Rigol DHO914S Bode plot

[blackdog]

Hi,

Here on the EEV forum today I made a topic about a simple Filter to learn how to test a scope with Bode Plot capability.

I provide a modified schematic of a bandpass filter that is somewhat less sensitive to load variations.
So that Bode plot characteristics of different types and brands of scoops can be better compared.

I tested it with two types of Siglent scoops, the SDS Plus and the SDS HD, a Rigol scope with Bode Plot capability I do not have.
My tests were done with an external Siglent Function Generator, but of course it can also be done with the Function Generator present in the scope itself, if you have the license for that.

Have fun with the Bode Plot function!

Link to the topic:
https://www.eevblog.com/forum/projects/bode-plot-test-filter/

Kind regards,
Bram

[Someone]

But considering that the whole point of having a DHO900S is more or less the advertized Bode Plot function, what sense does it make to purchase this scope while you've still got to do the "heavy lifting" in a PC.

As with bode plotting on all scopes at this point, they make so many assumptions and fixed decisions that they are unlikely to be the best solution to any given problem. There is a very large parameter/configuration/tradeoff space between different requirements that needs at least several more options (as spectrum analysers do). Using an external programmable something, be that PC or raspberry Pi, is still motivated for anything beyond toy examples.

[Cees]

Oh, sigh... I must have misunderstood Cees' post #60 then.

Here's hoping that Rigol do address this next time round. Getting one of the two selling points of the DHO 900 series wrong should be kind of important to them...

Based on the complaints I had regarding the auto ranging, this appears to have been resolved. Elsewhere I have indicated how much interference there is around the AWG and have seen the wrinkels as well. However, it appears that this also depends on the amplitude of the generator and the default value of 200 mV peak to peak is far too low.  I myself run at least with 2 volts or higher. Next what I have mentioned was the fixed vertical scales which is still not solved.

[ebastler]

Using an external programmable something, be that PC or raspberry Pi, is still motivated for anything beyond toy examples.

So what software do you (and others) use to acquire Bode plots? Are there good (and affordable) solutions which can control instruments from various manufacturers via SCPI, and have pre-programmed Bode plot acquisition and display with a good balance of flexibility and ease-of-use?

[moffy]

Using an external programmable something, be that PC or raspberry Pi, is still motivated for anything beyond toy examples.

So what software do you (and others) use to acquire Bode plots? Are there good (and affordable) solutions which can control instruments from various manufacturers via SCPI, and have pre-programmed Bode plot acquisition and display with a good balance of flexibility and ease-of-use?

Personally I have started experimenting with Visual Studio C++, and have written a small test program that sets the AWG frequency and amplitude, scales ch1 and takes a sample of the memory and downloads it to a file. It is pretty straight forward, and much easier than I anticipated. One can then analyse the data on the PC. Opens up a world of possibilities.

[moffy]

I captured the AWG output at 1kHz/5Vp-p on CH1/1V/div, 200us/div with 62.5Msps and 1Mpts (downloaded 100kpts) and the output is quite noisy of the order of +/- 1.25% or +/-15 counts for a peak of 1200 counts. It also has a noticeable DC bias around +150mV. Can't tell what is the ADC or the AWG yet with regards to noise.

[ebastler]

Can't tell what is the ADC or the AWG yet with regards to noise.

Take a capture with the same scope settings but a grounded input for comparison?

[moffy]

Can't tell what is the ADC or the AWG yet with regards to noise.

Take a capture with the same scope settings but a grounded input for comparison?

Thanks, I'll get there but diner first.

[moffy]

Did the same test, AWG not running, with a 51R resistor connected across the BNC input and got a similar result. The noise looks repetitive, possibly PSU related, when I get my USB C male breakout connector I'll try again with a battery as the supply.

[Someone]

Using an external programmable something, be that PC or raspberry Pi, is still motivated for anything beyond toy examples.

So what software do you (and others) use to acquire Bode plots? Are there good (and affordable) solutions which can control instruments from various manufacturers via SCPI, and have pre-programmed Bode plot acquisition and display with a good balance of flexibility and ease-of-use?

Is free affordable? 
https://www.eevblog.com/forum/testgear/faster-fra-from-scope-by-external-control-over-visa/
Easy enough to mix and match vendors/instruments with that sort of setup. Bugs are left for the reader to fix! Same package can also produce the pretty plots if desired.

