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Noise Floor of Oscilloscope and How to Specify DUT Noise.

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Mechatrommer:
related thread are perused... https://www.eevblog.com/forum/general-chat/rigol-1052e-noise-at-100mhz-3mhz-etc/msg4474/#msg4474 ...but this time its different, i dont believe its "outside radio interference" since i'm using 0ohm shorting BNC terminator (maintaining fully shielded input), but maybe something penetrating through/from inside the DSO body/circuit. Johnson Noise is also studied, 50ohm should give 9uV Johnson noise at 100MHz (4kTRB) thats why i think using 0ohm short terminator is wise to rule out Johnson noise at the input (but i maybe wrong that something else is not within my insight). opamp noise formula is also studied, but i prefer more practical way instead of relying on formula alone. from my calc, opamp noise is lower than i can measure, but i need a real life prove on it.

1st picture: apparently cleaner 0.8mVpp noise of Rigol DS1052E (hacked to DS1102E) is just deceiving from my observation, when not connected to anything.
2nd picture: connecting a simple such as T-connector increases the noise to 1.44mVpp.
3rd picture: putting a 0ohm (50ohm, 1Mohm terminator tried to the same result) shorting terminator, boom! 4.32mVpp noise!
4th picture: removing T, just direct terminator doesnt help much (3.6mVpp jumping up and down around the same 4mVpp)
5th-7th pictures: closer look at the offending 100MHz intermittent noise.
last picture: DS1102E frontend circuit as drawn by A-Hallene FWIW. thanks to him i hope he wouldnt mind i reproduce his work here.

now, this got to my attention when i tried to specify my circuit noise output (around 2.8-4mVpp read by the DSO). i thought its my circuit noise floor. but when doing the test above, now i cannot say about it. i simply cannot specify my DUT with this noisy system (DS1102E). So i'm opening for discussion, such as do you find this similar to your oscilloscope? what about the Agilent Infinity X000X? are they exhibit the same behaviour? any other DS1052-1102E can confirm this behaviour?

Is there anyway i can do to be able to specify my DUT with this noisy scope? is there any trick? or other more suitable device (preferably at hobby level budget? or pro device just for our knowledge) i'm aiming at DUT noise around 20µV (or at least at the DSO theoritical noise floor ie 800µV). so i see, from experience i'm going through, things get messy when we/i dealing with this micro level stuff. i also would like to hear comment from our admin Dave about this matter. how did he specify his µCurrent before he got his Agilent DSO. ISTR his µCurrent is using 100X gain amp. so 4mVpp back to 100X opamp can simply deceives us into believing that the opamp input is at 40µV noise level (4mVpp / 100). so any idea? friends and dave?

edit: i may find this thread also usefull... https://www.eevblog.com/forum/beginners/question-about-preamp-for-measuring-noise/ FWIW

jahonen:
Note that Johnson noise calculation gives RMS voltage (or standard deviation), not p-p. P-p of noise voltage is a tricky business to determine (although an estimate may be calculated from RMS value if the noise distribution can be assumed to be gaussian), RMS is much simpler. Also, noise bandwidth is generally not equal to -3 dB bandwidth, and is a topic of another discussion.

Regards,
Janne

Mechatrommer:
thanks janne for the reminder. yes i forgot it should be rms value. i tried 10x postamplifier as your suggestion in another thread on my circuit. and it showed 30-40mVpp, so i guess my circuit is indeed that... 3-4mVpp noise or 1.41mVrms (assuming Vrms = (Vpp/2)/sqrt(2).

free_electron:
Scopes dont have a large enough s/n or dynamic range for these kinds of measurements.
The 8 bit converter has a 10 millivolt resolution and for most low end scopes you only get 6 effective bits.
You need a spectrum analyser. those things have a much lower noise floor.

Mechatrommer:

--- Quote ---The 8 bit converter has a 10 millivolt resolution and for most low end scopes you only get 6 effective bits
--- End quote ---
i dont know how to calculate it, but how a dso cheat to show 0.8mVpp while not connected to anything, and then 4mVpp when shorted? as shown above?

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