New toy(?) scope Fnirsi DPOX180H, claimed 180MHz/500MSps (May 2023)

[Rooster Cogburn]

Update:
I was wrong, the input capacity only changes less than a half pF. It is more the coupling capacitor, which seems to have a too low value. At higher frequencies (e.g 10kHz) the problem vanishes.

Interesting, this is probably why I didn't notice it at first. I compensated my probe and then tried out the scope by looking at analog video signals, high-frequency noise of a SMPS, tried my funcgen closer to its BW limit, etc.

I repeated tests, seems you are right, for 1kHz AC decoupling capacitor has too low value (flat part of square wave is declining and cannot be compensated)

Hm, I can get the probe adjusted just fine for either AC or DC, just not for both with one setting?


What would be the most sensible probe calibration procedure for this scope then? Calibrate for either AC or DC at 1Khz? Use 10Khz or higher?

[RAPo]

Calibrate for 10x times DC at 1kHz square wave.
Then remember if you switch coupling-mode or frequency goes below 1kHz: recalibrate.

What would be the most sensible probe calibration procedure for this scope then? Calibrate for either AC or DC at 1Khz? Use 10Khz or higher?

[Rooster Cogburn]

Gotcha. I find that the error only goes to zero at ~20Khz. At 15Khz you can still see a difference between AC/DC, much less than at 1Khz, of course. 1Khz and below is really quite bad.

[infino]

Gotcha. I find that the error only goes to zero at ~20Khz. At 15Khz you can still see a difference between AC/DC, much less than at 1Khz, of course. 1Khz and below is really quite bad.

Personally, I do not consider this oscilloscope to be an accurate measuring device and do not overestimate the requirements for its readings. If the manufacturer can improve it - well, if not - well, let them be
The main thing is to understand where it fails to cope with constant measurements and where it shows the signal incorrectly.

[Rooster Cogburn]

It always depends on what you want to use it for. I don't design nuclear reactors. I want to see that the sync pulses coming out of my console look Ok. That my PSU isn't too crappy. That the clock line on this PCB does clock line stuff. How a voltage rail stabilizes when I turn a device on. Stuff like that. If there's a ~5% measurement error or some signal edges look a bit funky, that's alright.

[Anatol_i]

In a good way, the manufacturer at least should have warned about such a problem. The device under discussion, according to its characteristics, has not gone far from the ZEEWEII DSO2512G, in which there is no such problem. I wonder if the Fnirsi representative happens to be here? I think it would be in their interest to at least indicate how this problem can be solved at the hardware level. Measurements of the input capacitance of the device with the input open/closed showed its slight change in unit pF. It is likely that the problem really lies in the incorrect capacitance of the transient capacitor. Only it needs to be found somehow on the device board.

[Evi]

All that need to be done is:
- unmount input shields from the scope PCB
- find theese capacitors and replace with x10 values ones

[RAPo]

Well the pcb-image is here

Only it needs to be found somehow on the device board.

[vk6zgo]

Update:
I was wrong, the input capacity only changes less than a half pF. It is more the coupling capacitor, which seems to have a too low value. At higher frequencies (e.g 10kHz) the problem vanishes.

Interesting, this is probably why I didn't notice it at first. I compensated my probe and then tried out the scope by looking at analog video signals, high-frequency noise of a SMPS, tried my funcgen closer to its BW limit, etc.

I repeated tests, seems you are right, for 1kHz AC decoupling capacitor has too low value (flat part of square wave is declining and cannot be compensated)

Hm, I can get the probe adjusted just fine for either AC or DC, just not for both with one setting?


What would be the most sensible probe calibration procedure for this scope then? Calibrate for either AC or DC at 1Khz? Use 10Khz or higher?

If you have analog video signals, try looking at them at "field rate".
That was the "acid test" we did on early DSOs----they all failed!

[Rooster Cogburn]

Is there a difference between the V-p and V-max measurement on this scope? They sound the same to me and seem to show the same value.

If you have analog video signals, try looking at them at "field rate".
That was the "acid test" we did on early DSOs----they all failed!

Could you explain what you mean by that? Set the horizontal so long that I see entire fields? This scope doesn't have video triggers, so I unfortunately don't think I can trigger on vsync.

[Anatol_i]

https://youtu.be/jKF-cIToWKc
Another illustration of the open/closed input problem when investigating mains voltage.

[infino]

https://youtu.be/jKF-cIToWKc
Another illustration of the open/closed input problem when investigating mains voltage.

Input filters for low frequencies are incorrectly designed.
I asked Fnirsi if manual calibration would be available, they said no

[vk6zgo]

Is there a difference between the V-p and V-max measurement on this scope? They sound the same to me and seem to show the same value.

If you have analog video signals, try looking at them at "field rate".
That was the "acid test" we did on early DSOs----they all failed!

Could you explain what you mean by that? Set the horizontal so long that I see entire fields? This scope doesn't have video triggers, so I unfortunately don't think I can trigger on vsync.

Yes, you don't need video triggers, you only need to fit 20ms or so on the screen for one field, so depending on the shape of your screen, you can set your rate at 2ms/div or a bit less.
 
At that rate, you can trigger on the vertical syncs.
We never used built-in video triggers as the old Tektronix HP, etc., Analog 'scopes which could easily trigger on vertical syncs.

The early DSOs had no problem with triggering, either.
Their failing was that they had such a small memory that the only way to not overflow it was for the sample rate to fall radically at such rates as 2ms/div.

If the sampling rate fell below around 10MSamples/sec, aliasing would set in.
Many reduced the sampling rate quite a bit more than that, so the video field signal was not recognisable.

