Rigol DHO914S Bode plot

[Martin72]

Yepp,
Appx 18€ more. 
SDS1104X-E + SDG1032X are around 850€ together (incl.VAT), DHO914S cost 832€ (incl VAT), prices from batronix.
But this is only interesting when you need a good bode plot function(and a good 2-ch awg), you don't have 12 bit for it then.

[TimFox]

My DHO914S, direct from Rigol North America, cost $699 plus shipping.
I'm not anyone's sales representative, just trying to understand how to improve this measurement.

[Martin72]

Understand, back to your topic:
If your time allows you, can you make a short video from the bode menu ?
As written before the manual is a little bit meager.

[TimFox]

Understand, back to your topic:
If your time allows you, can you make a short video from the bode menu ?
As written before the manual is a little bit meager.

"Meager" is a polite word here;  unfortunately the series hasn't been out long enough for someone to write a relevant "dummies" book.
My next experiment is to build the simple network in a properly shielded box, but I'm busy elsewhere until next week.
I also need to figure out how to increase the drive voltage:  I believe the default I used here is only 200 mV pk-pk, but the generator can do 5 V pk-pk max.

[mawyatt]

You shouldn't need a shielded box for this type of simple Bode measurements, now if you are measuring the Open-Loop response of a op-amp in Closed-Loop form (injecting a small signal in the loop), then maybe shielding might be required.

Wonder if the Bode implementation utilizes any sort of frequency selectivity like other implementations?

Best,

[Martin72]

Really quick and dirty with the 1104X-E and SDG2122X, 8K2 and 2n2 cap...

[TimFox]

That's what I was expecting to see.
If I can't get my Rigol better, is there a contact at the company that is worth contacting?

[TimFox]

I've done a bit more experimenting, using exactly the same R-C setup while waiting for my shield box to be delivered.
I played with the bandwidth limit (125 or 20 MHz) and the amplitude (0.2 or 2 V) and the FIR filter (On/Off), increasing the points/decade from 100 to 200, keeping the log scan from 1 to 100 kHz.
No noticeable changes resulted.
I then did the "slow way" manual scan, using the voltage measurement function (pk-pk) and phase measurement function, at essentially the same frequency points (truncating to nearest Hz), manually entered, between 75 and 100 kHz.
The attached graph shows the dB and phase results from that subset of the Bode Plot results, and another graph for the slow manual result.
Both were taken at 20 MHz bandwidth limit and 2 V.
I added symbols to show the actual sample frequencies, and switched to a linear frequency axis for the limited frequency range.
It really looks like bad firmware on the Bode Plot, since the graphs with discrete frequency measurements look reasonable and are certainly monotonic.

Thanks to ebastier, KedasProbe, CosteC, and Martin72 for the suggestions I followed here.

[TurboTom]

The postman just brought me one of these "toys" as well and, unfortunately, I've got to chime in with the same findings: The Bode Plot function is pretty much useless as it is right now!

I found the same "wigglyness" of the traces while at least a monotony should be shown in case of a simple low pass. Maybe they designed too many "wiggles" in the traces on the instrument's PCB... 

Whatsoever, there are several points that are possibly worse (but easier to get right):
- The Bode Plot function won't start (and it also won't display any message) if the signal exceeds the range of the "Output" channel (didn't test this for the "input" channel). So it may be necessary to manually adjust the sensitivity of the channels correctly before starting the Bode Plot.
- The preset number of points relates to a decade, not to the whole measurement interval (this may be a convention and I'm not aware of it).
- The setting for the measurement frequency spacing ("Line"/"Log") only relates to a decade, within the next decade, the intervals may change even if the spacing is chosen "Line".
- The on-screen display of the plot is abysmal: Even though the markings suggest a Logarithmic frequency scale, the values are placed linearly within each decade which results in kinks in the diagrams, regardless of Lin/Log setting, see RigolDS7(log) and RigolDS8 (lin) ! I'm not sure if the situation is the same for Amplitude values since the scale of the Y-axis apparently cannot be adjusted to a range that allows a better resolution. Same for the frequency axis, only automatic scaling is available.

So right now, for displaying the bode plots, you've got to take the route via the CSV output. If you think, in its current configuration, the DHO900 is a good tool for high resolution Bode Plots, think again... I'm actually considering to return the instrument since this function was the one that I'ld needed the most for a project I'm currently working on. I'm pretty sure Rigol will get it right eventually, but how long that may take is written between the stars... I guess for the time being, I'm better off with my ADALM2000 and the "Scopy / NWA" tool.

[TimFox]

That looks like the same problem I reported, and I am also disappointed.
Every time I obtained a scan, I noticed the non-monotonic behavior as it was displayed ("wiggles"):  clearly visible even with the narrow display range.
The reason I graphed the results (.csv file) on a graphing program was that I could not adjust the scale on the screen:  I assumed that I needed to study the manual more.
In my graphs, the x-axis is the frequency values listed in the first column of the .csv file.
Rigol only advertises the Bode Plot function to be used with an isolation transformer to evaluate feedback stability, and it does announce the voltage and phase margins for that measurement.
The first scan I did (not posted here), I had an attenuator on Ch1 so it would look more like a feedback Bode Plot (gain above unity at low frequency), and those margins were calculated.
Is there a useful address at Rigol to report bugs?

