Rigol DHO914S Bode plot

[iMo]

This is the myGenerator hardware (an example only)..
0-60MHz 2Vpp sine (9851 DDS), couple of milliHerz step, controlled via USB..
Almost everybody here has something like that in his junkbox handy. No need to have AWG for bode..

[Fungus]

Almost everybody here has something like that in his junkbox handy. No need to have AWG for bode..

True, but it would be a nice feature and not much extra work.

[iMo]

If you can control the INTERNAL AWG via SCPI, then simply create a BODE_APP (android one) which will call AWG via scpi, do set sine, a freq and ampl, and then it will make a measurement in CH1 and CH2 simultaneously (with some param), do repeat for all freqs, you get N .csv files (best placed in the ramdisk as I wrote above) and the BODE_APP will then crunch out a nice picture from all those files Easy..

For DHO800 and none-S owners we may easily use an external generator (of any kind). We will not call internal AWG via scpi, but the external myGenerator via simple commands via the USB connector in the front panel (all under our control). We may add some relay clicking for an attenuator/amplifier in myBode black box.. Setting a DDS/PLL/etc.,etc. chip via SPI/I2C and banging with relays cannot be a big problem off the android via any MCU, imho..

The process and result in both cases shall be the same, btw..

[Fungus]

and then it will make a measurement in CH1 and CH2 simultaneously

How will you "make a measurement" without SCPI?

[TimFox]

If you can control the INTERNAL AWG via SCPI, then simply create a BODE_APP (android one) which will call AWG via scpi, do set sine, a freq and ampl, and then it will make a measurement in CH1 and CH2 simultaneously (with some param), do repeat for all freqs, you get N .csv files (best placed in the ramdisk as I wrote above) and the BODE_APP will then crunch out a nice picture from all those files Easy..

For DHO800 and none-S owners we may easily use an external generator (of any kind). We will not call internal AWG via scpi, but the external myGenerator via simple commands via the USB connector in the front panel (all under our control). We may add some relay clicking for an attenuator/amplifier in myBode black box.. Setting a DDS/PLL/etc.,etc. chip via SPI/I2C and banging with relays cannot be a big problem off the android via any MCU, imho..

The process and result in both cases shall be the same, btw..

As I mentioned above, be sure to allow enough time after each AWG frequency step to allow the measurements of magnitude and phase in the DSO to settle out.
Apparently, the coders who wrote the Rigol-supplied firmware were not aware of that problem.

[iMo]

..As I mentioned above, be sure to allow enough time after each AWG frequency step to allow the measurements of magnitude and phase in the DSO to settle out.
Apparently, the coders who wrote the Rigol-supplied firmware were not aware of that problem.

To write the BODE_APP in android is not an easy exercise, sure. There is a lot of stuff to be solved in order to get it running. Knowledgeable people here who would perhaps try to write the BODE_APP will certainly spend a lot of time with getting it right.

The fastest (and most reliable) way to tackle the issue is Rigol itself, of course. Adding like XXms delay for settling out the AWG signal is something they could do anytime. But it could be somethimg else as well.

The major problem here to be solved is to get in contact with somebody involved at Rigol..

[Fungus]

The major problem here to be solved is to get in contact with somebody involved at Rigol..

I'm sure Rigol are aware of it.

We've had two firmware updates in the last few months. Let's hope the streak continues...

[TimFox]

The major problem here to be solved is to get in contact with somebody involved at Rigol..

I'm sure Rigol are aware of it.

We've had two firmware updates in the last few months. Let's hope the streak continues...

I sent them a detailed memo and data a year ago to the main company (after short discussion with US office).
I got a polite acknowledgment of receipt, but nothing since then.

[iMo]

Could you try following - a bode of a parallel resonant circuit, like 100-300mH in parallel with 10-50nF (foil), driven by the AWG via a 50-300k resistor and sensed via A) the 10x probe(s) setting, and then B) with 1x probes(s) setting ??
I want to see the bode plot with higher dynamic range where some adaptive approach with AWG level would be needed..
Example below..
Could you measure the AWG output level during the process somehow?

