Author Topic: SDS800X HD Actual Use Cases  (Read 3496 times)

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Online mawyattTopic starter

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SDS800X HD Actual Use Cases
« on: April 22, 2024, 04:45:26 pm »
OK, this thread is only for actual use cases for the Siglent SDS800X HD, the Rigol DHO800X use cases can start another independent thread so as to keep the "Mines better than yours" pissing contest away!!!

Also along with the only for actual use cases above, this is not about finding bugs, perceived bugs, manual confusion/discrepancies, user wants/wishes, my brand X can do too (better than yours), and so on, it's about actual use cases of the SDS800X HD where one actually uses the DSO for measuring/displaying something useful ;)

Hopefully some potential DSO users will find the expected following use cases valuable, and not have to sift thru all the BS flying around when it comes to these new 12 bit low cost DSOs!

We'll start off with something that just came up.

Using a precision CMOS pulse generator to create a precise 0 to 5 volt low frequency pulse we noted some peculiarity on the pulse rising edge. To view this we needed to use the Zoom feature to expand the horizontal and the vertical offset to look into the waveform details of interest at 10mv/div while using a 5 volt offset. This is a fairly large range for the offset with a 10mv/div vertical scale factor. The waveform was bouncing around some due to the high and low frequency noise present, so we employed the math function to average the Zoom waveform not the usual Channel waveform (nice ability), to reveal the average of the Zoom and vertically expanded waveform. The pulse output is delivered directly to the DSO BNC without any cabling, and also deliverd to a KS34465A DMM with a ~350mm twisted pair. Power is from a cheap 12VDC SMPS Wall Wart.

First (#27) is the waveform as mentioned connected above, second (#28) is with leads to DMM reversed!!! Note the "ringing waveform" introduced by just flipping over the Dual Banana connector on the DMM!!!

Next we powered the pulse generator with a lab power supply, and repeated above. #29 is with DMM connected normal and #30 is reversed!! Note the lower level of "ringing" compared to Wall Wart Power Supply as expected, however some remnants of the "ringing" still appear!!!

Why this behavior, this is left as an exercise for the interested readers :-+

Anyway, hope this thread proves out useful with others contributing actual use cases, and folks being able to witness meaningful examples without all the BS. Would be nice to include the new Rigol (and others) but fear this will just turn into another pissing contest as we've all seen in other threads |O

Edit: Here we go again with this site messing up all the images :P

Best,
« Last Edit: April 22, 2024, 04:53:39 pm by mawyatt »
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Offline shabaz

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Re: SDS800X HD Actual Use Cases
« Reply #1 on: April 22, 2024, 05:50:30 pm »
With high-res, new ways are needed to see information that older 8-bit 'scopes didn't need.
Your 'scope traces reveal a particular need and your solution, which was the need to do "something" about the noise, because especially when zoomed-in, a single capture in some cases can be almost meaningless, and the solution in your case was the averaging. It's not something one normally realizes unless they actually try it for real, which goes to show that real-life examples are extremely useful.

Does the 'scope support other possible solutions? Averaging is for sure a good solution, but there may be cases where 'something else' is preferred, to not get rid of peaks and troughs for instance, but to still see the trend without as much noise. I'm thinking are there color grading or other options present in the zoom view, that would also be interesting to see for this use-case.

I don't have this 'scope (or the Rigol), but am curious what capabilities these instruments contain.
 
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Online mawyattTopic starter

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Re: SDS800X HD Actual Use Cases
« Reply #2 on: April 22, 2024, 06:35:58 pm »
Good point about using the advanced "Display" capabilities for revealing more useful information contained within the waveforms, something we're sure others are much more proficient with than us.

We took the above case using the Display Modes "Infinite Persistence" with "Dots" capability. Here you can see the discrete 12 bit quantization levels with the vertical scaled to 5mv/div with 4.98V Offset :-+

First #33 is with Color Grade OFF, second #32 is with ON, last (#34) is with Persistence changed to 30 second Color Grade ON.

