the test scenario for the PicoScope 5444 as close as possible, i.e. 1GSa/s, 1.4Mpts, 100µs/div and 2mV/div:


Siglent SDS1104X-E_100µs_2mV_100MHz_T50

Yes we get lower noise (63µV), but bandwidth is only 110MHz for this scope. The average noise density over the full bandwidth is about 6nV/sqrt(Hz) and I cannot see how this could be 5-10 times better than the ~9nV/sqrt(Hz) of the Pico 5444.

For the true high resolution models, like the 4262, we get much lower noise (4.33µV) because of the limited bandwidth, but also a lower noise density of <2nV/sqrt(Hz) at 2mV/div (zoomed to 100µV/div):


Pico 4262_Noise_10ms_2mV_5MHz_T50_Z100µV

The noise spectrum of the 4262 from 5Hz – 5MHz captured at a RBW of 14Hz looks like this:


Pico 4262_Noise_10ms_2mV_5MHz_T50_FFT

1/f noise is not very pronounced down to 50kHz and noise floor is below -156dBV (16nV) at 50kHz and even lower above. The strongest spurious signal measures -149.5dBV (32nV).

A closer look at the low frequency spectrum finally shows 1/f noise quite clearly:


Pico 4262_Noise_10ms_2mV_5MHz_T50_FFT_Z50kHz

We can see -156.5dBv (15nV) at 50kHz and -135.7dBV (164nV) at 100Hz. Given the FFT bin width of 4.768Hz and the flat-top window, we can expect a noise bandwidth of some 14Hz for the measurement.

This would result in 4nV/sqrt(Hz) at 50kHz and 44nV/ sqrt(Hz) at 100Hz, yet all these figures have to be taken with a grain of salt, because even at 5MHz, where we get -160dBV (10nV) the calculated noise density would still be ~2. 4nV/sqrt(Hz), which does not conform to the total noise density of <2nV/sqrt(Hz) as calculated earlier. I would tend to rather trust the FFT and believe that the RMS measurement in y-t mode might have given a slightly low reading.

[maxwell3e10]

Here is the comparison of PicoScope modes from Andreas as well as Rigol DS4014 data I got from rsjsouza.

As one might expect, at low mV/div setting the noise does not depend on the bit resolution, it is simply limited by the input amplifier noise.  The advantage of high resolution modes is that they would give the same noise even at higher mV/div setting (for example Owon 12 bit scope has similar noise on 5mv/div setting).

Also, small details, like how many divisions are in the full screen, do not matter when noise is limited by the input amplifier.

The 16 bit 200 MHz BW noise is higher due to aliasing with 62.5 MHz sample rate. I am surprised it is even allowed by the firmware.

[_Wim_]

Hi Maxwell3e10 and Performa01. Thanks for the plots above. Very interesting!

I am surprised it is even allowed by the firmware.

Yes, I am surprised about this to, I always thought that for reduced sample rates a corresponding hardware aliasing filter would have been activated. If only a single 20MHz filter is available, then only the 12-bit mode makes sense for the 200Mhz model.
Makes me wonder how they do the bandwidth limiting on the lower bandwidth models like mine? I always thought it was done in hardware, but seeing this that makes no sense. If it done in software, I normally should be able to create aliasing effect on the higher resolution modes. I will give that a try later today.


I wonder now if this is common on other scopes also. If four example you enable all channels, samples rate drops below nyquist for the lower end scope (ex. "upgraded" Rigol ds1054z has only 125MS/s for 100Mhz bandwidth"

[_Wim_]

Ok, done quick a few tests. Setup: CMU200 signal generator connected to Pico 5442B with 50 ohm input adaptor.

First: 8 bit mode (1GS/s), my scope has 60Mhz bandwidth

* reference signal at 5Mhz (500mV RMS)
* -3db found at 87Mhz
* -6db at 200Mhz
* -10db at 300Mhz
* -10db also at 400Mhz
* when switching to 200mV/div sensitivity, signal drops completely (this path has much lower bandwidth)

Second: 16-bit mode (62.5MS/s)

* reference signal at 5Mhz (500mV RMS)
* at 57.5 Mhz we see a 5 Mhz alias only 3db down (no bandwidth limit)
* at 57.5 Mhz with 20Mhz bandwidth limit we see a 5Mhz alias 10db down => 20Mhz bandwidth limit only 6db/oct or less filter slope?