[Someone]

Just pondering how they do the phase/amplitude calculation, the way that comes to mind is an FFT of CH1 and CH2 and extract the amplitude and phase from that. But they would be shortish sample sizes for speed and not synchronised to the input sine. This would lead to both phase and amplitude errors perhaps even the ripple seen. Not sure if that is how they do it or it is even the best way.

No mention in the manual of the method, nor can I find any algorithmic information on Google.

I'm not sure any of the scope manufacturers have detailed what measurement and processing techniques are being used to produce their results, so people have been trying to reverse engineer implementations for curiosity/understanding. A problem is the different possible methods all have their own plusses/minuses but it doesn't appear any of the scopes are offering different modes. Long captures and big FFTs vs hardware averaging, single tone vs chirp vs noise.

Some people think chirps are magical:
https://www.ap.com/blog/how-many-ways-can-we-measure-frequency-response/
But from actually building out systems measuring frequency response by optimised chirps, impulse/step response, and stepped sine, I keep coming back to stepped sine due to its dynamic range advantages (which is generally more pressing/extreme in electronics than in acoustics or mechanical/vibration).

[2N3055]

But considering that the whole point of having a DHO900S is more or less the advertized Bode Plot function, what sense does it make to purchase this scope while you've still got to do the "heavy lifting" in a PC.

As with bode plotting on all scopes at this point, they make so many assumptions and fixed decisions that they are unlikely to be the best solution to any given problem. There is a very large parameter/configuration/tradeoff space between different requirements that needs at least several more options (as spectrum analysers do). Using an external programmable something, be that PC or raspberry Pi, is still motivated for anything beyond toy examples.

What implementation do you refer to (as being too monolitic and not flexible enough) and what do you miss ?

[Someone]

But considering that the whole point of having a DHO900S is more or less the advertized Bode Plot function, what sense does it make to purchase this scope while you've still got to do the "heavy lifting" in a PC.

As with bode plotting on all scopes at this point, they make so many assumptions and fixed decisions that they are unlikely to be the best solution to any given problem. There is a very large parameter/configuration/tradeoff space between different requirements that needs at least several more options (as spectrum analysers do). Using an external programmable something, be that PC or raspberry Pi, is still motivated for anything beyond toy examples.

What implementation do you refer to (as being too monolitic and not flexible enough) and what do you miss ?

The obvious first step would be to offer some form of speed vs noise suppression tradeoff for the user to choose. Trying to pull noise out of a switching regulator control loop is a wildly different environment to checking the bandwidth of a filter out of circuit/use. The supplied tools are very rudimentary and this is a space with plenty of scope to build productivity improving solutions, question is there a market big enough for development effort to go into it? So far not so much.

[2N3055]

But considering that the whole point of having a DHO900S is more or less the advertized Bode Plot function, what sense does it make to purchase this scope while you've still got to do the "heavy lifting" in a PC.

As with bode plotting on all scopes at this point, they make so many assumptions and fixed decisions that they are unlikely to be the best solution to any given problem. There is a very large parameter/configuration/tradeoff space between different requirements that needs at least several more options (as spectrum analysers do). Using an external programmable something, be that PC or raspberry Pi, is still motivated for anything beyond toy examples.

What implementation do you refer to (as being too monolitic and not flexible enough) and what do you miss ?

The obvious first step would be to offer some form of speed vs noise suppression tradeoff for the user to choose. Trying to pull noise out of a switching regulator control loop is a wildly different environment to checking the bandwidth of a filter out of circuit/use. The supplied tools are very rudimentary and this is a space with plenty of scope to build productivity improving solutions, question is there a market big enough for development effort to go into it? So far not so much.

OK, speed argument is not a feature, but we already agreed that is valid point in a separate topic...

Apart from that?
What is so rudimentary it left you wanting..?

I look at my Siglent and see, just of top of my head:
- automatic adaptive /manual level stimulus
- Vary level stimulus (where you draw level/frequency graph to have stimulus vary with frequency). Up to 4 stored profiles.
- offset (for DC biased measurements)
- Amplitude unit and system impedance.
- 3 channels of analysis
- linear and log sweep
- number of points
- Cursor measurements
- Automated measurements of -3dB LP point, HP point and BW
- Automated measurement of GM and PM  (gain and phase margin)
- Full control of graphing (auto/manual) mode, axis types, units etc..
- Table data always visible with graph.
- Many more details
- Save CSV data for PC analysis. And with built in networking, you can save directly on PC/server (shared folder) without sticks cards etc.