Best,
« Last Edit: April 22, 2024, 06:48:02 pm by mawyatt »
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Offline Antonio90

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Re: SDS800X HD Actual Use Cases
« Reply #3 on: April 22, 2024, 08:27:02 pm »
I'm quite interested as to why the polarity reversal on the DMM causes the ringing.
Could it be the input capacitance of the DMM, charged by the previous pulse? Or is that a completely stupid gamble?

Edit: stupid gamble. I don't know.
« Last Edit: April 22, 2024, 08:37:41 pm by Antonio90 »
 
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Online mawyattTopic starter

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Re: SDS800X HD Actual Use Cases
« Reply #4 on: April 23, 2024, 01:56:53 pm »
Yes, it's the highly asymmetrical input capacitance of the DMM +- inputs when measured to ground reference :-+

The DMM - input has significantly more capacitance to ground than the + input, so when the pulse generator is reverse connected its' output "sees" a much larger effective capacitance thru the DSO input ground.

Best,




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Offline Antonio90

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Re: SDS800X HD Actual Use Cases
« Reply #5 on: April 23, 2024, 03:24:18 pm »
Yes, it's the highly asymmetrical input capacitance of the DMM +- inputs when measured to ground reference :-+

The DMM - input has significantly more capacitance to ground than the + input, so when the pulse generator is reverse connected its' output "sees" a much larger effective capacitance thru the DSO input ground.

Best,
I didn't know that at all. Thank you very much!
 

Online mawyattTopic starter

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Re: SDS800X HD Actual Use Cases
« Reply #6 on: April 23, 2024, 04:08:20 pm »
If you have a bench DMM try measuring the +- input capacitance to ground and report over here:

https://www.eevblog.com/forum/testgear/dmm-input-capacitance/

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Offline Njk

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Re: SDS800X HD Actual Use Cases
« Reply #7 on: April 24, 2024, 01:32:03 pm »
BTW that use case shows that a floating scope can't be always substituted with an isolating transformer.
 

Online mawyattTopic starter

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Re: SDS800X HD Actual Use Cases
« Reply #8 on: April 24, 2024, 05:24:42 pm »
Here's another interesting use case with the SDS800X HD. If one wishes to see a low frequency plot of capacitance vs frequency then an expensive LCR meter is usually required. There were various threads about using the built-in Bode Function to plot Impedance and Admittance using a DSO, see these for details.

https://www.eevblog.com/forum/testgear/capacitive-impedance-plots-with-sds2104x-plus-bode-function/msg4335745/#msg4335745

https://www.eevblog.com/forum/testgear/admittance-measurements-with-dso-awg-with-bode-function/msg4491952/#msg4491952

Following the analysis in the above threads, the Bode Plot is defined as:

Bode Function Display = Vo/Vi where Vi is the signal created by a coupled signal generator, and Vo is the voltage across the DUT.

If we add a series impedance Zr with the DUT (Z), then (neglecting DSO sensing currents) the DUT current is:

I = (Vi-Vo)/Zr, where Vo is the output or voltage across DUT, and:

I= Vo/Z, and equating I:

Vo/Z = (Vi-Vo)/Zr

Z = Zr(Vo)/(Vi-Vo)

If Vo is << Vi (meaning Zr >> Z), then:

Z ~ Zr(Vo/Vi)

Normally we make Zr a Reference Resistor, but what if we make Zr a Reference Capacitor, what then?

Z ~ Zcr(Vo/Vi), where Zcr is reference capacitance impedance and what if we are interested in Z being just capacitance, thus:

Zc ~ Zcr(Vo/Vi), where Zc = 1/jwC and Zcr = 1/jwCr

1/jwC ~ (1/jwCr)Vo/Vi, and:

C ~ Cr*/(Vo/Vi)

So if we can "fool" the Bode Plot and swap the Ch1 (Vi) with Ch2 (Vo), then:

C ~ Cr*Bode Plot!!

Of course we can just set the Bode Plot Input and Output to Ch2 and Ch1 respective and not need to reverse the probes and "fool" the Bode Plot!!

Ok, now that we've got thru all that, does it work!!

Yep, sure does :-+

Here's an example of a Precision Polystyrene 1uF Reference Capacitor (1.01594uF) and a 100uF Electrolytic Capacitor.