[Rooster Cogburn]

At 2ms/div I get at least 240 lines of actual video content on screen, not seeing much since this screen has about ~300 pixels horiz. dedicated to the waveform. But I can clearly see the vblank and the rest is pretty much smushed together. I can seen just about the vsync/hsync pulses but when I zoom in on the capture and try to look at stuff like the colorburst it's mostly a mess, guess the scope would need a higher sample rate to allow for that kind of resolution at such a time interval.

[vk6zgo]

At 2ms/div I get at least 240 lines of actual video content on screen, not seeing much since this screen has about ~300 pixels horiz. dedicated to the waveform. But I can clearly see the vblank and the rest is pretty much smushed together. I can seen just about the vsync/hsync pulses but when I zoom in on the capture and try to look at stuff like the colorburst it's mostly a mess, guess the scope would need a higher sample rate to allow for that kind of resolution at such a time interval.

That is not what you look for at field rate.

Even though the the signals are "Smushed together", you should be able to see the whole of the "Field group", including pre & post equalising pulses as well as the "serrations" on the vertical pulse itself.
The video part should not look like a unmown back yard, & you should be able to recognise it as video.

Using an analog 'scope with delayed/dual timebase, you would be able to "zoom in" & see the individual colour bursts, horizontal syncs, & so on.

I don't have a modern DSO in the same class as the relatively inexpensive Siglent & Rigol bench instruments (or, indeed, any DSO), but I am confident they can do the same.

If the DPOX180H cannot do this, it does point to a lack of memory, causing the sample rate to drop radically at long time/div settings, which is the same problem the early DSOs had.

[svetlov]

you can capture the video signal directly from the camera - like in this video
this test shows what your oscilloscope can do it should be carried out first of all to assess the capabilities of digital phosphor - color gradation and trigger response speed - display speed of oscillograms per second and much more

[vk6zgo]

you can capture the video signal directly from the camera - like in this video
this test shows what your oscilloscope can do it should be carried out first of all to assess the capabilities of digital phosphor - color gradation and trigger response speed - display speed of oscillograms per second and much more

Any halfway reasonable modern DSO can display analog lines, as all it needs is to save enough samples to correctly resolve around 5MHz over 64us or so, & the memory is usually large enough to handle this at an adequate sample rate. (Some early 1990s major brands & "El Cheapo" ones from the early 2000s & 2010s could not do this.)
Field rate is "another kettle of fish".

Neither of the displays in the video even do much of a job of triggering at horizontal rate.
Colour gradation adds little to the display of analog video waveforms.

[tautech]

I find some holdoff and careful trigger level selection helps.

[infino]

Neither of the displays in the video even do much of a job of triggering at horizontal rate.
Colour gradation adds little to the display of analog video waveforms.

Does a great job, single frame picture visible, color signal visible, sync signal visible, picture visible. You just have to either memorize the signal or change the synchronization level.

[imarcinszn]

I've recently bought the DPOX180H portable oscilloscope. I'm rather happy with my purchase, but there is one issue that is pretty serious. It was already described by the others that after enabling AC coupling readings on the lower frequencies are highly inaccurate. 

I've already tried to externally connect AC blocking capacitor to it with higher value (100nF) and it seems to solve the issue, but i would like to make this change more convenient and to resolder AC blocking capacitor inside of the scanner. Normally I would expect to see some kind of relay or solid state relay, but in this case I cannot find it.

Has anyone tried to change it, or found where AC blocking capacitor is located?

[BillyO]

Has anyone tried to change it, or found where AC blocking capacitor is located?

You should be able to trace back from the BNC connector.  It is usually VERY close to the input.  BTW, adding a capacitor in series with another capacitor will reduce the total capacitance, not increase it.  If things improve it might be due to conductive contamination on the PCB or a faulty blocking capacitor.

EDIT:  BTW, there is clearly a relay in each of the front ends.  There may also be some CMOS bilateral switches.

[ru_tash]

Has anyone tried to change it, or found where AC blocking capacitor is located?

You should be able to trace back from the BNC connector.  It is usually VERY close to the input.  BTW, adding a capacitor in series with another capacitor will reduce the total capacitance, not increase it.  If things improve it might be due to conductive contamination on the PCB or a faulty blocking capacitor.

EDIT:  BTW, there is clearly a relay in each of the front ends.  There may also be some CMOS bilateral switches.

i think he used DC input with external capacitor added.

[imarcinszn]

BTW, adding a capacitor in series with another capacitor will reduce the total capacitance, not increase it.

Yes, that is right, but I've tried with DC coupling mode so the external capacitor should be the only one. So I guess that higher value  still should help with that issue. Connecting external 22nF capacitor produced similar to the internal capacitor.

EDIT:  BTW, there is clearly a relay in each of the front ends.  There may also be some CMOS bilateral switches.

I wasn't specific enough. There is clearly one relay, but it is clicking when vertical scale is being changed. During DC -> AC coupling mode change there is no sound, so it needs to be changed by some kind of solid state circuitry.

I performed some measurements and i think that this is AC blocking capacitor:


At the input (just at the output of the relay) of this capacitor signal is the same, no matter if it is AC or DC coupling mode:


Output of this capacitor on DC mode:


And output of this capacitor on AC mode:

There is still some overshoot on DC mode but it is not visible on the FNIRSI scope, but at least it contains DC offset

[svetlov]

rise time 1.8 nanoseconds 

[Kon]

Подскажите какая микросхема отвечает за смещение для калибровки.Появилось остаточное напряжение на индикаторах.Что то вышло из строя.Штатная калибровка не помогает.Всё остальное работает.Или можно уже выкинуть осциллограф?Снимок без щупа.