[2N3055]

I assume the phase measurement is in there

edit: page 142 in the English DHO900 user guide

That is something else, a phase measurement in Measurements. It is not a plot but single point phase measurement.
Bode Plot is something completely different.

[2N3055]

Understand, back to your topic:
If your time allows you, can you make a short video from the bode menu ?
As written before the manual is a little bit meager.

"Meager" is a polite word here;  unfortunately the series hasn't been out long enough for someone to write a relevant "dummies" book.
My next experiment is to build the simple network in a properly shielded box, but I'm busy elsewhere until next week.
I also need to figure out how to increase the drive voltage:  I believe the default I used here is only 200 mV pk-pk, but the generator can do 5 V pk-pk max.

The more I read real user feedback now that specimens are starting to trickle in (and it has been few months already since it was "released") I see that it is even worse than I expected (I thought they would cleanup things a bit by now)..

These scopes are still in alpha stage of development as of today... I actually expected much better from Rigol. Hope soon this gets much better, otherwise users will not be happy...

[2N3055]

You shouldn't need a shielded box for this type of simple Bode measurements, now if you are measuring the Open-Loop response of a op-amp in Closed-Loop form (injecting a small signal in the loop), then maybe shielding might be required.

Wonder if the Bode implementation utilizes any sort of frequency selectivity like other implementations?

Best,

Mike,

I actually did tests (on my Siglent and Keysight scopes) where I injected both signals and wideband noise from additional AWG into circuit while doing Bode and got perfect results as long as  S/N ratio was something reasonable. In normal lab normal levels of injected noise from environment are simply not visible in measurements.
If Bode is done right it should not matter.

Best.

Siniša

[ebastler]

The more I read real user feedback now that specimens are starting to trickle in (and it has been few months already since it was "released") I see that it is even worse than I expected (I thought they would cleanup things a bit by now)..

These scopes are in alpha stage of development as of today... I actually expected much better from Rigol. Hope soon this gets much better, otherwise users will not be happy...

I am, frankly, not too surprised that functionality which is new vs. the DHO 1000 and 4000 series is immature. Those scopes don't have the function generator option and hence the Bode plot functionality. They don't have the 16 digital inputs either -- seems that nobody has tested the logic analyzer on the DHO 900 yet, but I am sure it holds its own surprises...

The fact that basic storage scope functions -- like zooming in and out on captured traces, or storing trace data on a USB stick -- also seem to have glitches is more disconcerting to me. The HDO/DHO 1000 and 4000 have been out for a year and they share that functionality. Do they share the bugs too?

[KedasProbe]

I assume the phase measurement is in there

edit: page 142 in the English DHO900 user guide

That is something else, a phase measurement in Measurements. It is not a plot but single point phase measurement.
Bode Plot is something completely different.

Since doing it the slow way (point per point and making your own graph) it is measuring stable/correctly (luckily), most likely they changed something so the timing for the measurements changed and screwed the automated measurement script that was probably working before.
So I'm sure it's an easy fix, just get the measure timings right. (it may end up slower though)

[TurboTom]

That looks like the same problem I reported, and I am also disappointed.
Every time I obtained a scan, I noticed the non-monotonic behavior as it was displayed ("wiggles"):  clearly visible even with the narrow display range.
The reason I graphed the results (.csv file) on a graphing program was that I could not adjust the scale on the screen:  I assumed that I needed to study the manual more.
In my graphs, the x-axis is the frequency values listed in the first column of the .csv file.
Rigol only advertises the Bode Plot function to be used with an isolation transformer to evaluate feedback stability, and it does announce the voltage and phase margins for that measurement.
The first scan I did (not posted here), I had an attenuator on Ch1 so it would look more like a feedback Bode Plot (gain above unity at low frequency), and those margins were calculated.
Is there a useful address at Rigol to report bugs?

The isolation transformer may be required if the DUT is a powerful amplifier (or a PSU) in order to eliminate ground loops or ground reference offset problems. In case of a low power passive network as a DUT, this is completely irrelevant since the Scope's source and measurement channels are rather had-linked to the same ground and only minuscule currents are flowing.

To verify this, I ran another test with a really high impedance network: A low pass of 100kOhm and 330pF -- same result.
I also tested the effect of enabling the FIR filter -- virtually none, but maybe it helps if there's noise on the signal or if there's heavy distortion (harmonics) that shouldn't be evaluated. But as long as the Bode Plot algorithm isn't refined substantially, all this is a moot point.

Btw, running the plot (1kHz~100kHz -- 2 decades; 50pts each) takes 52s (FIR disabled - RigoDS9) vs. 90s (FIR enabled - RigolDS10).