The DUT Capacitance is directly displayed in dB to the Reference Capacitor, so the resulting DUT Capacitance is the product of the display and Reference Capacitor. Note how the effective DUT capacitance begins to drop off with increasing frequency. Results agree well with Lab Bench LCR Meter (TH2830).

DUT Capacitance result is Cr * 10^(Reading/20)

Not a replacement for a plot displaying LCR meter, but in a pinch might help.

Anyway, hope some folks find this useful.

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Online mawyattTopic starter

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Re: SDS800X HD Actual Use Cases
« Reply #9 on: April 25, 2024, 01:41:30 pm »
Added a few more plots.

#37 is a 470uF Electrolytic that measures 472.6uF (TH2832) @ 100Hz, with C Reference (1.01594uF) is Cr*10^(53.28/20) or 468.7uF @ 100Hz.

#39 is 1000uF Electrolytic that measures 997.8uF and plot shows Cr*10^(59.84/20) or 997.4uF @ 100Hz.

#41 is 47uF Electrolytic that measures 44.8uF and plot shows Cr*10^(33.12/20) or 46.0uF at 100Hz.

Edit: Technique also works with inductors. Since the inductive impedance is direct rather than inversely proportional to value there's no need to "invert" the Bode Plot. Follow the above analysis and insert inductive impedance rather than capacitive to show how this works. Of course one needs a Reference Quality Inductor which is usually rare, so we just used a small SMD 2.4mH as Lref, see #43. Note the Reference Impedance must be >> than the DUT Impedance for the "assumption" mentioned above to work, so a larger value inductor is required for Lref, rather than the smaller value Cref when plotting capacitance.

Anyway, details left for the interested readers and here's an example with a 100uH inductor using a 2.4mH reference. Inductor measured 100.5uH with T2830 at 10KHz and plot shows 96.0uH. Better result should be expected with quality reference!!


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« Last Edit: April 25, 2024, 02:27:58 pm by mawyatt »
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Online mawyattTopic starter

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Re: SDS800X HD Actual Use Cases
« Reply #10 on: May 03, 2024, 03:18:54 pm »
Here's a few uses with the Bode plot showing filters.

These are interesting RC filters that exhibit a "greater than unity gain" yet have no inductors, nor transformers, transmission lines, switches, non-linear elements, or active devices, just a couple resistors and a couple capacitors.

One is a Low Pass and the other a High Pass, and both show ~0.66dBV gain. Probably not much use but interesting anyway, analysis left up to the reader :-+

Included some LTspice Sims for reference.

BTW a single JFET also exhibits a Passive Voltage Gain without any additional components, nor power supply!!

Gotta love this little SDS800, puts a grin on our face every time we use it :)

Will add more later if folks are interested.

Best
« Last Edit: May 03, 2024, 03:20:46 pm by mawyatt »
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Online joeqsmith

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Re: SDS800X HD Actual Use Cases
« Reply #11 on: May 03, 2024, 03:40:03 pm »
I wonder if you can't make PDN measurements with it?   I tried using the NanoVNA and get ok-ish results.  Do they support anything like this native?

Online mawyattTopic starter

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Re: SDS800X HD Actual Use Cases
« Reply #12 on: May 03, 2024, 03:47:36 pm »
Would expect so, as it's already been shown how to make Impedance and Admittance measurements utilizing the Bode Function.

One could use a current probe for the current measurement for V/I or I/V plots, but also a simple small sense resistor works. The 12 bit resolution, low noise, and large dynamic range (range scaling) come into play here.

If Siglent makes a firmware update that allows the math channels to be part of the Bode Function, then this opens up a lot for direct use cases without need for additional support probes/components/equipment.

As we noted on numerous occasions, contrary to some naysayers, this Bode Function is extremely useful in the hands of knowledgable users :-+

Best,
« Last Edit: May 03, 2024, 03:56:34 pm by mawyatt »
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Online KungFuJosh

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Re: SDS800X HD Actual Use Cases
« Reply #13 on: May 03, 2024, 04:05:37 pm »
this Bode Function is extremely useful in the hands of knowledgable users :-+

...and me too. 😉
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Online mawyattTopic starter

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Re: SDS800X HD Actual Use Cases
« Reply #14 on: May 03, 2024, 04:09:32 pm »
Here's a couple plots of the classic passive Twin T Filter, just a few resistors and capacitors for a ~47dB notch :-+

Fun stuff indeed  :)

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« Last Edit: May 03, 2024, 05:16:09 pm by mawyatt »
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Offline tszaboo

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Re: SDS800X HD Actual Use Cases
« Reply #15 on: May 03, 2024, 04:09:48 pm »
Would expect so, as it's already been shown how to make Impedance and Admittance measurements utilizing the Bode Function.

Best,
I've been thinking about a PDN measurement for a while with this scope. I think a low output resistance preamp would be needed for it. And a DC block. And a preamplifier with specific input impedances. And some way to calibrate the whole system. And some script to de-embed the results from the measurements. It's not going to be easy.
It's also kind of difficult to use the results. They would be limited to ~200MHz. Systems that define PDN impedances are typically high speed digital, where wider band measurements would be needed. Of course you can connect it to the Arduino and show that it's a bad design, but what is the design requirement for a microcontroller like that?
 

Online joeqsmith

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Re: SDS800X HD Actual Use Cases
« Reply #16 on: May 03, 2024, 04:13:43 pm »
I had made up some standards down to 100uohms.   The NanoVNA just doesn't have good enough performance to resolve it.  Still, $50.   

I think I documented everything if you go to my dropbox and select the NanoVNA directory, you will find the software users manual.   

One problem with the NanoVNA when making these measurements is you really have to slow things down.   It seems like a sweep would take me several minutes to run.   The other problem is it was limited to about 50kHz.  You could run it lower with different firmware but the performance was too poor to be useful.   

If you are looking for actual use cases as your thread suggests, and want to try it out, I'm interested to see this measurement  with the Siglent.   You may have to write some software to do it, but then again, wouldn't be surprised if it didn't already have it.

***
I did make a video demonstrating the basics and goes over some of the problems.   Also shows using the software to tame a PDN.  The three hurdles with that whole setup were breaking the ground loop, blocking the DC and standards to verify performance.   


« Last Edit: May 03, 2024, 04:23:11 pm by joeqsmith »
 

Online mawyattTopic starter

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Re: SDS800X HD Actual Use Cases
« Reply #17 on: May 03, 2024, 04:35:17 pm »
Would expect so, as it's already been shown how to make Impedance and Admittance measurements utilizing the Bode Function.

Best,
I've been thinking about a PDN measurement for a while with this scope. I think a low output resistance preamp would be needed for it. And a DC block. And a preamplifier with specific input impedances. And some way to calibrate the whole system. And some script to de-embed the results from the measurements. It's not going to be easy.
It's also kind of difficult to use the results. They would be limited to ~200MHz. Systems that define PDN impedances are typically high speed digital, where wider band measurements would be needed. Of course you can connect it to the Arduino and show that it's a bad design, but what is the design requirement for a microcontroller like that?

At present the upper end is limited to 120MHz, maybe Siglent can extend this if the AWG is capable (SDG6000X for example).

Here's a couple links showing the Bode Function involving Injection Locking, which shows how well the selectivity of the Synchronous Sampling/Detection or whatever algorithm Siglent has utilized behaves. Impressive IMO and could benefit other uses where large interference is present.

https://www.eevblog.com/forum/testgear/siglent-sds800x-hd-12-bit-dsos-coming/msg5441336/#msg5441336

https://www.eevblog.com/forum/projects/injection-locked-peltz-oscillator-with-bode-analysis/

Best recommendation we can provide is get one and play around, you can always return it, however we'll wager you'll keep it and it will put a grin on your face as well :-+

We are a very well seasoned semi-retired research scientist/engineer with a fairly extensive background. Grew up with nothing put HP/AG/KS and Tek equipment over the years, so it takes quite an instrument to impress us, the SDS800X HD has succeeded admirably!! We have a DHO814 as well, it's impressive and quite a good little DSO, but the SDS is just better is almost every way and much more "Pro Like".

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Online joeqsmith

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Re: SDS800X HD Actual Use Cases
« Reply #18 on: May 03, 2024, 05:08:14 pm »
Plan to buy a new low end one at some point just to try out.  The 800 has a smaller display than my tablet.  It's difficult anymore to see these small screens without glasses.   Doesn't appear the 800 supports an external monitor as well.   Does look like a nice scope otherwise and it's certainly affordable.

Online mawyattTopic starter

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Re: SDS800X HD Actual Use Cases
« Reply #19 on: May 03, 2024, 05:09:59 pm »
I had made up some standards down to 100uohms.   The NanoVNA just doesn't have good enough performance to resolve it.  Still, $50.   

I think I documented everything if you go to my dropbox and select the NanoVNA directory, you will find the software users manual.   

One problem with the NanoVNA when making these measurements is you really have to slow things down.   It seems like a sweep would take me several minutes to run.   The other problem is it was limited to about 50kHz.  You could run it lower with different firmware but the performance was too poor to be useful.   

If you are looking for actual use cases as your thread suggests, and want to try it out, I'm interested to see this measurement  with the Siglent.   You may have to write some software to do it, but then again, wouldn't be surprised if it didn't already have it.

***
I did make a video demonstrating the basics and goes over some of the problems.   Also shows using the software to tame a PDN.  The three hurdles with that whole setup were breaking the ground loop, blocking the DC and standards to verify performance.   




That looks like an interesting project, however software isn't our bag. Also seems this will likely get quite involved, so we'll let others more capable than ourselves tackle this.

Maybe you might consider acquiring a SD800X HD, doubt you'll be dissapoined :-+

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Online mawyattTopic starter

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Re: SDS800X HD Actual Use Cases
« Reply #20 on: May 03, 2024, 05:13:06 pm »
Plan to buy a new low end one at some point just to try out.  The 800 has a smaller display than my tablet.  It's difficult anymore to see these small screens without glasses.   Doesn't appear the 800 supports an external monitor as well.   Does look like a nice scope otherwise and it's certainly affordable.

We are 76 and bad eyes (cataracts and such), but can read the DSO reasonably well. The remote Web server works really well and isn't too slow and we use this often.

Best,
« Last Edit: May 03, 2024, 07:14:59 pm by mawyatt »
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Online joeqsmith

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Re: SDS800X HD Actual Use Cases
« Reply #21 on: May 03, 2024, 08:12:28 pm »
Maybe tszaboo will make an attempt at the PDN project. 

Another subject that had recently came up again was measuring SRF of an inductor.  I show measuring it with the cheap VNA but then also with a scope.  Again a pretty good actual use case for this particular scope.  Even if you can't drive a sharp edge, you can certainly square it up easy enough.  Your only dealing with a single component and much easier to setup and measure.   

https://www.eevblog.com/forum/beginners/measuring-the-self-resonant-frequency-of-an-inductor/msg5112405/#msg5112405

I'll see if I can find a video review showing that remote interface.   

Online mawyattTopic starter

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Re: SDS800X HD Actual Use Cases
« Reply #22 on: May 14, 2024, 04:11:09 pm »
Another use for this DSO and the Bode Function is for regulator PSRR measurements. This is a measure of the regulators ability to attenuate unwanted signals/fluctuations from the input to the output.

We have a couple popular +-5 volt linear regulators, the 78L05 and 79L05 respectively.

The setup is rather simple with a series R (10 ohms) for the input power source and shunt C (0.1uF) for stability connected to the regulator input. Also a larger coupling C (100uF) from the regulator input to the Bode Signal source AWG output, this C provides DC isolation for the AWG. One could also use a coupling transformer common for use in closed loop measurements, but pay attention to the DC thru current as it can saturate the transformer core.

The regulator output is loaded with a shunt R (1K) and shunt C (0.1uF) for stability.

Bode connections are CH1 to the Regulator Input and CH2 to the Regulator Output. This will produce a dB ratio of output to input voltage and thus a negative dB representation. PSRR is generally specified as a + dB term, so one can swap the Bode inputs either physically or thru the Bode Configuration if necessary, however we prefer the negative connotation as this indicates an output less than input more of a Gain type display.

