Number of samples, Sample rates, timebase .....

Ask any questions about using PicoScope Automotive software here.
martinr
OneWave
OneWave
Posts: 12
Joined: Tue Mar 06, 2018 4:06 pm

Number of samples, Sample rates, timebase .....

Post by martinr » Mon Apr 16, 2018 2:09 pm

Not sure if this is more a software or hardware question.

PicoSocope 4425 and PicoScop6 6 Automotive Beta 6.13

Trying to get a better understanding of sample rates and limitations etc, I drew up a table of total waveform time (time per division x10) from 50ns to 50,000s and noted down the maximum possible number of samples for 1, 2, 3 and 4 channels, as indicated in the Properties column. Also included was the fastest possible sample rate, sample interval, and sample interval as a % of the waveform.

The conclusion I came to is that, with a waveform total time of between

1. 50ns to 200ms, the maximum possible sample rate is applied (400 MS/s on a single channel). As a result, the sample interval as a percentage of the waveform gets bigger as the timebase gets faster (so at 5 ns/div the sample interval would be 50% of a division with a single channel and 100% with 2 channels, that is 2 samples/div on one channel and one sample/div on 2, 3 or 4 channels)).

2. 500ms to 50,000s, the sample interval as a percantage of the waveform length is fairly constant (~0.00001% of a division), with the sample rate increasing accordingly as the timebase gets faster, until the the limit of 400 MS/s is reached (single channel at 20ms/div or 200ms total waveform time).


The maximum number of samples I see is 100M (single channel, 500ms to 50,000s total waveform time). However, the maximum possible number of channels indicated in PicoScope 6 Automotive software is 2GS - yes, it does say it will try to capture that many samples and the actual number might be less - but I see a maximum possible number of samples that is 20 times less that the maximum indicated by the software. So under what conditions, then, might it be possible to capture beyond 100M samples and up to 2G samples?

Any other thoughts/observations would be much appreciated.

Martin

User avatar
Autonerdz
TwoWaves
TwoWaves
Posts: 343
Joined: Wed Nov 20, 2002 4:19 pm
Location: Washington State USA
Contact:

Re: Number of samples, Sample rates, timebase .....

Post by Autonerdz » Mon Apr 16, 2018 4:30 pm

Hi Martin,

Here is a capture for your consideration. This is CAN high and low using a ten second capture time.

We have captured 250 million samples at 12.5 MS/s. This is fast enough to render high speed CAN with good resolution.

This is a Pico 4425 pedal to the metal. PicoScope version 6.12.7.2724. We don't use the current beta.
Attachments
10secan2.jpg
Ten second CAN High and Low.

martinr
OneWave
OneWave
Posts: 12
Joined: Tue Mar 06, 2018 4:06 pm

Re: Number of samples, Sample rates, timebase .....

Post by martinr » Mon Apr 16, 2018 5:56 pm

Many thanks, Autonerdz. That’s a thought: I’ll run it through the PicoScope 6.12.

I should have said I’ve never had any problem seeing detail no matter how far I’ve wanted to zoom in. This is more of a theoretical exercise brought about by my trying to understand the ‘scope better. I’d never really given a thought to sample size until I watched ScannerDanner’s Pico Basics Part 2 video and I then realised I was totally ignorant in the area of sampling, buffer size and timebase, so, for example, I may well have been looking at the different heights of secondary ignition firing lines not realising that these were possibly a result of aliasing through sample intervals not being small enough. (I then found that without a reasonable understanding of the relationship between timebase, sample rates and buffer size, I couldn’t get my head properly around other topics such as dead time and glitch capture.)

martinr
OneWave
OneWave
Posts: 12
Joined: Tue Mar 06, 2018 4:06 pm

Re: Number of samples, Sample rates, timebase .....

Post by martinr » Tue Apr 17, 2018 7:13 am

PicoScope 6.12 gives the same results as 6.13 beta.

Also, the Resolution Enhancement is set to 12 bits. Increasing that reduces the number of samples and the sampling rate - as one would expect, perhaps.

But I think I have the answer. The specs for the 4425 https://www.picoauto.com/products/autom ... ifications give the buffer memory as 250M samples (and max sampling rate, one channel in use, 400MS/s). The 2G Sample setting in PicoScope 6 Automotive might cater for other Pico 'scopes that can use that software, or perhaps for future developments/models.

Martyn
Pico Staff Member
Pico Staff Member
Posts: 259
Joined: Thu Aug 11, 2011 7:43 am
Location: Cambridgeshire

Re: Number of samples, Sample rates, timebase .....

