Hacking the Rigol DHO800/900 Scope

[Mechatrommer]

rigol dso is already capable of aliasing without you modify anything out of factory (if you dont switch peak detect on). just switch to large enough timescale and sample rate will go down way below nyquist limit for a scope.. even with 20MHz limit is turned on, sample rate can go down even further iirc. is there entry level scope from any brand can do automatic BW reduction based on current sample rate used by user? to satisfy this nyquist limit problem? i havent heard one.

[ebastler]

If you read carefully what RX3AM wrote on that forum, you can see that he wrote about the pass resistance of 50 Ohm, and even recommended not to modify all four channels, and even pointed out the approximate error in the measurements. If you don't need it, then you don't have to do it.  In my opinion, the purpose for which he did this is clearly described there.  I consider this man extremely competent in what he did, unlike some of the “great theorists” who often speak out here.

External 50 Ohm termination is good to ~200 MHz. I would not trust step response measurements etc. at 400 MHz unless the scope offers internal 50 Ohm termination. And even if modifying a single channel only, one has to be aware of the increased aliasing problem for that channel going forward.

I'm not saying that nobody should make that modification; it may have its uses e.g. in amateur radio. But I think it is justified to point out the limitations and side effects, without getting slighted for that.

[S2084]

Guys!  500€....And a real opportunity to see the spectrum up to 625 Mhz....And even measure it relatively truthfully....In my opinion, we are all forgetting about the price of this device...

[ebastler]

Guys!  500€....And a real opportunity to see the spectrum up to 625 Mhz....And even measure it relatively truthfully....In my opinion, we are all forgetting about the price of this device...

Even cheaper if you buy a DHO802, which would be all you need in "400 MHz mode" anyway. Too bad they don't make an 801 version. 

[norbert.kiszka]

The optimal bandwidth for this scope is 125 MHz maximum  in 1-2 channel mode, and 70 MHz in 3-4 channel mode. In extreme cases, 250 MHz in single channel.
There is no point in expanding it.

250 MHz bandwidth makes scope usable to 25-50 MHz. Above that, square wave will become sinus - more or less.

500 MHz will allow to see up to 100 MHz - unless ringing or background noise will come into play.

Probes included to DHO924S have a nominal bandwidth 350 MHz. Practically it should have little bit more, around 380-500 MHz. However 10 pF in a probe can be a little pain - You know where.

[S2084]

Here guys is the result of my modification.  My assumption that reducing the resistance of the resistor in the divider will lead to a decrease in the fan speed turned out to be correct.  The resistance of the pre-soldered resistor I measured was 55.6 kOhm.  In parallel, a 1MOhm variable resistance resistor was connected to it, this gave a very large range of fan speed adjustment.  I adjusted the noise level so that it is not audible at all, even up close.  With this setting, the temperature on the board did not exceed 61°C for about one hour of operation at maximum load.  Photos are attached.

My modification is V 2.0.  the second resistor in the divider was replaced with a 10 kOhm NTC thermistor.  I left the 1 MOhm parallel variable resistor for adjustment; it still regulates the fan speed.  Now everything works as I originally planned, the speed increases very smoothly along with the temperature.  A variable resistor makes it possible to adjust the desired speed at the desired temperature.  With these ratings, it turned out to be possible to set the maximum voltage on the fan at 10V, which is even higher than what was in the factory version.  Damn this works great!!!

[Fungus]

...and even recommended not to modify all four channels

Just modify channel 4 and save it for special occasions! 

[shapirus]

Just modify channel 4 and save it for special occasions! 

Sampling rate is halved if channel 4 is enabled (as well as any but channel 1), even if it's the only channel enabled. So if any channel is to be made special, then it has to be channel 1.

[S2084]

This is wrong,!!!

[ebastler]

Sampling rate is halved if channel 4 is enabled (as well as any but channel 1), even if it's the only channel enabled.

No, you are mistaken there. Most likely you neglected to change the trigger input to channel 4 as well. If you leave triggering on channel 1, that channel also remains active, since it's a digital trigger system.

[Fungus]

Just modify channel 4 and save it for special occasions! 

Sampling rate is halved if channel 4 is enabled (as well as any but channel 1), even if it's the only channel enabled. So if any channel is to be made special, then it has to be channel 1.

Try setting trigger on channel 4...

[S2084]

Also, if some measurements are enabled on other channels

[AndyBig]

You just need to switch the trigger to the only active channel. And the frequency will again become maximum.
P.S. For some reason I didn’t see the messages above before mine...

[ebastler]

Now that we have said this on four channels, I think we can move on. 

[AndyBig]

By the way, the noise of channels 1 and 4 is not much different. According to the average value AC.RMS on channel 1 is 18.67 µV, on channel 4 - 17.90 µV.

[norbert.kiszka]

...and even recommended not to modify all four channels

Just modify channel 4 and save it for special occasions! 

Im waiting for some screenshots, how good/bad it works.

[shapirus]

Good catch guys. Yes I can confirm that 1.25 Gsa/s is actually possible on channel 4 .

And I've been caught by this trigger channel thing more than once already! It's actually caused not only by the trigger setting, but also if any of the other channels are used in measurements, counter, and, I think, math as well. In other words, other channels, even if they are turned off, must not be referenced by anything to allow for full sampling rate to be active. This can easily be considered a bug, or at least a serious UX issue: when a channel is off and its respective button is not lit on the front panel, then it should not matter whether anything else references it. If it's off, it can't be used by anything anyway, and it should be automatically disabled for everything that may be concerned.

[Mechatrommer]

Good catch guys. Yes I can confirm that 1.25 Gsa/s is actually possible on channel 4 .

thanks for letting us know we have been with rigol since the beginning... like 20 years ago?

[ebastler]

This can easily be considered a bug, or at least a serious UX issue: when a channel is off and its respective button is not lit on the front panel, then it should not matter whether anything else references it. If it's off, it can't be used by anything anyway, and it should be automatically disabled for everything that may be concerned.

But the channel can be used! I am pretty sure that triggering works, and according to the manual, so does the DVM (and the counter?). I can't double-check this anymore since I no longer have my DHO.

If a channel is "off", i.e. its front panel button unlit and the settings field on the screen not coloured, that primarily means that it is not displayed on the screen. It can still acquire data, and hence take up part of the scope's sampling bandwidth.

I think this is useful functionality. You may want to trigger on a separate channel, but don't need to see it on the screen. In that case it is nice that you can disable the trace to reduce clutter on the small screen. Same for counter and DVM measurements (assuming that they actually work on a non-displayed channel).

[shapirus]

But the channel can be used! I am pretty sure that triggering works, and according to the manual, so does the DVM (and the counter?). I can't double-check this anymore since I no longer have my DHO.

If a channel is "off", i.e. its front panel button unlit and the settings field on the screen not coloured, that primarily means that it is not displayed on the screen. It can still acquire data, and hence take up part of the scope's sampling bandwidth.

Agreed, these are all valid points. I've just checked: yes, trigger, measurements, and counters all keep working when the respective trace display is off.

Then there has to be a way to disable the channel trace display and data acquisition separately :). It can still be annoying for the user to see that the sampling rate is not at the max level and sit there wondering why for a moment, then remember that it has to be dereferenced in measurements etc., and find and turn those off (sometimes one by one, when it's not desired to remove all).

[ebastler]

Then there has to be a way to disable the channel trace display and data acquisition separately . It can still be annoying for the user to see that the sampling rate is not at the max level and sit there wondering why for a moment, then remember that it has to be dereferenced in measurements etc., and find and turn those off (sometimes one by one, when it's not desired to remove all).

If you see a channel colour highlighted, either in one of the channel indicators on the lower left, the measurements on the right, or the trigger indicator on top, the channel is "active". I don't see an issue with the operating logic there.

You can quickly deactivate individual measurements by swiping them to the right. And you should be aware which channel you are triggering from anyway -- apart from taking up part of the sample rate, it has a certain influence on what you see on the screen.

[Mechatrommer]

Then there has to be a way to disable the channel trace display and data acquisition separately . It can still be annoying for the user to see that the sampling rate is not at the max level and sit there wondering why for a moment, then remember that it has to be dereferenced in measurements etc., and find and turn those off (sometimes one by one, when it's not desired to remove all).

at least you will be wondering only once. and then you ask people why and now you know and are educated and become a better man... if you dont want that, buy good brand dso fully automatic, Lecroy maybe? or Rohde & Schwarz? tell us what brand name dso that doesnt cause confusion to beginner? in any aspect?

[shapirus]

If you see a channel colour highlighted, either in one of the channel indicators on the lower left, the measurements on the right, or the trigger indicator on top, the channel is "active". I don't see an issue with the operating logic there.

There may be a lot of measurements that don't fit in the narrow vertical space, much of which is additionally wasted by the useless icons, frames, and padding. It will then require the user to scroll through the list and find the unnecessary channels referenced -- on a small screen with small letters. This is a UI/UX issue.

You can quickly deactivate individual measurements by swiping them to the right. And you should be aware which channel you are triggering from anyway -- apart from taking up part of the sample rate, it has a certain influence on what you see on the screen.

The fact that there is a workaround does not mean that the issue does not exist.

at least you will be wondering only once. and then you ask people why and now you know and are educated and become a better man... if you dont want that, buy good brand dso fully automatic, Lecroy maybe? or Rohde & Schwarz? tell us what brand name dso that doesnt cause confusion to beginner? in any aspect?

This has nothing to do with the Rigol's UI/UX problems that I mentioned. The fact that I have never used a Rigol product before makes my observations more valuable -- having a previously unaware person estimate the usability of a user interface is an essential step in designing one. I only pointed out some of issues, that's it, there's no reason to start a personal discussion or do a comparison with other products. Maybe some day someone from the Rigol UI/UX team (if they even have one) will see this and take it into account.

[Mechatrommer]

Maybe some day someone from the Rigol UI/UX team (if they even have one) will see this and take it into account.

send a PM to them... we both pray for that.. except i prayed long time ago... what i learnt, other than simple pluses spelling mistake, we have to learn the rigol's way. i hope your msg will reach them and they accepted and implemented. cheers.

[shapirus]

i hope your msg will reach them and they accepted and implemented. cheers.

I'm pretty sure it will not, lol. Or if it will, it will fall on deaf ears. That's why I'm simply leaving it here so that if eventually Rigol hires someone who will care, they will be able find it here along with all the other issues.