Anyway, here's couple plots showing the 78L05 (+5V) #2 and 79L05 (-5V) #3 PSRR. Note the inferior rejection of the negative 79L05 wrt the positive 78L05, both setups were identical except for polarity. The "response peaks" are partially attributed to setup (Protoboard), altho the 79L05 still appears worse in that respect.

For those interested the PSRR is influenced by input-output voltage differential and load current, so one could create a "family" of curves if desired.

Edit: Added a 78L15 #4 and 79L12 #5.

Best
« Last Edit: May 14, 2024, 05:04:15 pm by mawyatt »
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Online mawyattTopic starter

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Re: SDS800X HD Actual Use Cases
« Reply #23 on: May 14, 2024, 06:01:37 pm »
Now that we've got the Regulator PSRR, how about taking a look at the Regulator Output Impedance Z as a function of Frequency. We can apply the Bode function for this as well!!

Configure the Regulator setup with decoupling C on Input and Output, use a large value (100uF) for the Input and small for the Output (0.1uF) so we can view the Regulator Output Z as a function of Frequency.

Attached the AWG output to the load Resistor (1K) normal Ground end and DSO CH1 to same end, then DSO CH2 to Regulator Output which connects to the load R.

If we call the AWG output as V2 and the Regulator Output as V1, then:

Z (Regulator Output Impedance) = V1/I, where I is the R load current.

I = (V2-V1)/R

Z = V1/((V2-V1)/R), or R(V1/(V2-V1))

If V1 is << V2, implying R is >> Z, then,

Z ~ R(V1/V2)

Since Bode is in dB, and Y axis is 20Log(V1/V2)

Then Z is R* 10^(Bode Display)/20

Does it work??

Yep sure does :-+

Here's a plot #6 of a 78L05 output Z using a 1K load. Just add 20Log R (60dB) to Y axis, and then it's in dB ohms.

So at 100Hz the 78L05 Output Z is 13.8 milliohms, and 69.5 milliohms at 100KHz. Note the inductive nature (+ Slope) of the output Z beginning at ~10KHz.

Plot #7 is the 79L05 negative Regulator, 11.5 milliohms at 100Hz and 4 ohms at 100KHz  :o

Anyway, this seems like another useful capability of these new "Entry Level" DSOs!!

Best,
« Last Edit: May 14, 2024, 06:12:31 pm by mawyatt »
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Offline Performa01

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Re: SDS800X HD Actual Use Cases
« Reply #24 on: May 14, 2024, 07:36:10 pm »
Since Bode is in dB, and Y axis is 20Log(V1/V2)

It is worth mentioning that Bode Plot doesn't have to be in dB. It can also use Vpp and Vrms.
 
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Online mawyattTopic starter

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Re: SDS800X HD Actual Use Cases
« Reply #25 on: May 14, 2024, 09:07:49 pm »
Since Bode is in dB, and Y axis is 20Log(V1/V2)

It is worth mentioning that Bode Plot doesn't have to be in dB. It can also use Vpp and Vrms.

Yes, but because of the expected range of the measurement (~1000X) better displayed in dB than in linear.

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Re: SDS800X HD Actual Use Cases
« Reply #26 on: May 14, 2024, 11:26:07 pm »
Setups for PSRR and Output Z measurements & plots.

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« Last Edit: May 14, 2024, 11:51:40 pm by mawyatt »
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Re: SDS800X HD Actual Use Cases
« Reply #27 on: May 15, 2024, 04:32:24 pm »
Here's a KIA7810AP +10V regulator in TO-220 case (#10), we isolated the AWG (see setup above) with 2000uF and used 100 ohms for the sense resistor R in the output impedance setup. Load was established with Electronic Load (SDL1020X-E) at 100 ohms for a load current of 100ma.

So output impedance starts at 100Hz as Z ~ 100*(10^(-100.6/20)) or 933 micro-ohms and at 1MHz is Z ~ 100*(10^(-62.8/20)) or 72.4 milli-ohms.

