Rigol DS2072 Problem

[cutthebluewire]

Hi all,

Very long time lurker here, but never had a real need to post as most of my questions had already been asked and answered. Search button = time saver.

But I now have an issue I need advice/feedback on. I have one of the original Rigol DS2072 scopes. As I am not in the business it has sufficed for the occasions I needed a scope. Work and family commitments never left me much time to play anyway. Now I am retired and getting back into Ham Radio and currently updating the shack workbench.

To the issue at hand. I hope I can make this coherent.

When I turn on a measurement such as Vmax, Vpp there is a value displayed that is much higher than I would expect, even accounting for the intrinsic noise of the scope. It is also there when a signal is supplied. 

On CH1 At 10V/Div it shows a Vpp value of 800mV (sometime flickering to 1.20V), doesn't matter if coupling is DC or AC, or if the inputs are open or shorted. Even tried a 50Ohm terminator and no difference.

If I input 10V DC from my Agilent 8001 PS (Confirmed the 10V accuracy with my Agilent 1241 and R&S HMC-8012, I don't have anything more accurate), the scope will show 10.8V.  But as near as I can tell I do not see this extra 800mV signal on the trace. If I change coupling to AC the DC input goes as expected and I have just the 800mV.  I thought it was noise but the only values showing if I display all measurements are Max, Vpp, Vrms, Avg, Area, and Vmin (though this tends to be always 0. None of the others like Vtop, Rise, Fall, have values.
This 800mV changes when changing V/Div.

I ran through all the settings from 10V/Div to 500uV/Div with both channels on all measurements displayed. No input signals.

CH1
5V/Div - 400mV
2V/Div - 160mV
1V/Div - 80mV
500mV/Div - 40mV
.......... 
5mV/Div - 800uV
This pattern continues until we reach 2mV/Div where its starts fluctuating around 500uV, At 1mV/Div most if not all the other measurements start to show, duty, period, freq. At 500uV/Div all show and we are in the noise floor. I thought I read somewhere that these scopes go to 2mV/Div then use digital magnification or something to get down to 500uV.

This pattern is similar on channel 2 also, although sometimes it will show 600mV instead of 800mV, but it divides down same ratio as CH1. Values a little bit different but consistent as V/Div changed. I am beginning to think it is firmware but I updated it to the latest available with no change. Also did a number of calibrations. I did note on one that CH2 came back showing half the values of CH1, ran it again and it then matched CH1. Once I turned off CH2 and CH1 value halved from 800mV to 400mv, re-enable Ch2 back to 800mV and it wouldn't do it again.

I noted that with a signal applied to CH1 turning off CH2 changes the "added value".
It was a bit different inputting a sinewave. I input a 1Khz sine at 5Vpp. It skewed the values of each half of the waveform. + went up by 40mV and - down by 20mV. Too small to see on the trace with V/Div at 1V/Div. Oddly changing to 500mV/Div and using vernier adjustment to bring the trace back onto the screen fully, the values were much closer to the input signal (if the Sig-gen is correct of course). I gather a lot of these values are done based on whats on screen or it seems that way.

I even removed it from the mains and ran it off a battery and inverter. I get a lot of QRM around here and wanted to check if anything was getting in via the mains. The inverter is not a good one but I don't think it was introducing anything as there was no change in the values.
I have even gone old school and counted the division/sub-divisions and the input voltage looks correct. The traces look correct. Just the damn measurements are off.  A signal from the DG4000 Sig-gen "looks" ok and frequency is measured correctly. I don't have another scope to compare to as my Tek 2215A died and this is where I need the DSO to fix it.

I am at the point where I can view signals but don't trust the measurements. It's painful to check the offset for that setting and allow for it.

Does this sounds like a hardware or software issue?. I don't have the experience to decide either way. I hope the above doesn't sound like the drivel of an idiot.

I am going to buy a new scope anyway, and no I am not going to start one of those threads.

Regards

Rob
 

[TurboTom]

Welcome, Rob -

may I first ask what version of firmware is installed on your scope? If it's an original DS2072 and has never been updated, it's very likely an early, buggy firmware version. Current version for the DS2000(A) series is 03.06.00.00. which is mature and stable now. You may want to update the firmware and re-check.

Cheers,
Thomas

[JDubU]

1)
Update firmware to 03.06.00.00.  It can be downloaded from Rigol at:
https://www.rigolna.com/firmware/

Be sure to follow the firmware update instructions originally described in this post:
https://www.eevblog.com/forum/testgear/first-impressions-and-review-of-the-rigol-ds2072-ds2000-series-dso/msg158684/#msg158684

"For those planning to upgrade/downgrade firmware:

Do the upgrade ONLY during bootup - not from the GUI/Menus/OS asking for file/etc. or you might lock up the scope - losing any trial options you have remaining - and requiring you to do the upgrade again anyway using the method listed below:

You do this by using two hands when booting up - one thumb on the 'Power On' switch - one thumb on the 'Help' button. When you press 'Power On', all of the scope LEDs will light for ONE SECOND - during that brief period, you must PRESS AND LET GO of the 'Help' button. It can be a little tricky, but if it works, bootup will stop before the Rigol logo with the 'SINGLE' button lit (if it doesn't, turn off power and try again until you get it). Then insert the USB stick with the file on it. The CH1 LED will flash as the DSO loads the file.

