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Oscilloscope input noise comparison
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2N3055:

--- Quote from: _Wim_ on December 28, 2018, 05:18:34 pm ---
--- Quote from: Andreas on December 28, 2018, 05:13:01 pm ---I guess that also a ferrrite on the USB-cable gives some improvement.

--- End quote ---

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.

--- End quote ---

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_:

--- Quote from: ADT123 on December 28, 2018, 08:10:13 pm ---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.

--- End quote ---

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.
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