In the next plot (#11) is the PSRR with a load current of 100ma. The range between 100 and 1KHz shows the PSRR improving, this isn't likely and caused by the setup coupling capacitance.

Edit: Added PSRR #13 which extends to 10MHz and shows some fixture setup resonates at ~3.5MHz.

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« Last Edit: May 15, 2024, 04:49:58 pm by mawyatt »
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Re: SDS800X HD Actual Use Cases
« Reply #28 on: May 21, 2024, 03:18:49 pm »
Another interesting use of the SDS800X HD features, the Multi-Channel Bode Plot capability.

Here we have a 3rd Order Butterworth Active Low Pass Filter implemented with Equal Valued Components, see note #23 below.

https://www.eevblog.com/forum/beginners/calculation-of-the-3rd-order-rc-filter/msg5343509/#msg5343509

This circuit was implemented with 3 equal resistors and 3 equal capacitors and a single dual op-amp (LM358).

The Bode function allows one to "see" the voltage waveforms as they progress down the 3rd Order Filter Chain, from the 1st section C2 (note amplitude peaking which allows this overall transfer function to implement the 3rd Order Butterworth Response), followed by the 2nd section (C3) and the final result (C4) with the steeper roll-off response.

Edit: Added a High Pass by simply swapping the Rs and Cs in the Equal Valued 3rd Order Butterworth Active Filter.

Also, note how the op-amp artifacts (LM358) begin to effect the stop band LP performance and the HP upper frequency response. A better (faster) op-amp would yield better LP stop band and HP upper frequency responses.

The Rs were 1K 1% and the Cs were all 0.1uF 5%, which should yield a Low Pass and High Pass 3dB corner of 1.59KHz.

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« Last Edit: May 21, 2024, 04:29:01 pm by mawyatt »
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Re: SDS800X HD Actual Use Cases
« Reply #29 on: May 21, 2024, 04:56:43 pm »
Here we've changed the R to 10K with same C (0.1uF), this should move the 3rd Order Corner down to ~159Hz. Here you easily see the 60dB/decade stop band response of the classic Butterworth Low and High Pass responses!

Edit: For those with some Complex Variable/Circuit Analysis interest, the Low Pass has a normalized transfer function of :

Vo/Vi = 1/[S^3 + 2S^2 + 2S + 1] and thus = root(2)/2 or -3dBV at S=j {w=1} at -135 degrees phase shift and at {w=0} Vo/Vi = 1 (0dBV)

For the High Pass:
Vo/Vi = S^3/[S^3 + 2S^2 + 2S + 1] and at S=j,  root(2)/2 or -3dBV at +135 degrees and at {w=0} Vo/Vi = 0

Check the posts below for R = 10K and C = 0.1uF, tau of 1ms {w = 1000, f = 159Hz} and -3dBV points, note phase shifts :-+

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« Last Edit: May 21, 2024, 11:55:09 pm by mawyatt »
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Re: SDS800X HD Actual Use Cases
« Reply #30 on: May 22, 2024, 01:42:40 am »
Here's a couple plots showing how the op-amp dynamic output impedance limits the stop band rejection in active low-pass filters imploying the classic Sallen-Key (VCVS) configuration. The 2nd order VCVS type active low pass filter has a means for the input to couple thru to the output bypassing the op-amp by way of the feedback capacitor. The op-amp output impedance forms a voltage divider which limits the amount of input signal coupling thru to the output, thus lower output Z produces less coupling and better overall stop band rejection.

The above 3rd Order Low Pass Active Filter employs the mentioned Sallen-Key VCVS configuration to implement the 2nd order section of the 3rd order Low Pass Butterworth filter.

The first plot shows the Bode result, note the stop band limited attenuation in the overall filter C4 Green trace, the C3 Blue trace is the output of the VCVS section, and the C2 Magenta trace is the 1st op-amp output.

The second plot shows the same results except with a single added shunt resistor (1K) to the VCVS output op-amp section to the Vee supply rail. Note the dramatic improvement in overall stop band rejection (Green) and the VCVS (Blue).

