Rigol DHO914S Bode plot

[TimFox]

I just received my DHO914S, printed out the full user manual, and am just starting to learn about thumb-drive saving and other functions.
To try a simple Bode plot, I made a passive R-C filter between two BNC-binding post adapters on Ch 1 and Ch 2, driving Ch 1 through a BNC Tee from the back-panel generator output.
The resistor from 1 to 2 is 9100\$\Omega\$/ 1% metal film and the capacitor from 2 to ground is 1640 pF/2% polystyrene, which calculates to -3 dB and -45 deg at 10.66 kHz.
I am surprised at the ripple on the resulting measurement, on both ratio and phase (see attachment).
The Bode plot was set for 100 points from 1 kHz to 100 kHz, log frequency span, other parameters per default setting.
The 11,610 Hz frequency on the graph is 9% high, which seems like too much error for the 1% resistor and 2% capacitor, especially since parasitic capacitance on Ch 2 binding post and any effect of the source resistance in series with the 9100\$\Omega\$ resistor should lower the frequency. 
A 1 megohm input impedance at Ch 2 would only reduce the 9100\$\Omega\$ by about 1%.
The graph was done on my favorite graphing program (Grapher from Golden Software), directly from the .csv file I transferred through a thumb drive, with my own formatting and notes.
Even near 11 kHz, I eyeball about 5 deg pk-pk ripple on the phase angle.
Is this reasonable for the measurement, and is there any improvement I can make with the settings for the Bode plot acquisition?

[rf-loop]

And oscilloscope screen displayed Bode Plot result is?

[TimFox]

Very similar, but harder to read on the small screen.
There is no discrepancy between the screen and my graph of the data.
I first tried several different capacitors, but always saw the "ripple" in magnitude in phase.
To see the non-monotonic behavior better, I saved the data to the thumbdrive.

[CosteC]

Can you add photo of setup? Generator has 50 Ohm impedance, but this cannot explain ripple. Is result repeatable? I got spikes in bide plot dut to microphoning - just hitting table, not very hard. Different scope however.

[ebastler]

A firmware-based artefact is conceivable: They must do some kind of curve fitting to determine the incoming amplitude and phase, and if that is implemented in a sloppy way, the result might vary with the number of full and partial periods sampled.

Does the scope offer any parameters for the Bode plot which you can change -- sweep rate for example, which one could decrease to work from a larger number of sampled periods?

EDIT: I checked the manual and could not find any options beyond setting the number of frequency points per decade, which is not promising. But the "FIR Filter" option looks ominous. It is probably meant to reduce edge effects at the start and end of the captured wave trains? Does switching it ON or OFF affect the ripple on the Bode plot?

[TimFox]

Thanks for the suggestions.
I'll load a photo of the setup later today:  everything is right on the front panel, with a coax cable from the generator output on the back, but no 50 ohm terminator.  No connection to the ground terminal on the rear, with the supplied power module (three-prong plug) connected to an outlet strip on my computer desk.
The appearance of ripple is very repeatable:  I was trying different components and frequency ranges (all below 100 kHz).  The only parameter I could find (not an exhaustive search yet) is the number of points and log sweep:  I should try a linear frequency sweep to see if that differs and try testing the FIR on/off suggestion.
I'll also check the .csv file to see if there is some odd/even effect in the recorded data.

Edit:  I checked for odd/even effects in gain and phase curves, but there is nothing obvious.
However, there may be a periodic effect, with a period of 6 or 7 data points (eyeball analysis).
There are 200 data points, in a log sweep, at 100 per decade.

[nctnico]

The million dollar question: how long does it take to do a plot?

[TimFox]

The time to do the plot varies with the frequency range:  when starting at 100 Hz, the plot is quite slow due to the long time required to measure the low frequency sinusoids.
Otherwise, it seemed reasonable for the discrete measurements being made.
I first noticed the wiggles while watching the Bode plot advance during the measurement.

[TimFox]

Here is a photo of the setup on the front panel of the DHO914S (no tripod, so depth of field suffers).
The resistor runs out of the photo plane from the left binding post adapter on Ch1 to the right binding post adapter on Ch2, and the capacitor is on the right adapter.
The 36" RG-213/U cable runs from the back panel around to the BNC Tee on Ch1, under the adapter, without terminator.