Once updating is finished, several of the LEDs will light up - and all flashing, etc. will stop. That means it's finished, so you can remove the USB stick and reboot - but please take notice of the following:

Note: Rigol has changed the the structure of unit settings stored in FRAM, so if you have upgraded/downgraded between the major versions of v.01 & v.02 (i.e. v.01.XX.XX.XX to v.02.XX.XX.XX - or vice-versa), it's a good idea to hold in the left-menu F6 button (sixth gray button down on left side of LCD) during the first reboot after loading (to clear FRAM) - otherwise the DSO can hang when switching between various menu items."

The DS2000 can be a bit picky about USB flash drives. Best to use the smallest capacity one that you have.

2)
After updating to the latest firmware, run a self calibration of the scope.  It is best to place the scope in a low QRM  environment at normal room temperature and let it warm up for at least 30 minutes before running the calibration.  Also, I seem to get the best results by tightly covering all of the inputs with a single piece of aluminum foil during calibration.

[cutthebluewire]

The first thing I did was to upgrade the firmware, its at 03.06. I would assume it should fairly stable by now. If so then I likely have a hardware issue. I was wondering if something was getting in from the internal supply.

I was thinking that since the "error" changes with adjustment of the vertical then it might be subject to the same processing as an incoming signal, so maybe something is getting in there. I presume the front end introduces attenuation and gain as needed at the various vertical settings. I think I need to go read up on the general processing flow of a DSO before I provide more examples of my lack of knowledge in this area.

I could send it off to be looked at since I am limited with what I can do. But it would probably be sent to China and cost more than the scope is worth now.

[Circlotron]

I got mine from Emona Instruments in Sydney. I’m sure they would look at it for you, even if you didn’t get it from them.

[TurboTom]

Rob -

I just went through your starting post again and replicated the settings on my "improved" MSO2072A step-by-step and find the figures on my machine to be even a little worse (maybe as a result of the bandwidth that I opened up to the full 300MHz). Actually, this has never been a concern to me since round about 1/10 of a division is actually the "noise smear" of the trace. You will get slightly (and sometimes much...) better figures if you change acquisition mode to "high-res" (boxcar averaging) or "averaging" over a considerable number of (repetitive) sweeps.

Since on the DS2000 and contemporary Rigol scopes (don't know for the new ones), the measurement function basically uses the frame buffer memory (and not the sample memory), you would be able to do the measurement (i.e. Vpp) with the cursors as well and should get the same result if you wouldn't use "optical averaging"...

Anyway, the measurement function of a scope cannot be considered a highly accurate system, already the quantization depth of 8 bits with < 7 ENOB (equivalent number of bits) of the ADC used should tell you what you may expect from the instrument.

Today, there are scopes available that are less noisy, but this basically affects the highly sensitive input range settings. Since similar and sometimes even still the same coverters are used, ENOB won't improve that much. Due to the improvements in digital processing, mathematical methods may reduce the diplayed noise level, though.

So I'm afraid, the figures that you find with your scope are completely normal, but at least now you know that you won't need to send it in to be checked...

Anyway, I still consider the DS / MSO 2000 (A) a pretty decent "allround" scope. I don't see a reason to replace it anytime soon (except maybe being a bad case of "TEA"  ).

Cheers,
Thomas

[cutthebluewire]

Rob -

I just went through your starting post again and replicated the settings on my "improved" MSO2072A step-by-step and find the figures on my machine to be even a little worse (maybe as a result of the bandwidth that I opened up to the full 300MHz). Actually, this has never been a concern to me since round about 1/10 of a division is actually the "noise smear" of the trace. You will get slightly (and sometimes much...) better figures if you change acquisition mode to "high-res" (boxcar averaging) or "averaging" over a considerable number of (repetitive) sweeps.

Since on the DS2000 and contemporary Rigol scopes (don't know for the new ones), the measurement function basically uses the frame buffer memory (and not the sample memory), you would be able to do the measurement (i.e. Vpp) with the cursors as well and should get the same result if you wouldn't use "optical averaging"...

Anyway, the measurement function of a scope cannot be considered a highly accurate system, already the quantization depth of 8 bits with < 7 ENOB (equivalent number of bits) of the ADC used should tell you what you may expect from the instrument.

Today, there are scopes available that are less noisy, but this basically affects the highly sensitive input range settings. Since similar and sometimes even still the same coverters are used, ENOB won't improve that much. Due to the improvements in digital processing, mathematical methods may reduce the diplayed noise level, though.

So I'm afraid, the figures that you find with your scope are completely normal, but at least now you know that you won't need to send it in to be checked...

Anyway, I still consider the DS / MSO 2000 (A) a pretty decent "allround" scope. I don't see a reason to replace it anytime soon (except maybe being a bad case of "TEA"  ).

Cheers,
Thomas

Thanks Thomas for confirming where I thought it was measuring from, It certainly looked like it was the screen. I did note that it seemed to get better if I used vernier to push the signal full screen.  I had "liberated" the scope a long time ago to 200Mhz all options. I uninstalled all of it and returned to it to original 70Mhz. No change. Put it back to where it was. Haven't bothered with 300, maybe I will just to see.

I just don't remember it being that off is all. Though it had been a while since I used it regularly, maybe I am just seeing something that isn't really there so to speak and my "ocd" is making it a big deal. I have this thing about accuracy and tend to see things a bit black and white. It just looks odd showing a 10V DC signal and it having an additional 800mV show on the measurements. Its funny because it doesn't show on the trace, even with 2V/Div you cannot see that 800mV. The traces look like they always have including that small amount of noise "smear". Maybe I'll read off the graticule and take those odd measurements with a grain of salt.

I am after a 4 channel scope anyway so I'll keep this as is.

Thank you for your input.  Now I am off to ask some stupid noob questions re Bode plots.

Cheers

Rob