So indeed much higher bandwidth noise can come back into the spectrum under test, and the software should at least active the 20Mhz bandwidth limit automatically when running at 62.5MS/s only.

[MrW0lf]

So indeed much higher bandwidth noise can come back into the spectrum under test, and the software should at least active the 20Mhz bandwidth limit automatically when running at 62.5MS/s only.

And what if you want to actually use alias?
The problem cannot be handled properly with 20MHz limit anyway so better leave it alone.
AFAIK they disabled FFT plot > official analog bw not to confuse dumbusers. So now you cannot look 101MHz peak on -3dB@~140MHz scope etc

[_Wim_]

So now you cannot look 101MHz peak on -3dB@~140MHz scope etc

Why not, you know were it will fold back to, so you still can but just need to calculate a bit...

[MrW0lf]

Why not, you know were it will fold back to, so you still can but just need to calculate a bit...

...with scope running at 1GSa/s? I would sort of get it if they handicapped dedicated FFT only which has auto control over sampling rate, but no. It's also imposed on FFT viewport. Also FFT is completely disabled for ETS So sorry but these boxes already have enough of dumb (or perhaps marketing derived?) limitations.

[_Wim_]

...with scope running at 1GSa/s? I would sort of get it if they handicapped dedicated FFT only which has auto control over sampling rate, but no. It's also imposed on FFT viewport. Also FFT is completely disabled for ETS So sorry but these boxes already have enough of dumb (or perhaps marketing derived?) limitations.

Ok, but now you are suggesting they deliver you a 500Mhz product for the price of a 60Mhz product, I am off course also a fan of that (but would be bad marketing).

I think almost all base scopes  are just a software cripled version of the high bandwidth model of the same series, this is not only so for pico, but is the case for all brands.

[_Wim_]

Here is the frequency response of the 20 Mhz BW Limit filter. Setup: Pico FFT with peak hold, CMU200 sweep frequencies between 10Mhz and 60Mhz.

[MrW0lf]

Ok, but now you are suggesting they deliver you a 500Mhz product for the price of a 60Mhz product, I am off course also a fan of that (but would be bad marketing).

But it is not the case. Scope still has unhackable analog bandwidth and FFT plot would drop accordingly. I know of no other (low end) scopes where FFT is artificially crippled like that. AD2 will honestly go to SR/2 so all the others I know of.
When using ref compensation one can flatten the plot nicely, but it would be still limited to reduced resolution according to the actual analog bw. So it's not like you suddenly getting more than you paid for. In Pico & FFT case you get less what you paid for if compare to other scopes. But thats OT - it's just a little soft spot suppose - I've hassled a bit too much to overcome various artificial limitations is PS6. To bad it does not have in-GUI scripting like AD2 which is real limitation-killer. Going SDK for each little hack is cumbersome.

Edit: So sorry if it did seem I jumped on you, it just "do not give them any more ideas what to limit" reaction

[srce]

Here are some results for a Keysight MSOS204A.

Vertical: 1mV/div - 8 divisions - 10bit
Horizontal: 5uS/div - 20GSa/s - 1Mpts

Yellow is 20MHz B/W (s/w filter) - 50 Ohm - 44uV AC RMS - 350uVpp
Blue is 2.1GHz B/W - 50 Ohm - 115uV AC RMS - 1.1mVpp
50 Ohm terminators on BNC inputs.

2GHz data 50 Ohm

20MHz data 50 Ohm

[maxwell3e10]

Thanks, srce, I was looking for data from a high-end Keysight scope. Are these with 50 Ohm impedance, as the screen shot says?

Could you also take data for 1 MOhm setting to compare.

[srce]

Yellow is 20MHz B/W (s/w filter) - 1MOhm - 140uV AC RMS - 1mVpp  - 1mV/div

Blue is 500MHz B/W (s/w filter) - 1MOhm - 355uV AC RMS - 3.3mVpp - 5mV/div (note b/w is limited to 200MHz at 1mV/div)

20MHz 1MOhm data

500MHz 1MOhm data

[rsjsouza]

maxwell3e10, thanks for adding the DS4014 to the graph. It seems a bit noisier than some of the other models (although the R&S seems unbeatable).