That is why I asked: what specifically? If it's a good suggestion, I will pass it on to Siglent. They are always listening to ways how to make their products more useful for customers...

[TurboTom]

...
That is why I asked: what specifically? If it's a good suggestion, I will pass it on to Siglent. They are always listening to ways how to make their products more useful for customers...

Really? I got the impression when reviewing and analyzing the SDG6000X that very little of my (IMO reasonable) suggestions have been considered by them. Possibly their Oscilloscope department is more responsive to user / customer wishes.

[2N3055]

...
That is why I asked: what specifically? If it's a good suggestion, I will pass it on to Siglent. They are always listening to ways how to make their products more useful for customers...

Really? I got the impression when reviewing and analyzing the SDG6000X that very little of my (IMO reasonable) suggestions have been considered by them. Possibly their Oscilloscope department is more responsive to user / customer wishes.

I don't work for Siglent and cannot comment on that. I still have same SDG6000X as you, paid with my own money.
They did fix many original complaints but not all of the suggestions.
I guess there is hardware/software distinction as to what can or cannot be done sometimes to consider.
Anyways, it is for Siglent to answer.

OTOH as you can see from SDS2000X HD topic, there are literally dozens and dozens things implemented as enhancements as per user requests. Fixed memory/sampling mode, more Math channels etc. etc.. In Bode plot alone more than dozen things were added as per user request.. Where it is possible. Higher screen resolution is not gonna happen on existing scope...

[TurboTom]

Yes, this is basically what makes me assume that there are completely differend divisions of the engineering department working on the gear: The Scope and RF (SA/VNA) groups are pretty decent and maintain good "customer relations" while the AWG section -- let's put it like this -- leaves room to improve  . But that's off topic here, so I'll shut up on this now.

[moffy]

I did an FFT plot up to 1MHz for the DHO914s CH1 input with 51R resistor and there are some peaks in the region of 10-100kHz. The vertical scale is db, horizontal Hz, the bins are about 963Hz apart so the high level below 1kHz is just DC.

[TimFox]

The guy at Rigol has sent my data to the relevant group.
He asked me for data from a "direct" connection, so I disconnected the LP filter network and its BNC to binding post adapters, driving Ch2 through a 12 inch coax from the TEE on Ch1.
Obviously, the resulting gain and phase curves have much less variation over the 1 to 100 kHz, 100 pts/decade scan than had the LPF curves.
Note the dots on the phase curve, showing the actual frequencies for each data point.
The gain curve was reasonably flat (+/- 0.25 dB), but the phase curve shows a similar ripple, with a somewhat longer period.
See attached curves (vertical scales adjusted for new range), which I sent him along with the .csv file.

[iMo]

I would try to connect the AWG to the CH1 and do an AWG sweep from 1kHz to 100kHz (w/ proper 50ohm termination).
Look at the CH1 signal envelope - are there the (AM) wiggles as well?
Not sure it will be clearly visible when your wiggle amplitude (aka "AM modulation index") is 6% only..
PS: when none wiggles visible the issue could be a math bug in Bode, otherwise the AWG could be the issue..

[ebastler]

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?

[TimFox]

I would try to connect the AWG to the CH1 and do an AWG sweep from 1kHz to 100kHz (w/ proper 50ohm termination).
Look at the CH1 signal envelope - are there the (AM) wiggles as well?
Not sure it will be clearly visible when your wiggle amplitude (aka "AM modulation index") is 6% only..
PS: when none wiggles visible the issue could be a math bug in Bode, otherwise the AWG could be the issue..

I'll try this later;  I'm not sure how to set up a useful frequency sweep.  In all the results I have posted here, there was no 50 ohm termination, for frequency < 100 kHz.
Earlier, I used a 50 ohm termination to make sure of the output impedance and drive capability:  5 V pk-pk (maximum AWG setting) gives 5 V pk-pk without, and 2.5 V pk-pk with, the 50 ohm termination.
I don't think the Bode Plot sweep retains Ch1 ("input") data separately, only the ratio Ch2/Ch1 (in dB) and phase difference.
On the narrow-band manual data (70 to 100 kHz) I posted above,  https://www.eevblog.com/forum/testgear/rigol-dho914s-bode-plot/msg5102484/#msg5102484 , again without 50 ohm termination, the measured pk-pk voltage on Ch1 was quite constant at 2 V pk-pk (probably < 1% pk-pk), and the equivalent graph of voltage ratio and phase shift was acceptably monotonic, using the same frequency points (in that narrow range) as the Bode Plot function used over the wider range.  (Those data were obtained before the recent firmware update, but I haven't seen any important difference to the Bode Plot results from that update.)