[mawyatt]

Your setup looks fine, let's hope the horrible looking Bode Plot is due to something simple.

If this is due to the DSO and firmware, not good!!

Sure hope the 814 we've ordered doesn't exhibit this kind of behavior!!

Best,

[Martin72]

Very similar, but harder to read on the small screen.

Coud you share a screenshot from it ?

[TimFox]

I'll save one after I try the suggestion about FIR sometime in the next couple of days.
However, I'm sure there is no substantial difference, only that my graph of the data makes the wiggles easier to see.

[KedasProbe]

There are many things you can do to find the problem
Replace cap with resistor (or no filter present test)
Also just do it the slow way and measure it manually (not with the automated bode plot) and compare a few freq.
Then you know what it should be (not theoretical)
also check is ripple error the same for the same freq every time or random.
Also check if it changes with different frequency steps.

my 914S only arrives in 3 weeks but will also compare.

[TimFox]

There are many things you can do to find the problem
Replace cap with resistor (or no filter present test)
Also just do it the slow way and measure it manually (not with the automated bode plot) and compare a few freq.
Then you know what it should be (not theoretical)
also check is ripple error the same for the same freq every time or random.
Also check if it changes with different frequency steps.

my 914S only arrives in 3 weeks but will also compare.

Slow way:  I haven't found any way to calculate the phase difference between Ch1 and Ch2 in the scope firmware (at a fixed frequency).
I have tried different capacitor values, which shifts the corner frequency, but see essentially the same thing.

Also:  my first setup had an external wire between the ground terminals of the two binding post adapters, with the same apparent results.

[KedasProbe]

I assume the phase measurement is in there

edit: page 142 in the English DHO900 user guide

[TimFox]

Thanks:  I looked for and found that "other" menu under "measure" that has the four phase measurements.
(My problem with menu-driven systems is knowing which menu to look for.)
I will get back to this probably next week.

[Njk]

It's a low frequency experiment but the equipment is wideband-capable. There are a circuits running at a GHz clock speeds inside the plastic housing. While the DUT is represented by a through-hole components with long leads and it is located very close to the front panel of the oscilloscope. What if there is some kind of EMC issue because of that? It would be interesting to make sure the results are the same when the DUT is located at some reasonable distance from the scope

[TimFox]

If nothing else works, I can make up a shielded enclosure with BNC connectors to house the discrete components.

[CosteC]

At what level you made this test? Was test signal 10 mVpp or 1 Vpp? Low signals are more noisy, which may lead to errors... Not sure what can be configured in DHO914S.
EDIT: amplitude and point/decade is adjustable. Page 199 of manual, paragraph 15.3

[Martin72]

Manual tells you can choose between fixed and variable amplitude - but no values are mentioned.

(Edit: but the datasheet tells..)

Apart from that, I don't find the representation of the Bode mode all that great.

[tautech]

Manual tells you can choose between fixed and variable amplitude - but no values are mentioned.

(Edit: but the datasheet tells..)

Apart from that, I don't find the representation of the Bode mode all that great.

Only one channel used ?
What if you need Bode plot a multistage filter ?

[TimFox]

The Bode plot function allows one to select the input and output ports from the four front connectors (on the 914S), but only one pair at a time.

[TimFox]

Manual tells you can choose between fixed and variable amplitude - but no values are mentioned.

(Edit: but the datasheet tells..)

Apart from that, I don't find the representation of the Bode mode all that great.

Yeah, I've had problems with the documentation.
For example, that excerpt shows "points per octave", but the 100 points I entered were "points per decade":  I ended up with 200 frequencies between 1000 and 100,000 Hz.
I used the default amplitude values--more experiments next week when I have time.

[tautech]

The Bode plot function allows one to select the input and output ports from the four front connectors (on the 914S), but only one pair at a time.

Shame.

Here's a pic from an old post where 2 traces are shown from a 2 stage passive filter:


Starts here and that pic ^ is a few posts later.
https://www.eevblog.com/forum/testgear/siglent-sds1204x-e-released-for-domestic-markets-in-china/msg1435854/#msg1435854

[TimFox]

That could be useful.
Note that the Siglent unit in the picture requires an external AWG, so the total costs more than the Rigol.