Oscilloscope input noise comparison

[2N3055]

I guess that also a ferrrite on the USB-cable gives some improvement.

I have a ferrite on both USB and power cable, but when I installed it, I did not see a whole lot of difference. I also looked at the common mode noise "delivered" via the power cable, but did not find any correlation between the spurs and the common mode noise. Should do the same for the USB also. I will try to post some results of this later on.

I did see a bit of improvement on 4262 by making two loops of USB cable trough ferite... But not much..

[ADT123]

The PicoScope 5000 contains 8 x ADCs each of which can sample at 125MS/s.  In 12 bit mode time interleaving is used to increase the sampling rate - matching between ADCs is not quite perfect hence the interleaving spurs.  These are small / within spec so this is normal.

In 14 bit mode each channel has 2 ADCs in parallel (to increase resolution / reduce noise) so there are no interleaving spurs.  In spectrum mode 14 bits is great unless you need > 62.5MHz. 

More generally for all scopes using FFTs its best to increase the number of points to spread the noise between as many bins as possible.  Also its best to measure to a higher bandwidth than you need.  Eg if measuring just audio to 20kHz then measure to say 10MHz and zoom in on the 0 to 20kHz range - this ensures higher frequency noise does not get folded back into the range you are measuring / displaying.  Add in some averaging and perhaps also the hardware bandwidth limit and its amazing what you can detect.

[_Wim_]

These are the common mode profiles for both USB and power cable. These look "bad", but this is due to the sensitivity of the used current probe (>40V/A from 10Mhz and up). When I compare this with other result I have got in the past, the peaks are quite low and also did not match with the 31.25MHz. 
The probe used is a Tegam 95242-1 (some model number is also sold by ETS-lindgren, I suspect they are identical).
I also made a comparison plot (with shorted input) when the Picoscope power adaptor was plugged into a very clean power outlet (isolation transformer output) and from a normal "noisy" power outlet. Some very small differences were seen. The wider trace is due to shorter averaging time. For me this means that the spurs are mainly internally generated by the ADC (because they practically disappear when the "precision 14 bit mode is used".
P.S: the above current probe is normally intended to inject common mode current, but so far I have no suitable high bandwidth power amp for this.

[_Wim_]

The PicoScope 5000 contains 8 x ADCs each of which can sample at 125MS/s.  In 12 bit mode time interleaving is used to increase the sampling rate - matching between ADCs is not quite perfect hence the interleaving spurs.  These are small / within spec so this is normal.

In 14 bit mode each channel has 2 ADCs in parallel (to increase resolution / reduce noise) so there are no interleaving spurs.  In spectrum mode 14 bits is great unless you need > 62.5MHz. 

More generally for all scopes using FFTs its best to increase the number of points to spread the noise between as many bins as possible.  Also its best to measure to a higher bandwidth than you need.  Eg if measuring just audio to 20kHz then measure to say 10MHz and zoom in on the 0 to 20kHz range - this ensures higher frequency noise does not get folded back into the range you are measuring / displaying.  Add in some averaging and perhaps also the hardware bandwidth limit and its amazing what you can detect.

Hi, thanks for your explanation, I didn't notice before you had posted also (got no notification some-one posted while I was typing). Great to have somebody (previously) related to Pico here on the forum.

Can you provide some more info on the differences in clock specs between the 5442B and the 5444B? Is it only because of the higher ETS sample rate?

[ADT123]

_Wim_ the crystal oscillator in the higher end model is simply a more expensive one that is calibrated at final test.  Other than the accuracy of frequency measurements there should be no other differences.  Other specs such as jitter are the same.

[_Wim_]

_Wim_ the crystal oscillator in the higher end model is simply a more expensive one that is calibrated at final test.  Other than the accuracy of frequency measurements there should be no other differences.  Other specs such as jitter are the same.

Thanks for this info, if it is only about absolute frequency accuracy, that less of interest to me.

[maxwell3e10]

I got a more recent model of Owon XDS3062A scope (due to a sale at Arrow.com). The input noise spectrum of it looks somewhat better than an earlier version of the same scope. White noise level is quite low at about 3.5 nV/sqrt(Hz). The rms noise is 46 uV and it remains the same up to full scale of 100mV, thanks to 12 bit ADC. But there is significant 1/f noise and a number of prominent noise  peaks, in particular one near 1.3 MHz. 

The scope also came with a built-in function generator and it has a basic frequency response analysis function (gain and phase) that seems to work OK. The function generator has the same clock as the scope, which can also have some advantages. Not bad for less than $200!

[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

And here's another one with the same settings:

8.4GHz - 50 Ohm - 248uV AC RMS - 2.4mVpp

8.4GHz data 50 Ohm

[Nanoman]

The Picoscope details I have seen show an integrated CMOS transimpedance buffer intended for DSOs from TI that by itself has 100 times the noise of a discrete front end at low frequencies.  I mean literally 1000nV/SqrtHz where a discrete design could be 10nV/SqrtHz.

So a broadband RMS noise of 125.7uV over 200MHz does not surprise me at all and that is about 5 times worse than my 40+ year old 200MHz analog oscilloscopes.

3000/5000 series ADA4817-1 (4nV), recent series of scopes. Obsolete versions (early 2000's) employed discrete jfet front ends.
2000 series          ADA4891-1 (9nV) or AD8065 (7nv) (25MHz versions)
4000 series          ADA4891-1 (8 channel scope) and AD8065 on older 20MHz scopes and 4817 on 16bit scope
6000 series          BF998 mosfet (~6nV, obsolete device), 6404 is ~12nv due to attenuator configuration

Either Analog Devices parts or discrete jfet/mosfets. The ADA4891 measures closer to 12nV due to 1/f contribution.

I used to design scopes at Pico and wanted to clear up this misinformation, noise is a critical scope spec and we'd use the best parts whenever possible.

[ILIJA]

Hi
Can you share script you used  to compute nV/sqrt(Hz)?
Thank you