[iMo]

I would try to connect the AWG to the CH1 and do an AWG sweep from 1kHz to 100kHz (w/ proper 50ohm termination).
Look at the CH1 signal envelope - are there the (AM) wiggles as well?
Not sure it will be clearly visible when your wiggle amplitude (aka "AM modulation index") is 6% only..
PS: when none wiggles visible the issue could be a math bug in Bode, otherwise the AWG could be the issue..

I'll try this later;  I'm not sure how to set up a useful frequency sweep.  In the results I have posted here, there was no 50 ohm termination, for frequency < 100 kHz.
Earlier, I used a 50 ohm termination to make sure of the output impedance and drive capability:  5 V pk-pk (maximum AWG setting) gives 5 V pk-pk without, and 2.5 V pk-pk with, the 50 ohm termination.
I don't think the Bode Plot sweep retains Ch1 ("input") data separately, only the ratio Ch2/Ch1 (in dB) and phase difference.
On the narrow-band manual data (70 to 100 kHz) I posted above, again without 50 ohm termination, the measured pk-pk voltage on Ch1 was quite constant at 2 V pk-pk (probably < 1% pk-pk), and the equivalent graph of voltage ratio and phase shift was acceptably monotonic, using the same frequency points (in that narrow range) as the Bode Plot function used over the wider range.

My test above is not made with the "Bode" o'scope functionality.
It shows you the "envelope" of the AWG sweep.

Thus the setup is simply AWG->Ch1 (50ohm terminated), do the sweep, watch the "envelope" of the signal in CH1.

Your Bode wiggles mean the CH1 signal envelope shall be 6% amplitude modulated, provided the Bode wiggle issue comes from the AWG.

When the envelope of the sweep signal is clean (none AM modulation), your wiggles in Bode come from math bug in Bode.

[2N3055]

I would try to connect the AWG to the CH1 and do an AWG sweep from 1kHz to 100kHz (w/ proper 50ohm termination).
Look at the CH1 signal envelope - are there the (AM) wiggles as well?
Not sure it will be clearly visible when your wiggle amplitude (aka "AM modulation index") is 6% only..
PS: when none wiggles visible the issue could be a math bug in Bode, otherwise the AWG could be the issue..

I'll try this later;  I'm not sure how to set up a useful frequency sweep.  In the results I have posted here, there was no 50 ohm termination, for frequency < 100 kHz.
Earlier, I used a 50 ohm termination to make sure of the output impedance and drive capability:  5 V pk-pk (maximum AWG setting) gives 5 V pk-pk without, and 2.5 V pk-pk with, the 50 ohm termination.
I don't think the Bode Plot sweep retains Ch1 ("input") data separately, only the ratio Ch2/Ch1 (in dB) and phase difference.
On the narrow-band manual data (70 to 100 kHz) I posted above, again without 50 ohm termination, the measured pk-pk voltage on Ch1 was quite constant at 2 V pk-pk (probably < 1% pk-pk), and the equivalent graph of voltage ratio and phase shift was acceptably monotonic, using the same frequency points (in that narrow range) as the Bode Plot function used over the wider range.

My test above is not related to the "Bode" o'scope functionality.
It shows you the "envelope" of the AWG sweep.
Thus the setup is simply AWG->Ch1 (50ohm terminated), do the sweep, watch the "envelope" of the signal in CH1.
Your Bode wiggles mean the envelope shall be 6% amplitude modulated, provided the Bode wiggle issue comes from the AWG.
When the envelope of the sweep signal is clean (none AM modulation), your wiggles in Bode come from math bug in Bode.

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.

[iMo]

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 in the amplitude with 6% (0.5dB) modulation index..

PS: the 3rd issue could be there is a phase shift (or jitter or something like that) between CH1 and CH2 when doing Bode..