Post by Martyn » Wed Apr 18, 2018 7:18 am

Resolution Enhancement shouldn't be reducing the number of samples but it will have an effect on the data as it uses a moving average filter.

The effects are
  • Widened and flattened impulses (spikes)
    Vertical edges, such as those of square waves, turned into straight-line slopes
    Inversion of the signal, sometimes making it look as if the trigger point is on the wrong edge
    A flat line when there are not enough samples in the waveform
The Samples setting at the top which can be increased to 2GS, is specifying what you would like to try and capture in the specified screen time. The 2GS reflects our deepest memory Test and Measurement scope, the 6404D, as the software is the same under the hood.

How many samples you actually can get on a given screen will depend upon the screen time you have specified, and the memory available on the scope. The properties panel on the right hand side will show exactly how the software has setup the scope to meet your request.
Martyn
Technical Support Manager

martinr
OneWave
OneWave
Posts: 12
Joined: Tue Mar 06, 2018 4:06 pm

Re: Number of samples, Sample rates, timebase .....

Post by martinr » Wed Apr 18, 2018 7:28 am

Many thanks, Martyn. Since discovering the Properties panel, I always leave it on and I check the sample interval against the timeframe of the signal I’m measuring especially if I want to capture detail, to make sure it’s small enough, but also to check it’s not so small that I might be wasting memory unnecessarily. Often, to visualise and get a perspective on the sample interval, I’ll use 2 cursors to see how that interval compares with the timebase of my signal. A big improvement on the days when I accepted whatever default sample interval was set without a second thought.

Thanks again.

Martin

martinr
OneWave
OneWave
Posts: 12
Joined: Tue Mar 06, 2018 4:06 pm

Re: Number of samples, Sample rates, timebase .....

Post by martinr » Wed Apr 18, 2018 11:53 am

Martyn wrote:Resolution Enhancement shouldn't be reducing the number of samples but it will have an effect on the data as it uses a moving average filter........
Hi Martyn,

I tried it again to see if I'd missed something and managed to repeat it:

at 200msec/div (2 sec total screen time), with Channel A only on.

At default 12 bit, the max no of samples is 33.3M, with sample rate of 16.7MS/s and interval 60ns.

Increasing the resolution to 16 bit gives:

max no of samples 5M, with sample rate of 2.5MS/s and interval 400ns.


This was with both 6.12 and 6.13beta.

Martyn
Pico Staff Member
Pico Staff Member
Posts: 259
Joined: Thu Aug 11, 2011 7:43 am
Location: Cambridgeshire

Re: Number of samples, Sample rates, timebase .....

Post by Martyn » Thu Apr 19, 2018 6:52 am

Sorry I misunderstood your point. When applying Resolution Enhancement, Lowpass Filtering or Math Channels to a running signal, the software will limit the samples per screen to a maximum of 5MS. This is to control the amount of data that the software needs to receive over the USB bus when capturing a signal.

However if you capture the data raw, then apply any processing post capture, the number of samples will not reduce.
Martyn
Technical Support Manager

martinr
OneWave
OneWave
Posts: 12
Joined: Tue Mar 06, 2018 4:06 pm

Re: Number of samples, Sample rates, timebase .....

Post by martinr » Fri Apr 20, 2018 7:33 pm

Martyn wrote:Sorry I misunderstood your point. When applying Resolution Enhancement, Lowpass Filtering or Math Channels to a running signal, the software will limit the samples per screen to a maximum of 5MS. This is to control the amount of data that the software needs to receive over the USB bus when capturing a signal.

However if you capture the data raw, then apply any processing post capture, the number of samples will not reduce.
Thank you for explaining that; I really appreciate that valuable bit of information, especially that resolution enhancement can be applied to the raw data after capture. I knew that applied to low-pass filtering but didn’t know about resolution enhancement. After this little exercise, I feel I know my PicoScope a lot better than before.
Last edited by martinr on Sat Apr 21, 2018 6:43 am, edited 1 time in total.

User avatar
Autonerdz
TwoWaves
TwoWaves
Posts: 343
Joined: Wed Nov 20, 2002 4:19 pm
Location: Washington State USA
Contact:

Re: Number of samples, Sample rates, timebase .....

Post by Autonerdz » Fri Apr 20, 2018 8:06 pm

After this little exercise, I feel I know my PicoScope a lot better than before.
Excellent. If you manage to duplicate the capture shown in my image above, you will learn a lot more.

I hope no one spoils your fun by telling you how it was done. :roll:

Post Reply