Products > Test Equipment

Dithering on 8-bit scope?

<< < (4/5) > >>

EggertEnjoyer123:
My Lecroy Waverunner oscilloscope can get up to 3 bits through averaging (at a significant cost to bandwidth).

Interestingly the FFT isn't different (at least at frequencies below the cutoff). This is expected because the extra bits are obtained from filtering (so all the lower frequencies are still let through). So if you are looking for a signal at a low frequency that's obscured by the noise... well you're out of luck.

Edit: Oops I forgot to turn off the bandwidth limit

Berni:
Did a quick test and it looks to reduce noise across the board on the Infiniium scopes. I fed it a tiny -40dBm sinewave to make an example of a case where you are not far from the noise floor.

It looks lower all over, even removes some of the spurs (tho this is more likely due to the ADC running faster)




Here is an example of squeezing the most dynamic range out of it. it is being fed a +6dBm sine wave while the noise floor is down near -130dBm. Tho most of this comes from doing a huge FFT window (as you can see it captured 15 seconds of waveform, and yeah you can get a cup of coffee while you wait for it to calculate). This scope has a very good FFT implementation. Tho the harmonics are likely mostly coming from the scopes front end, no amount of averaging will remove non linearity errors.


And as for just how it looks like, here is the sine wave zoomed in. This is with bandwidth wide open to 500MHz (max it can do in 12bit high res). As you can see the waveform is razor sharp, but still has some fuzzynes around it (since it is not just a 1 pixel wide line like you would get with averaging)


Someone:

--- Quote from: Berni on August 22, 2023, 11:52:10 pm ---Did a quick test and it looks to reduce noise across the board on the Infiniium scopes.
--- End quote ---
Note the acquisition sample rate is lower than the peak ADC sample rate, so there is "excess" sample rate in that case to reduce the noise. The reverse explanation is the "normal" mode is using decimation from the ADC to the acquisition memory and throwing away data.


--- Quote from: Someone on August 22, 2023, 02:25:42 am ---
--- Quote from: switchabl on August 21, 2023, 04:43:41 pm ---High resolution/ERES can be quite useful in some cases but it is not a replacement for native 12bit scopes. It does nothing to improve linearity and spurious tones and it costs a lot of bandwidth. As a rule of thumb, to gain 1 bit/6dB in SNR, you need to down-sample by a factor of 4.
--- End quote ---
in some cases it won't even consume bandwidth, as the ADC sample rate could be high enough to grab a few bits for "free" (Keysight 50x oversampling that was described as "excessive" by a particular opinionated poster).

--- End quote ---

Berni:
Yes good point, In this case high res mode was actually working with more samples.

I tried it again on a 10MHz sinewave while forcing it use as high of a sample rate as possible. Indeed it leaves the FFT noise floor untouched, just cutting it off above 500MHz.

However these was still something to gain from it. The FFT slows down a lot when it has to compute captures this big (10s of Mpts). So while around the 10 MHz sine wave the FFT looks identical, the scope could compute it in about 0.5 s using highres mode, but it takes an agonizing 10 s to update in normal mode(since it has way more samples to number crunch). So that is a huge speed up for free. (or i could use the extra speed to do an even longer FFT before it gets too slow to use)

Okay but what about more normal scopes? Like the ol classic MSO6000 does 2GSps and has 8Mpts of memory, well the memory is filled up in only 0.004 seconds. If you use 2 channels and continuous run mode that is only 1ms of acquisition time. So as soon as you are using more than about 100us/div time scale the scope is forced to throw away ADC samples. So you quickly again get into a situation where the ADC is too fast to use all samples. With FFT it is even worse since most scopes are usually limited in the number of FFT points to something much smaller than the size of the sample memory. Another example is the good ol 20MHz BW limit, while that is turned on you could high res downsample a 1GS/s ADC down to 100MS/s and still have plenty sample rate to cover the bandwidth.

So there is a lot of use cases where you do have excess sample rate to work with even on entry level scopes. Yet it is very rare to see such a downsampling feature in its proper implementation.

Someone:

--- Quote from: Berni on August 23, 2023, 08:01:00 am ---So there is a lot of use cases where you do have excess sample rate to work with even on entry level scopes. Yet it is very rare to see such a downsampling feature in its proper implementation.
--- End quote ---
Working mostly with audio/MHz stuff, hi-res acquisition mode is on 90% + of the time when I'm using a Megazoom scope, averaging mode 2nd, normal mode 3rd! There are some quirks/non-ideal side effects but they outweigh the noise reduction and resolution benefits for my use cases.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod