Rigol DHO914S Bode plot

[Fungus]

The guy at Rigol has sent my data to the relevant group.
He asked me for data from a "direct" connection, [...]

Good to read that you reached someone who seemed to care and came back with a relevant follow-up question. Who did you contact? Is there an email address you could share?

Can you persuade them to follow the bugs thread?

[TimFox]

Bode plot is RELATIVE measurement. You can have AWG amplitude vary (and in fact that is being used by Bode plot implementation if it is done right) all over the place, it shouldn't make a difference.

Yep, but he has got wiggles with straight AWG->CH1-Ch2 connection (see the above graph).
There is always 1:1 ratio.
Thus with my test you get whether the wiggles come from Bode math bug, or the AWG wobbles with 6% (0.5dB) modulation index..

I am new to DSOs with built-in AWG, so I'm not sure how to set up a frequency sweep on this unit.
When I scanned the same circuit manually, relying on the Vpp measurement tool on Ch1 and Ch2, there was very little amplitude change on Ch1 (no 50 ohm termination, just 9100 ohms in the LPF), although Ch2 (after the filter) followed the expected variation.
On manual point, by mistake, used a different AWG output setting, but the result in the voltage ratio was negligible.

[TurboTom]

...
Bode plot is RELATIVE measurement. You can have AWG amplitude vary (and in fact that is being used by Bode plot implementation if it is done right) all over the place, it shouldn't make a difference.

And this exactly isn't the case here, see these two 1kHz~100kHz sweeps, 50pts/dec with equal signals to both inputs AWG -> 30cm Coax -> (BNC-Through-Terminator) -> T-Piece central leg -> 2 * 30cm Coax to each input.

In RigolCSV11, the terminator is installed, so the effective input voltage to the scope was 2.5Vpp (5Vpp AWG output), while in RigolCSV12, the terminator was left out and the coax from the AWG was directly coupled to the T-Piece central leg, so both scope inputs were seeing 5Vpp.

I verified before that the signals were identical and even adjusted the "Channel Skew" at 25MHz so there wasn't any delay between the channels (had to adjust channel 2 by ~700ps) which weren't caused by the cables (reversal of the inputs didn't make any change). After that, I ran the Bode Plots and found what's shown below. The funny thing about this all is that the channel-to-channel phase measurement, if used separately, only shows a difference between the channels of <0.1°, basically "dithering" in the noise. I hope Rigol can make sense of this... 

[iMo]

What would happen if you do the above measurement with say 5V and 2.5V AWG amplitude and terminated 50ohm in both cases?
Thinking loud - whether 50ohm termination on CH1 and CH2 is required in this rig when doing Bode with the AWG's 50ohm output..
Even though 1-100kHz are pretty low freqs to mess up the matching.. Or perhaps the 50ohm matching plays a role in the math inside the box?

[mawyatt]

Tom,

Could you check the AWG amplitude stability over the frequency sweep with a good SA using peak hold? Not much help with phase but could show if the source of this "ripple" is the AWG and not the DSO.

Anyway, just a thought.

Edit: Think you found a correlation between scope trace position selection/movement and offset from AWG that may be related to unstable internal scope bias/rail voltages. Could this "ripple'" effect be related to such??

Best,

[moffy]

Interesting that the amplitude and phase error curves are so similar, and I think 2N3055 is correct that it's probably not the AWG since it is a relative measurement. It's not noise because it is too repeatable that leaves math i.e. their algorithm.

[TimFox]

There was a suggestion early-on that I should use a shielded network for this test to eliminate possible high-frequency pickup from the "naked" wires and components.
Therefore, I made a simple box (after I discovered I was out of BNC connectors and had to wait for an order).
Also, I carefully measured the actual R and C values to get the theoretical corner frequency f_c (-3 dB and -45 deg).
The C was measured with the BNC included, and 15 pF added for the input capacitance of CH2 on the 'scope.
The R value was measured between BNCs, and (slightly) corrected for the 50 (series) output resistance of the AWG and the 1 megohm (parallel) input resistance of CH1.
C = 1,987 pF.  R = 8,859\$\Omega\$,  Therefore, f_c = 9,041 Hz.

Summary:  the curves still wiggle, perhaps a bit less than before.
However, the eyeball value of f_c remains closer to the older eyeball value of 11.6 kHz (which should have been 10.66 kHz with the older component values) than the new calculated value of 9.04 kHz.
(See fiducials at older value of 11.6 kHz and new calculated value of 9.04 kHz.)

[Martin72]

Maybe a Rigol MSO5000 owner can present a bode plot of a RC element with similar values here.
The bodeplot function was implemented there after my MSO5000 time, i.e. it was not part of the actual development of the scope.

[TimFox]

Another investigation of my DHO914S with the shielded low-pass filter between CH1 and CH2.
I had carefully measured the actual R and C values for the box (including the effects of the AWG output resistance and CH2 input resistance and capacitance) for my previous post, giving a calculated corner frequency of 9040 Hz.
In the data set below, I manually stepped the frequency from 8,000 to 12,600 Hz in 200 Hz increments (plus one at 9,040 Hz), noting the CH2 V(pk-pk) and phase (CH1 - CH2) from the "Avg" results posted by the 'scope.
The solid lines are the measurements (with dots on the measured gain curve to show data points), while the dashed lines are calculated directly from the 9,040 Hz corner frequency.
While the solid lines do not have the "wiggles" found on the automated Bode Plots I did before, there are noticeable systematic discrepancies between the measured and calculated curves.
Fiducials were added at 9,040 Hz (expected) and 10,000 Hz (eyeball fit), as before.
To verify the connections, I reduced the frequency to 100 Hz and measured 0 dB and about 0.5 deg, as expected.
I have sent these results to Rigol for their consideration.
I am also open to suggestions about how to do these tests;  I already found that I needed to reduce the "counts" setting for the measurement results from default of 1000 to 50, to speed up the average's settling after changing frequency.

[moffy]

The plots look reasonably good to me. I previously mentioned that I did some noise and AWG SIN wave measurements, by downloading the memory data to my PC. The noise on my scope was around +/-15 counts out of 4096 counts or +/- 0.4% of fullscale. If the waveform isn't fullscale on the scope then the noise will be relative to the signal larger. When searching for zero crossings and peaks a degree of filtering was necessary and uncertainty remained. It's not terrible but good filtering is necessary for good results. You might want to increase the waveform averages, but that will slow down results, as well as the memory depth, this will increase the sample rate and might give a little more resolution.

[Martin72]

The plots look reasonably good to me.

I would like to see a screenshot of this.

[TimFox]

The plots look reasonably good to me. I previously mentioned that I did some noise and AWG SIN wave measurements, by downloading the memory data to my PC. The noise on my scope was around +/-15 counts out of 4096 counts or +/- 0.4% of fullscale. If the waveform isn't fullscale on the scope then the noise will be relative to the signal larger. When searching for zero crossings and peaks a degree of filtering was necessary and uncertainty remained. It's not terrible but good filtering is necessary for good results. You might want to increase the waveform averages, but that will slow down results, as well as the memory depth, this will increase the sample rate and might give a little more resolution.

I'll look at memory depth.  I didn't see any appreciable effect from measurement averages (in the manual scans) except to slow the measurement settling time.
With 2 V pk-pk out of the AWG and 0.5 V/div, the voltages are a reasonable fraction of full-scale.
I think there must be some firmware bugs in the measurement display, as well:  after changing the frequency, and waiting for the averaging to settle, the Max and Min values did not change and are much farther apart than the "Dev" would indicate.
The manual does not give any useful details about these variables.

(There are no screenshots for this latest graph of mine:  data were entered manually from the "Result" listing into Excel, thence to Grapher.)

[moffy]

The plots look reasonably good to me. I previously mentioned that I did some noise and AWG SIN wave measurements, by downloading the memory data to my PC. The noise on my scope was around +/-15 counts out of 4096 counts or +/- 0.4% of fullscale. If the waveform isn't fullscale on the scope then the noise will be relative to the signal larger. When searching for zero crossings and peaks a degree of filtering was necessary and uncertainty remained. It's not terrible but good filtering is necessary for good results. You might want to increase the waveform averages, but that will slow down results, as well as the memory depth, this will increase the sample rate and might give a little more resolution.

I'll look at memory depth.  I didn't see any appreciable effect from measurement averages (in the manual scans) except to slow the measurement settling time.
With 2 V pk-pk out of the AWG and 0.5 V/div, the voltages are a reasonable fraction of full-scale.
I think there must be some firmware bugs in the measurement display, as well:  after changing the frequency, and waiting for the averaging to settle, the Max and Min values did not change and are much farther apart than the "Dev" would indicate.
The manual does not give any useful details about these variables.

(There are no screenshots for this latest graph of mine:  data were entered manually from the "Result" listing into Excel, thence to Grapher.)

That is a shame, I thought averages would improve the resolution, they certainly should. It looks like you are doing everything well, not that you need my approval. The only thing left is to download the raw samples to a PC and do all the calculations yourself, but that kind of defeats the whole purpose! I don't expect sample depth/rate to make much if any difference but it's worth a try.

[TurboTom]

After having decided to return the scope and prepared the shipment, I'm looking at the whole situation somewhat more relaxed  . Of course, I very well understand the urge to understand what's going on here -- I've been in the same boat just a few days ago and contributed with quite some observations.

But viewed from the "other side" of having made a decision, I actually can only recommend not to spend your time on a product that's not ripe for the market yet and let Rigol do their job, don't do it for them!

If the distributor where you purchased your scope offers a return policy, return it ASAP. This will make Rigol feel that there's something wrong with their approach to supply unfinished products to paying customers, and maybe finally change their attitude (though it's likely they won't...).

IMO, it's not worth spending valuable time on a product that shouldn't have been supplied as faulty as this one without any warranty that it will get fixed eventually  .

[moffy]

After having decided to return the scope and prepared the shipment, I'm looking at the whole situation somewhat more relaxed  . Of course, I very well understand the urge to understand what's going on here -- I've been in the same boat just a few days ago and contributed with quite some observations.

But viewed from the "other side" of having made a decision, I actually can only recommend not to spend your time on a product that's not ripe for the market yet and let Rigol do their job, don't do it for them!

If the distributor where you purchased your scope offers a return policy, return it ASAP. This will make Rigol feel that there's something wrong with their approach to supply unfinished products to paying customers, and maybe finally change their attitude (though it's likely they won't...).

IMO, it's not worth spending valuable time on a product that shouldn't have been supplied as faulty as this one without any warranty that it will get fixed eventually  .

Personally I am quite happy with my purchase in spite of its foibles. I do expect Rigol to remedy many of the issues, but I also agree it looks rushed for sale. Being an engineer as we are, we like to investigate and we are all benefiting from TimFox's investigations even Rigol indirectly or directly.

[TimFox]

Update on my communications with Rigol:  (Rigol NA had escalated my case back to the R&D department).
Today, I received a request from the FAE department in China for information on my tests, so I wrote a 9-page organized memo with my relevant results, and sent it to them this afternoon, with attached data files.

[Fungus]

Update on my communications with Rigol:  (Rigol NA had escalated my case back to the R&D department).
Today, I received a request from the FAE department in China for information on my tests, so I wrote a 9-page organized memo with my relevant results, and sent it to them this afternoon, with attached data files.

Did you report anything other than Bode plot?

[Fungus]

Personally I am quite happy with my purchase in spite of its foibles. I do expect Rigol to remedy many of the issues,

Me too.

but I also agree it looks rushed for sale.

The holiday season is here, they need to get them on shelves. 

AFAIK NO manufacturer can get oscilloscopes right on initial release.

So... if supposedly knowledgeable people buy a cheap-as-chips oscilloscope that's so new that there's still a waiting list to get one, from a manufacturer that isn't famous for speedy/regular firmware updates...

...forgive me for finding it amusing when thay act all indignant about having a few bugs then post several times about how they're sending theirs back because they have some important work that needs doing.

If you have work to do and need a specific function then buy something you know will do it. Anything else reflects more on you than on Rigol.

[2N3055]

Personally I am quite happy with my purchase in spite of its foibles. I do expect Rigol to remedy many of the issues,

Me too.

Good for you.

but I also agree it looks rushed for sale.

The holiday season is here, they need to get them on shelves. 

AFAIK NO manufacturer can get oscilloscopes right on initial release.

So... if supposedly knowledgeable people buy a cheap-as-chips oscilloscope that's so new that there's still a waiting list to get one, from a manufacturer that isn't famous for speedy/regular firmware updates...

...forgive me for finding it amusing when thay act all indignant about having a few bugs then post several times about how they're sending theirs back because they have some important work that needs doing.

If you have work to do and need a specific function then buy something you know will do it. Anything else reflects more on you than on Rigol.

If you're referring to Thomas, he certainly is very knowledgeable about electronics.
Again ad hominem attack from you.

Are you suggesting if you are smart you should have known that: Rigol will release buggy and half functional product and Rigol does not care to fix bugs because people who buy Rigol products don't do any important work and Rigol products are not meant to work properly anyways.
Is that what you are saying? I certainly would not say that about Rigol, despite their shortcomings.

Also, you used word "indignant" as derogative, something that you project as a character flaw.
While, in fact, it means "feeling or showing anger or annoyance at what is perceived as unfair treatment."
Which is how everybody should feel if you are cheated. And to make it clear, Thomas wasn't "indignant" because scope did not have "some special function".
He bought it for functions that are in the datasheet and are stated as being there. DHO900 claimed it had specific functions he needed, it just was so badly made it was unusable. He paid for it with his own money. And he found it wanting, felt cheated and sent it back.


Your statements certainly reflect more on you than having anything to do with Rigol products.
I like that one..

[TurboTom]

...
So... if supposedly knowledgeable people buy a cheap-as-chips oscilloscope that's so new that there's still a waiting list to get one, from a manufacturer that isn't famous for speedy/regular firmware updates...

...forgive me for finding it amusing when thay act all indignant about having a few bugs then post several times about how they're sending theirs back because they have some important work that needs doing.

If you have work to do and need a specific function then buy something you know will do it. Anything else reflects more on you than on Rigol.

I assume you are pointing fingers at me here, Fungus, and you may be partially right. I purposely ordered the scope at a distributor who offers the no-questions-asked return policy for this situation.

But we may argue about your terminus "few bugs". I'ld rather consider the findings major shortcomings than just bugs. Please keep in mind that I would have spent round about 1000€ on the DHO914S including the digital porobe set.

I considered buying only the scope and making the probe set myself as per the instructions found here and elsewhere but finally decided against that since Rigol's digital probe set comes with proper connector housings, fast adjustable threshold comparators, a wide input voltage range (so I can even use it on ancient p-mos circuitry -- sometimes I'm a sucker for ancient calculators, right now I've got a broken Compucorp 326 calculator with a faulty ROM chip on my hobby table) and comes with resistive interconnects and 32 quality microclips which adds up to combined representing some value for the money, considering the time required for a DIY solution and the limited performance (but that's OT).

I several times told that I still consider the DHO800 a good choice, especially after Rigol will eventually have addressed the problems found so far. Btw, IMO the sampling / aliasing findings are not a real issue (for the DSO only modes -- the additional cut by half when the digital channels are enabled is a no-go). Maybe they would be for someone new to DSOs, but once familiar with with the matter, you just have to observe the sampling frequency and know what kind of signals you can measure accurately and what not, and if unsure, start with a single channel and enable additional channels as possible/required. It's just a question of common sense and some discipline.

But if I intend buy a new product for a sum that's not too far from other, proven working products, I at least expect it to provide the promised features in a way that they are useable / useful. I gave Rigol a chance despite knowing of their history of slow problem solving, assuming / hoping they have learned from the past, only to find that basically all the added functionality that I intended to buy the DHO900 for, are unfinished construction sites or hardware-limited to a degree that wasn't forseeable from analyzing the teardowns and early reports of the product. So I decided to pull the safety-line, that's it.

I've got enough equipment that covers the basic oscilloscope functions, but if I've got (or just want) to replace one of these in future, the DHO800 is probably still high on the list, provided its performance matches the requirements.

If this approach amuses you, well, I'm glad you found some "joy" in my contributions 

All the best,
Thomas

[iMo]

IMHO, the Rigol will not start to fix faster and will not start to produce better sw/fw quality products unless the buyers will simply start to return their products back. It could easily be their leaders consider their products perfect as they do not see such a tangible buyer's feedback in their $$ numbers.. 

[ebastler]

I considered buying only the scope and making the probe set myself as per the instructions found here and elsewhere but finally decided against that since Rigol's digital probe set comes with proper connector housings, fast adjustable threshold comparators, a wide input voltage range (so I can even use it on ancient p-mos circuitry -- sometimes I'm a sucker for ancient calculators, right now I've got a broken Compucorp 326 calculator with a faulty ROM chip on my hobby table -- and comes with resistive interconnects and 32 quality microclips which adds up to combined representing some value for the money, considering the time required for a DIY solution and the limited performance (but that's OT).

Nice to see someone with a similar obscure interest here!   The large input voltage range was what got me interested in the Rigol logic probe (and potentially the DHO900) as well. The LGP-30 I am occasionally working on has logic levels of -20V and 0V... And it's a train trip away from me, so a portable scope would be nice.

Even with the compromised sampling rates, the DHO900 logic analyzer (with or without use of the analog channels) still has better specs than the Saleae Logic Pro 16, which costs 1.5x the price and does not support the large input voltage range. So I am still on the edge: I am very disappointed with Rigol that they did not publish an honest spec sheet, but even the compromised specs are quite competitive.

I will definitely wait and see whether Rigol gets the Bode wiggles fixed (and quickly!). Back when the DS1054Z was released, I bought mine right after Rigol had fixed the "jitter bug" it had originally been released with, which took them about three months. Let's see whether they repeat that story with the Bode plot on the DHOs. And let's see whether Siglent make up their mind on a price for the SDS1000X HD in the meantime, which might make the decision easier -- or more difficult...

EDIT: It only took 6 weeks from publishing the "5µs jitter bug" in an EEVblog video (#683, 14 Nov 2014) to having a firmware fix from Rigol. They actually published a Beta fix on the forum even earlier, but the first attempt did not work properly. The fix released around New Year did the trick then.

Maybe we should ask Dave to make a video about the "Bode wiggle bug" too, to speed things up? 

[ebastler]

IMHO, the Rigol will not start to fix faster and will not start to produce better sw/fw quality products unless the buyers will simply start to return their products back. It could easily be their leaders consider their products perfect as they do not see such a tangible buyer's feedback in their $$ numbers.. 

An EEVblog video about the "Bode wiggle bug" would probably be a more effective way to get Rigol's attention. As mentioned above, it got them to fix the "5µs jitter bug" in the DSO1054Z within 6 weeks after video #683 was published. (That bug was even nastier than the Bode problem, since it affected the core scope functionality.)

I think the percentage of returned scopes will always be small, since most users will not do a detailed evaluation right after purchasing. And those who do return it may do so for various reasons -- including "harmless" ones like finding the screen too small for their hands or eyes etc. I would not expect the resellers and Rigol to even collect information about the reasons for returns, hence specific bugs might not get direct attention that way. To put the spotlight on a really obnoxious bug, nothing beats public visibility...

EDIT: Fixed the mangled video reference, it's #683.

[dmulligan]

Has Rigol been providing timely fixes for bugs reported on their other DHO platform scopes?  I would have expected the second model series released to have less bugs and the DHO800 and 900 series to have even less.  If not less, then I would expect the bugs to be limited to UI layout and other series specific differences.
I hope Rigol's software team is using a common code base and stabilizing the whole platform rather than working on each one in isolation.  I also hope that Rigol has developed a quality automated software unit test suite to help them do this.  Are these things too much to hope for?

[TimFox]

Update on my communications with Rigol:  (Rigol NA had escalated my case back to the R&D department).
Today, I received a request from the FAE department in China for information on my tests, so I wrote a 9-page organized memo with my relevant results, and sent it to them this afternoon, with attached data files.

Did you report anything other than Bode plot?

I also reported the measurements I made manually, using the magnitude and phase measure functions, stepping the frequency through narrow ranges.
For the second set, I had shielded the filter components and carefully measured the actual resistance and capacitance (including connectors) to calculate the -3 dB point.
The measured frequency for -3 dB and -45^o was about 10% higher than the calculated frequency;  otherwise, the curves were smooth.