Products > Test Equipment

Quality PC-Based oscilloscope?

<< < (10/10)


--- Quote from: nctnico on January 26, 2013, 04:18:03 pm ---To put it simple: if there is not enough noise the oversampling simply will not work.

--- End quote ---
of course you right here. I've made some other pictures - to finally end up the "discussion" -
and to show a nice Hi-Res trap:

Let's take a 1MHz sine wave (R&S SMX used for that), with amplitude high enough to not add
"any" trigger jitter (which could cange our results):

Tektronix TDS is using 2 vertical pixel (in 8bit mode) for one LSB. One DIV is 50pix high, zoomed 5 times
we have 16mV/40mVper DIV, so 0.4 x 10pix, this makes 4pixel in not zoomed which is equal to max. 2 LSB moise:

The zoomed sine wave looks of course not really clean:

With hi-res it looks better:

But here is the Hi-trap. I have enabled 20MHz BW filter (to have only max. 2LSB noise),
one can think "this will optimize the waveform", well no ... this kills H-Res, the waveform is less smooth:

as here where the full BW is enabled (sure, total amplitude deviation "seems" to be higher, but that't the
result of broader freq. specturm. The hi-res is giving smooth edges and fine steps between values, that's the deal):

The reason is of course the amount of noise, here we have 24mv/40mv per DIV, so 0.6 x 10pix, this makes 6 pixel
which is equal to 3 LSB of the waveform:

Therefore, properly used, hi-res make sense and it is possible.

--- Quote from: nctnico on January 26, 2013, 04:18:03 pm ---You can't change the laws of physics. Oscilloscope makers clearly cut a corner here.

--- End quote ---

Well, normally there will be always some noise available, if not - in most case - it is enough to disable all BW filters,
then you will pickup enough noise to let Hi-res work. In case this is still not enough, apply some random ^^

nctnico then:

--- Quote from: nctnico on January 20, 2013, 01:35:52 am ---... And the people from Picoscope think they can get 12 bit from an 8 bit ADC and they are able to get 250MHz into a 1M Ohm 15pf input.

--- End quote ---

nctnico now:

--- Quote from: nctnico on January 26, 2013, 04:18:03 pm ---To put it simple: if there is not enough noise the oversampling simply will not work. You can't change the laws of physics.

--- End quote ---

I'm glad to see that you now acknowledge that pico, or any other scope vendor can indeed achieve 12 bits resolution from an 8 bit converter through oversampling of the signal+noise typically present in broad band scopes.

You'll be relieved to know that neither I nor Agilent violated any laws of physics in capturing the images posted here:

No one is trying or claiming to violate any laws of physics. It is those very laws that explain why noise is proportional to bandwidth. In a broad band device like a scope, lack of noise is rarely if ever an issue. We should all be so lucky as to have scopes with such low noise levels that oversampling were not possible. I would make that trade in a heartbeat, but that option is not available. Scopes have noise. Manufacturers have wisely made good use of that noise to enable a very useful hires mode. But like many scope settings, if you try hard enough you'll find a way to get an invalid measurement.


[0] Message Index

[*] Previous page

There was an error while thanking
Go to full version