[2N3055]

I assume the phase measurement is in there

edit: page 142 in the English DHO900 user guide

That is something else, a phase measurement in Measurements. It is not a plot but single point phase measurement.
Bode Plot is something completely different.

Since doing it the slow way (point per point and making your own graph) it is measuring stable/correctly (luckily), most likely they changed something so the timing for the measurements changed and screwed the automated measurement script that was probably working before.
So I'm sure it's an easy fix, just get the measure timings right. (it may end up slower though)

I said it is not done that way. Bode plot function does not simply automate manual procedure.. It uses different principle.
If they used that Measurement function for BODE, that could be explanation for weird results.

As for what is easy, I disagree.. We have no data of internals and have no clue if this is something simple or whole bode architecture is wrong... It might look easy, but look at the numerous bugs reported... I guess it is not that easy...

[TimFox]

I believe the transformer is required in order to inject the input signal into the "guts" of a feedback amplifier, without disconnecting the feedback, to get a Bode Plot of the loop gain/phase for stability analysis.
The measurement itself, however, should be a frequency scan measuring the relative gain and phase of two channels.
The measurement menu has enough functions to measure relative gain and phase at a single frequency.

[Cees]

I own the 924S and had some similair things. Keep in mind that the default voltage of the generator is peak to peak which is rather low. Besides using the delivered power supply I was using a Lenovo power supply which is quite noisy compared with the one supplied. In addition, I had very rough plots that were absent with the supplied power supply. Furthermore, the earth was important for reducing the noise. I also discovered many bugs such as the fixed vertical scales that make reading more difficult and the automatic gain does not work well at low frequencies. Hereby a plot of an audio transformer. 

[moffy]

Thanks TimFox for the info, I got my DHO914s a few days ago and was about to try some Bode plots, I'll see how it goes.

[tv84]

I found the same "wigglyness" of the traces while at least a monotony should be shown in case of a simple low pass. Maybe they designed too many "wiggles" in the traces on the instrument's PCB... 

 

[TimFox]

I found the same "wigglyness" of the traces while at least a monotony should be shown in case of a simple low pass. Maybe they designed too many "wiggles" in the traces on the instrument's PCB... 

As foul-minded 8th-graders back in 1963, referring to our young female typing teacher, we declared "all that wiggles is not Jell-O".

[moffy]

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.

P.S. If the sampling rate is a 2^n multiple of the test frequency then the error would tend to zero if such a method is used. You want to sample complete cycles.

[TimFox]

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.

As someone else mentioned on one of these threads, an actual "Bode Plot" is an approximation, using straight lines on semi-log graph paper (log freq on the x-axis, and dB or degrees of phase on the y-axis) as a quick drawing method for paper graphs of loop gain in a feedback circuit:  I learned this from H Bode's book Network Analysis and Feedback Amplifier Design Van Nostrand 1945, and used it manually before we had PCs on our desks to estimate loop gain/phase from the sums of individual piecewise linear curves for each time constant.
The "Bode Plot" function in this unit should be an accurate plot of gain and phase over a desired frequency range, with a measure of the gain and phase margins for the loop gain.
My next step is to carefully measure my network's R and C values, including connectors and 'scope input impedance, to see if the -3 dB / -45^o point is at the correct frequency.
In my first graph, a few posts above, I found that even with the wiggles, -3 dB and -45^o coincided, but at a noticeably higher frequency than expected: 11,610 Hz instead of calculated 10,660 Hz, a 9% discrepancy with nominal 1% resistor and 2% capacitor tolerance .

[mawyatt]

I believe the transformer is required in order to inject the input signal into the "guts" of a feedback amplifier, without disconnecting the feedback, to get a Bode Plot of the loop gain/phase for stability analysis.
The measurement itself, however, should be a frequency scan measuring the relative gain and phase of two channels.
The measurement menu has enough functions to measure relative gain and phase at a single frequency.

Yes a transformer is usually utilized. The negative feedback loop is broken in a location where the transformer secondary impedance doesn't effect the loop, and the DC bias is unaffected and can pass thru. When a signal is applied to the primary this introduces an "outside disturbance signal" in the loop via the secondary. If the loop remains stable and "pseudo linear" then the loop produces a countering signal in an attempt to neutralize the injected disturbance signal, the ratio of the loop generated correcting signal to the injected signal is the total loop gain and phase around the loop regardless of the injection point. Of course certain injection points are better than others, and some experimentation is often required.

There are a number of posts related to Bode Plots, techniques and injection transformers. As many, we used some Common Mode Filters rewound as injection transformers, recall they worked well over 3~4 decades.

This is extremely disappointing wrt the Rigol Bode Plot implementation in these new 12bit DSOs, we were hoping for much better reports now, that knowledgable folks are getting their hands on such.

We have an 814 due Thursday, for evaluation for a client that doesn't require these features, but all this doesn't "bode" well (pun intended) for these new Rigol 12 bit DSOs!!

Best,