Abberations display square wave signal on a PS3224

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blackdog
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Abberations display square wave signal on a PS3224

Post by blackdog »

Hi, :-)
I have been using a "3224" for low frequency spectrum analysis for over 10 years.
I have no other complaints about this, but since the beginning of the purchase of the 3224, the squairwave display is not as it should be.

The problems are not in my measurement setup since I have several function generators and also several digital and analog scoops with more than enough bandwidth.
Also, the 3224 is connected via a 50 ohm coaxial cable with a 50 Ohm BNC terminator on the BNC of the 3224.

The behavior of the 3224 I have also seen in some cheap USB scoops from China and it also occurs to a much lesser extent in my USB scoop from OWON VDS1022I, which I sometimes use when I need galvanic separation.
Lets show some pictures of this problem, both channels have the same problem!

5KHz Sine
3224-5KHz-Sine-Auto-Trigger-01.jpg
.
5KHz square wave, The problem is already somewhat visible as overshoot on the rising edge.
3224-5KHz-SQR-Auto-Trigger-01.jpg
.
50KHz sine as reference
3224-50KHz-Sine-Auto-Trigger-01.jpg
.
50KHz square wave, Now the abberation is visible on both flanks.
3224-50KHz-SQR-Auto-Trigger-01.jpg
.
As a reference for the frequency characteristic a 500Khz Sine.
3224-500KHz-Sine-Auto-Trigger-01.jpg
.
And this is the 500KHz square wave!
This is absolutely not how it should look and should be nicely displayed with the sample frequency used.
3224-500KHz-SQR-Auto-Trigger-01.jpg
.
Is there anyone here on the forum who can explain to me what is going on here?
My knowledge as far as it goes says it has something to do with the calculations being performed on the signal.
This is for example to improve the bandwidth of sine wave signals and that is usually at the expense of the quality of square wave signals.

But correct me, if I draw wrong conclusions! :D

Kind regards,
Bram

bennog
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Location: Netherlands

Re: Abberations display square wave signal on a PS3224

Post by bennog »

You mean the below option
2021-07-28_13-23-40.png
Benno

blackdog
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Re: Abberations display square wave signal on a PS3224

Post by blackdog »

Hi Benno, :-)

I played a bit with the settings you showed, the settings are quite "buggy".
If I adjust the value of 2000 in your picture to 100 nothing happens and I can't press "Apply".
Only after I have pressed the arrows 1 or more times to make the value higher or lower can I click Apply.

As I mentioned, I also have a cheap OWON USB scope that shows the same artifacts, but to a much lesser degree.
For a single channel the sampling frequency is higher but at lower time base settings the artifacts disappear on the square wave display on the OWON scoop.
I think I will leave it at that for the 3224 as I think it is in the programming of e FPGA in the 3224.
----------------------------------------------------------------
I do have another question, why is the FFT using a scale of 12dB/Div for level, because the default scale is 10dB/Div on almost all instruments.

I also played with version- of the picoscope software this afternoon.

That doesn't work so well, of course many bugs, FFT is actually unusable, this even to test as a beta version.

I don't like the new interface as it is now, have to do too many clicks to get to frequently used functions.
As it is now as a user interface I much prefer to use the old interface from version-6.
Not because I am so conservative, but I have a much better overview of my settings.
The fact that the interface of version -6 looks a bit dated is not important to me.

When I am doing measurements, I should in principle not be busy with the interface of my measuring instrument, but with the measurements I want to do.
The interfce should disturb this as little as possible while doing measurements.

For example, while looking at a waveform I want to be able to simply switch from positive to negative triggered, as is possible in version 6 of your software.
To let the user search in the menus in version-7 for this, is a bit "silly".

Another one that is not well thought out in version-7 in my opinion.
The time base and the input sensitivity "knobs".

I think this is the commonly used way....
Clockwise rotation makes the time base setting shorter!
Clockwise rotation but makes the input channel more sensitive!
Both functions are reversed in software version-7, why is that?

And I could go on like this for a while :lol:

I hope my 2 cents comments help a little.

Kind regards,
Bram

bennog
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Re: Abberations display square wave signal on a PS3224

Post by bennog »

Ok max sampling rate of your scope is 20MS/sec if 1 channel is used and 10MS/sec if 2 channels used.
2021-07-28_17-58-13.png
If the Sin(x)/x interpolation is disabled your signal would look like the above. Only 110 samples across the screen.
If you enable Sin(x)/x interpolation there are extra points calculated between the real samples.
2021-07-28_18-33-57.png
Above is the exact same sample and the interpolation calculated 10 times more samples than you had originally.

P.S. there is a bug in picoscope software if you enable Sin(x)/x then it is instant (or switch 1 screen back and forward). If you disable it you can see the effects after you stop and start sampling again.

Benno

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