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New toy(?) scope, DSO154pro, 1ch, claimed 40MS/s

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Fungus:

--- Quote from: Dave_g8 on March 26, 2024, 05:14:27 pm ---The DSO154Pro is fine, but you may not be seeing a true representation of the signal.

--- End quote ---

Square waves are the sum of a lot of sine waves.

You're seeing the sum of the sine waves that an 18MHz 'scope can see.

At higher frequencies the same thing will happen on your Rigol.

Dave_g8:
Hi,
The analogue bandwidth of the DSO154 will be much greater than 18MHz.

Attached is a capture on the DSO154Pro with a 1Hz (2V Pk-Pk) square wave input from a signal generator (fast edge speed).



You can see the same ringing before and after the transition which is a result of the display reconstruction.

I don’t want to repeat all the discussions on sin(x)/x, Gibbs, but in my opinion the displayed waveform is not always representative of the input signal.

If I sample a 1Hz square wave at 40MSa/s (25ns) and then join the dots, I will not see the pre and post edge ringing.
For completeness, the Excel graph below shows an example of sampling a square wave.
The dotted yellow line is joining the dots between samples and the red line is the sin(x)/x reconstruction. The frequency of the ringing is half of the sampling frequency.





sizziff:

--- Quote from: Dave_g8 on March 27, 2024, 08:53:06 am ---I don’t want to repeat all the discussions on sin(x)/x, Gibbs, but in my opinion the displayed waveform is not always representative of the input signal.

--- End quote ---

I also hold the opinion that the wave at the top of the pulse is the result of the Sinx/x interpolation
On the Hantek 2d72 oscilloscope (250Ms), it displays a square wave of more than 10 MHz very similar.
The straight line of linear interpolation is replaced by sinx/x arcs, which makes it possible to plausibly restore the sinusoidal signal, but gives such artifacts on the square wave signal.

Dave_g8:
Hi,
The attached link provides a summary of the pros and cons for different interpolation methods.
https://saving.em.keysight.com/en/used/knowledge/glossary/oscilloscopes/what-are-interpolation-methods

The sin(x)/x is great for band limited signals, but not so good for signals with fast transitions.

Dave_g8:
Hi,
For general information I noticed an undocumented feature in the DSO154Pro.
Since the feature is not covered in the documentation, it may be firmware/hardware dependant and not fully tested.
Whilst in the “Disp” Menu, press the “Stop” button.


Two options are available “F_Meter” and “Counter” as shown below.
Note: The options are mutually exclusive.


Enable the “F_Meter” by moving to the “ON” icon and pressing the “OK” button.


The “F_Meter” works irrespective of the time-base setting as shown below, unlike the normal frequency reading used in “Meas” options which is based on the waveform shown on the screen.


Notes
- The “F_Meter” seems to use its own auto trigger level setting, irrespective of the setting in the trigger menu.
- The “F_Meter” reading is updated whilst in run mode and keeps the previous value when the “Stop” button is pressed.
- The “F_Meter” seems to work correctly from around 30Hz up to 12.5MHz. The resolution is much better than the normal frequency reading used in “Meas” options.
     Below 30Hz the reading is erratic, perhaps due to the auto threshold detection.
     Above 12.5MHz the reading does not seem to be valid.
- The “F_Meter” will not operate correctly if the oscilloscope voltage range is set to high.

Enable the “Counter” by moving to the “ON” and pressing the “OK” button.


Notes
- The “Counter” seems to count the number of rising edges crossing the trigger level, irrespective of the rise/fall setting in the trigger menu.
- The “Counter” seems to use the same trigger level setting as the oscilloscope (Auto/Manual).
- The “Counter” increments in run mode and stops when the “Stop” button is pressed.
    Whilst in stop mode, pressing the “Auto” button does not restart the “Counter”.
- To reset the count to “0”, reselect the “Counter” from the options menu.
- The “Counter” will not operate correctly if the oscilloscope voltage range is set to high.

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