Noisy Waveform and then No Noise?

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Noisy Waveform and then No Noise?

Postby hexibot43 » Tue Jul 24, 2012 5:45 pm

Ok, what am I doing wrong?

I was thinking it was a problem with filtering, but I'm not sure. Running Beta 6.7.2.1. I am using one channel with the PDI CA-600 600 amp current clamp I turned the 1k low pass filter on and didn't see any effect in the pattern being generated on the screen. I was noticing that when the pattern would reach the end of the screen there would be like a hick-up and the pattern would briefly look like what I was hoping to be seeing.

Here is what I'm seeing while the scope is running....
NoisySignal.JPG


And here is what I see briefly when the scope is in transition. I've also been able to see it like this occasionally when I stop the scope.
WithoutNoise.JPG


Lastly I attached a copy of the saved waveform. The Pattern looks like what I would expect when I load it to view on the screen, but when I was originally capturing the pattern it looked like my noisy view.

Any ideas??

Thanks,

Mab
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Re: Noisy Waveform and then No Noise?

Postby Autonerdz » Tue Jul 24, 2012 6:02 pm

Mab,

That would be normal operation in streaming mode. The filtering is not applied until the trace reaches the right side of the screen. If you stop the scope then it is also applied and will be seen applied in the previous buffer screens.

Little reason to run live with filter though as you can apply a measured amount of low pass filter to the saved capture after the fact. This way you can dial in just the desired level of filtering.
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Re: Noisy Waveform and then No Noise?

Postby Alan » Tue Jul 24, 2012 7:45 pm

Hi Mab,

Tom is right, many PCs do not have the power to collect the data (stream across USB) and filter at the same time. If you go to Tools / Options / Sampling and enable the slow sampling display you can still review the old (filtered) waveform whilst its collecting the next one. Not sure I have explained that very well so best to just try it :D

The PDI clamp looks similar to the Pico one but as far as I know is not internally screened nor does it have the screened cable. As such it picks up a fair bit more noise.
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Re: Noisy Waveform and then No Noise?

Postby hexibot43 » Tue Jul 24, 2012 10:26 pm

Thx for the quick reply.

I'm left wondering why I got other than Pico current clamps...I'm going to have to figure out how my Matco tool supplier got these for me. It all came with a pico case. I got a big white box with everything separate. I had to put it all together. I had had him buy the scope first to see if it was going to work for me, and after I'd paid that off we got everything else.

Alright I knew there was going to be something different. I am very use to using my old Tektronix 2232 DSO. I have to admit I find it odd looking at the noisy signal. I will try the slow sample and see how that goes. And I am very use to looking at everything real time. I'll just have to get over it. I know there are inherent issues to pushing data over USB.

Thanks,

Mab
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Re: Noisy Waveform and then No Noise?

Postby Alan » Wed Jul 25, 2012 6:04 am

hexibot43 wrote:Thx for the quick reply.
Alright I knew there was going to be something different. I am very use to using my old Tektronix 2232 DSO. I have to admit I find it odd looking at the noisy signal. I will try the slow sample and see how that goes. And I am very use to looking at everything real time. I'll just have to get over it. I know there are inherent issues to pushing data over USB.


All digital scopes (PC or USB) have different operating modes depending on the timebase. If you have a short amount of time across the screen (anything less than about 0.1 seconds) then the trace is captured and then displayed. It can if you want be filtered before display so you only ever see the filtered data. Having said that Tom is right, with automotive diagnostics its often better to capture the unfiltered data and then filter later. Some problems are caused in vehicles by interference or intermittent shorts / grounding issues and filtering can actually hide them.

At longer time durations you don't want to have to wait for data to appear so the scope collects and plots data at the same time. It can not do this and filter at the same time. PCs are however getting faster and it may be something we can do in the future.
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Re: Noisy Waveform and then No Noise?

Postby Robski » Wed Jul 25, 2012 7:32 am

PCs are however getting faster and it may be something we can do in the future.


Alan,

What speed PC would be required :?:
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Re: Noisy Waveform and then No Noise?

Postby Alan » Wed Jul 25, 2012 7:38 am

Robski wrote:
PCs are however getting faster and it may be something we can do in the future.


Alan,

What speed PC would be required :?:



Its too early to comment really. Technically speaking we can stream 12 bit data into the PC using the latest beta releases at 10 million samples per second. If we filter at the same time then this will have to drop. As above, bear in mind that its not always the best solution to only view a filtered waveform though.
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Re: Noisy Waveform and then No Noise?

Postby Robski » Wed Jul 25, 2012 8:21 am

bear in mind that its not always the best solution to only view a filtered waveform though.


I don't :wink:
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Re: Noisy Waveform and then No Noise?

Postby hexibot43 » Wed Jul 25, 2012 3:41 pm

I understand and agree that you don't always want to filter out the noise. Many times it is the noise you want to see, because that is where the problem lies. You don't want to filter out what it is you are looking for! I would just like the option of doing it live, because there are many times where you aren't looking for the noise. The reason I was asking this question was because I was in one of those times - I had a circuit with 5 door lock actuators where an excessive draw was occurring. I wanted to simply look at the current draw knowing I had at least one actuator with excessive draw which was blowing the fuse. It wasn't very easy to see with all the noise in the way. I wanted to look at this "real time" while I was hitting the remote to lock and unlock the doors. I can think of a lot of times where you are simply looking for time base events where that is all you want to see.
I tried the "Slow Sample Display" and Wow! That is weird. I think I'll take the noise for now.

How are you all recording events to look at later? And now that I'm thinking about that - Ok, you turn on the low-pass filter. Now once the screen is captured the filtering is done. Is the data recorded modified? Or is it simply being filtered for viewing? I've got to get use to this way of doing things and make it work for me.
I have to admit this was the first time I'd had to use the 600a current probe. I was looking at a circuit that was topping out at about 24 amps. It had just acceded the range of my 20a current probe. Noise like I was seeing on a probe meant for 600amps I would guess would be considered acceptable.

I appreciate all the banter. I'm learning.
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Re: Noisy Waveform and then No Noise?

Postby Autonerdz » Wed Jul 25, 2012 4:55 pm

Is the data recorded modified?


No, you can unfilter after the fact as well as filter.

If you really want to see less noise live during streaming, you can turn the sample rate down.

Keep in mind that the trace moving across the screen when in streaming mode is more of a progress indicator than a finished product with or without filtering.
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