PS: when I google the issue I get only this discussion..
Edit for clarity..

[iMo]

Out of curiosity I've simulated one of your measurements and the results match pretty well (considering the parasitics and tolerances) - see below.

The only annoyance there I see are the "wiggles".
But those wiggles are there because you have to smooth out the points as they are inherently noisy (and not to let interpolate the points with a sinc or a similar fit). Mind the scope is 12bit only, with a much lower ENOB, there are many sources of noise (internal and external). Most bode graphs are made upon 3-5 points per decade/octave and then a nice smooth fit is made.

PS: would be interesting to compare Rigol's and Siglent's raw Bode data on their noisiness.
That would require to have both o'scopes hanging on the same DUT, and of course, you have to be confident you are really getting the "raw" data, and not the smoothed data instead.

[TimFox]

Those "wiggles" are actually rather large:  about 2 dB and 7 deg pk-pk.
Also, the "corner" frequency (-3 dB and -45 deg) is off by about 10%.

With a straight coax connection (no passive filter), the Bode function gave 0.4 dB and 3 deg peak-peak about 0 dB and 0 deg.

The interesting bit is that the "wiggles" appear to be periodic (with respect to samples) over the frequency range.
I used 100 points/decade for the scan.

With 12 bit "resolution" at 1/10 of full scale (-20 dB), 1/409 voltage ratio is -50 dB or so.

I analyzed this problem quite thoroughly into a memo that I sent to Rigol, which included my results with a slow manual scan through a limited frequency range.
If you are interested, please DM me with an e-mail address and I will send a .pdf to you.

[iMo]

I've had a look at the thread again (after a year) and yes, you've made a lot of work with that issue, indeed.

Still thinking whether the use of the transformer can have some impact (the notorious AWG transformer decoupling from the Rigol's example).

Imagine the RC filter above. The AWG is noisy as "moffy" reported (he indicated some periodic noise) and you have the CH1 at the input of the RC filter and CH2 at the output of the RC filter.

Now when the AWG noise event appears, it takes some time the event propagates from the point CH1 (the AWG output) via RC to the point CH2 (because there is a propagation delay via the RC filter). Thus you get an error in the CH1/CH2 ratio as well as in phase, imho (correct me if I am wrong).

Now imagine there is the CH1 and CH2 wired to the secondary wires of the transformer as in the example. The AWG noise event comes to the both channels in the same time, imho. So it is a "differential wiring" where the "common mode noise error from the AWG" will not apply and the CH1/CH2 errors should not appear then.

Just a thought..

[TimFox]

As I understand the Bode plot function, it measures the amplitude and phase difference between Ch1 (input) and Ch2 (output) of the device under test.
The delay between input and output of the filter is included in its phase shift.
The two channels should be sampled simultaneously (or very close in time).
If the sampling is premature compared with the stepping in frequency of the AWG, then the measurement would be corrupted.
In my manual scanning (excruciatingly slow, done over a narrow frequency range), I had to wait for the individual measurements to settle to get a reasonable result.

[iMo]

The Bode measures the "ratio" of the amplitudes and "difference" in phases.
So when there is an AWG noise disturbance (superimposed on the set sine wave) the propagation of such a disturbance via the DUT (from CH1 via DUT to CH2 as described above) can have different phase/amplitude parameters compared to the pure sine wave you set.. And you may get errors in the moment of sampling (the same as when you had to wait till the stuff settles).
As the result you may get the wiggles (because the AWG noise has got some periodic characteristic)..
When the AWG noise disturbance (superimposed on the set sine wave) appears at the both channels at the same time (when using the transformer as in the Rigol's example), the error coming from the disturbance will not apply.
PS: still a thought experiment

[TimFox]

That's why I suggested that Rigol or anyone writing their own firmware needs to pay attention to the settling time of the measurement.

[Fungus]

There's SCPI commands to measure phase and amplitude - all the "crunching" has been done for you, no need to download any data. 

[mawyatt]

Don't know how Rigol does the Bode Function. Others utilize a Synchronous Sampling or Detection method where a Frequency Selective Filter is placed around the measurement frequency to mitigate external influences. This is extremely important when attempting Bode Functions on a SMPS for example to evaluate Open and Closed Loop Stability. We utilized this for other uses such as Open Loop Oscillator Gain and Phase measurements as well as investigating Oscillator Injection Locking effects.

Best

[iMo]

It could easily be Rigol runs SCPI scripts for Bode with some math in between. Perhaps there are the SCPI strings to be found on the filesystem hidden somewhere

[TimFox]

There's SCPI commands to measure phase and amplitude - all the "crunching" has been done for you, no need to download any data. 

In my detailed reports, I used the built-in magnitude (peak-peak) and phase functions to create the manual-scan plots (from the .csv files).

[iMo]

So, you would do (a simplification)

:MEASure:ITEM VPP, CH1, CH2
:MEASure:ITEM FRPHase, CH1, CH2

and it returns Vpp for CH1 and Vpp for CH2, and phase difference between rising edges CH1 vs. CH2, all done in memory??

[ebastler]

So, you would do (a simplification)

:MEASure:ITEM VPP, CH1, CH2
:MEASure:ITEM FRPHase, CH1, CH2

and it returns Vpp for CH1 and Vpp for CH2, and phase difference between rising edges CH1 vs. CH2, all done in memory??

That could be done, but it would return sub-par results as explained by mawyatt above. You want to apply narrow-band filtering in both channels (e.g. via synchronous detection), to ensure that you only pick up the DUT's reponse to the test frequency and block any interfering signals.

Maybe Rigol has implemented that properly in their built-in Bode plot on the DHO900S, maybe not. But I think it will be impossible to make a DHO800 do it properly via its onboard measurement and filtering options. The bandpass filters available in the Math menu are far too wide. -- Hmm, maybe one can cobble a Math function together which correlates and averages the CH1 and CH2 signals in a suitable way? But off the cuff I don't see how.

[iMo]

As the first step we need the data fast (it means no messing with .csv files). Therefore my Q on whether it runs off the memory. The next step is to play with the math and processes..

[Fungus]

So, you would do (a simplification)

:MEASure:ITEM VPP, CH1, CH2
:MEASure:ITEM FRPHase, CH1, CH2

and it returns Vpp for CH1 and Vpp for CH2, and phase difference between rising edges CH1 vs. CH2, all done in memory??

I've never done it, but... that's what the functions are for, yes.

That could be done, but it would return sub-par results as explained by mawyatt above.

He's talking about extreme cases.

TimFox has said they're OK and the real problem is in settling time. 

Either way, they're a easy way to kickstart the coding and to experiment with.

[ebastler]

That could be done, but it would return sub-par results as explained by mawyatt above.

He's talking about extreme cases.
TimFox has said they're OK and the real problem is in settling time. 
Either way, they're a easy way to kickstart the coding and to experiment with.

You can't say that "these functions are ok" -- whether they are adequate will depend totally on the measurement conditions.

In a perfect world without background noise they will work; in many real-world scenarios they won't. Note that the output from the DUT can become a rather low-level signal outside of its passband, while any mains hum, switching-power-supply interference etc. will stay at a constant level during the scan. You don't want your amplitude and phase measurements to respond to these.

But I agree that it should be straightforward to experiment with these -- just write a Python script which talks to your signal generator and your scope, drives the frequency sweep and captures measurements from the scope.

[iMo]

I've done it today manually with my ds1062ca (it shows perhaps 2 valid digits for voltage and time diff, can do scpi too, interesting) and hp3010b w/ 33k/47n LP off the junkbox (50/500/5kHz, 6Vpp) and got results within 10% (against sim) first shot without much messing with it..
E: hp3310b