I forgot that you were measuring other modes as well. I can add five more measurements to the mix:

BW limit	Impedance
20MHz	50Ω
200MHz	1MΩ and external 50Ω terminator
200MHz	50Ω
500MHz (no limit)	1MΩ and external 50Ω terminator
500MHz (no limit)	50Ω

[egonotto]

Hallo,

@_Wim_: "Makes me wonder how they do the bandwidth limiting on the lower bandwidth models like mine?"

My PicoScope 100MHz 5243A has no bandwidth limit at all.

It has full 200MHz.

I ask this in the picotech forum:

https://www.picotech.com/support/topic33731.html

They don't explain what is matter.

Best regards
egonotto

[_Wim_]

But it is not the case. Scope still has unhackable analog bandwidth and FFT plot would drop accordingly. I know of no other (low end) scopes where FFT is artificially crippled like that. AD2 will honestly go to SR/2 so all the others I know of.
When using ref compensation one can flatten the plot nicely, but it would be still limited to reduced resolution according to the actual analog bw. So it's not like you suddenly getting more than you paid for. In Pico & FFT case you get less what you paid for if compare to other scopes. But thats OT - it's just a little soft spot suppose - I've hassled a bit too much to overcome various artificial limitations is PS6. To bad it does not have in-GUI scripting like AD2 which is real limitation-killer. Going SDK for each little hack is cumbersome.

Edit: So sorry if it did seem I jumped on you, it just "do not give them any more ideas what to limit" reaction

Ok, this makes indeed more sense. I also agree the software could use an update. I still like the pico overall, and have used it quite a bit. So far I did not use the SDK at all, but it is still nice to know this is an option.

[_Wim_]

My PicoScope 100MHz 5243A has no bandwidth limit at all.
It has full 200MHz.

That's nice!. I am also happy with only -6db @200Mhz for my 60Mhz scope. I am still wondering if there is actually a hardware difference between the scopes of the 5000 series (with same channel count).

[maxwell3e10]

Here is the plot for the Keysight MSOS204A. It is actually not that great compared with R&S scopes. At low frequencies, its even worse than low-cost scopes! I am surprised the noise is so poor on their "high definition" 10 bit scope.

Does anyone have any other high-end Keysight scopes to compare?

[Andreas]

Does anyone have any other high-end Keysight scopes to compare?

Unfortunately not; only a Tek 4104 MSO (from work).

The device has 1 GHz bandwidth and 5 GS/s maximum sample rate.
so this gives 20us/div * 10 div = 200 us for 1 MS on the x-axis.
the most sensitive range is 1mV/Div * 10 Div so +/-5mV. @ 8 Bit giving 40uV/LSB.

First set of measurements is done with internal 50 Ohms input.
with full 1 GHz BW and  250 MHz /20 MHz limited band width.

Interestingly the noise is much smaller with the 50 Ohms input (int) than with a external 50 Ohms terminator (ext).
(see next post).
So it seems that there are different signal paths for the 1 Meg and the 50 Ohms input resistance.

with best regards

Andreas

[Andreas]

And now the measurements with external 50 Ohms terminator. (End cap).

with best regards

Andreas

by the way: the RMS-value on the scope measurement seems to be including DC-Offset
If I calculate standard deviation the values are much lesser.

internal 50 Ohms:
1 GHz bandwidth      SD = 90 uV
250 MHz bandwidth  SD = 68 uV
20 MHz bandwidth    SD = 29 uV

external 50 Ohms: (1Meg input).
1 GHz bandwidth      SD = 136 uV
250 MHz bandwidth  SD = 108 uV
20 MHz bandwidth    SD =  46 uV

[maxwell3e10]

Here is a comparison of the 1 GHz+ scopes that I have so far. For full bandwidth I used 50 Ohm input, for 20 MHz bandwidth - 1 Mohm input. So far Rohde&Schwarz still looks the best.

[_Wim_]

Hi, could it be that in the above graph for the RTM3004 the 20Mhz curve is for 1M input, and the 1Ghz one is for 50 ohm? If not, why is the low frequency noise higher with the 20Mhz limit enabled? 

I should read also, and not only look at the picture... 

[JPortici]

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