What's happening is the shunt resistor draws static bias from the op-amp output npn transistor, forcing it into a more Class A rather than Class B bias condition, this significantly lowered the dynamic op-amp out Z and improving stop band rejection as shown. This simple technique is very effective with GP op-amps such as the LM358 type with the relatively weak Class B type output structure.

About 4  decades ago this simple concept was published in EDN.

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Re: SDS800X HD Actual Use Cases
« Reply #31 on: May 22, 2024, 04:12:29 pm »
Here are some LTspice simulations utilizing a generic LM358 model. The 1st plot shows the result above with 10K and 0.1uF components (compare with #28), the 2nd shows the added "shunt" 1K bias resistor (compare with #27).

Note how the simulations agree well with the actual circuit in the pass band, but begin to deviate as you move into the stop band.

This is a common occurrence with generic SPICE models, where the model doesn't represent the physical component well in certain areas, especially extreme cases.

The LM358 and most GP op-amps, have a very complex output impedance and rely on negative feedback to force the impedance to a low level. As the op-amp open loop gain deteriorates with increased frequency, the output impedance rises in an inductive fashion and can create issues with some circuits as shown here.

As most less seasoned tend to run to the simulations without totally understating the circuit under inspection, caution is advised, especially when "pushing" the limits for critical components.

In the case here the LM358 model is totally inadequate in the higher stop band region, and should be used with caution.

Anyway, more fun things for folks where you can compare physical and simulated circuits and begin the understand the limitations of both :-+

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« Last Edit: May 22, 2024, 04:18:50 pm by mawyatt »
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Re: SDS800X HD Actual Use Cases
« Reply #32 on: May 23, 2024, 01:30:47 am »
The above begs the use of the Bode Function to plot the op-amp closed loop output impedance as shown in #23 for use with Linear Regulators.

Set up the op -amp with a bipolar supply and in unity gain configuration. Connect the + op-amp input to ground and "drive" the op-amp output with resistor R, connecting the signal source (AWG) to R.

From #23 then:

Z is R* 10^(Bode Display)/20

Here we have in 1st plot R at 10K with an LM358 with +-15V supplies, AWG at 10Vpp (1mapp). Note the inductive (rising with positive phase) output Z with frequency, starting at ~73 milli-ohms at 10Hz, rising to ~31 ohms at 10KHz and reaching a peak of ~1.24K ohms at 200KHz.

Changing R to 1K in 2nd plot for 10mapp "drive" signal yields ~82 milli-ohms at 10Hz, ~5 ohms at 10KHz and ~176 ohms at 200KHz.

Next plot is a simulation of an LM358 Spice Model, note the difference between the actual LM358 and model.

These effectively show how the output impedance of the LM358 is far from ideal, even at modest frequencies, and highly dependent on frequency and signal level in real life!!! Exactly why simulation models should be carefully evaluated and understood before committing to a design based upon such :-+

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« Last Edit: May 23, 2024, 02:14:04 am by mawyatt »
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Re: SDS800X HD Actual Use Cases
« Reply #33 on: May 23, 2024, 02:50:31 pm »
Adding a shunt resistor to increase the output static DC bias current for the op-amp npn output transistor as mentioned above in post #30, where this reduces the LM358 output impedance which improves the Low Pass Filter Stop Band performance as shown, lets look at the LM358 output Z with and without the added DC bias.

1st is the output Z without the additional shunt R (1K) bias resistor, 2nd is with R from the output to Vee. Note the dramatic improvement in Z, from 5 ohms to 0.148 ohms @ 10KHz, a 30dB improvement :-+

Maybe some folks at Siglent are watching and will realize the tremendous value of having Math functions available in the Bode Function, also would be nice to be able to superimpose previous plots (like reference plots), so one could show "before" and "after" comparisons  ;)

Anyway, this little 12 bit DSO really packs some serious capability and can do much much more than just display a time domain waveform like the analog scopes of old  :clap:

Edit: If anyone is interested will be happy to explain what's going on with the various posts above wrt to the LPF and op-amp characteristics.

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« Last Edit: May 23, 2024, 02:55:51 pm by mawyatt »
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Re: SDS800X HD Actual Use Cases
« Reply #34 on: May 24, 2024, 02:33:32 pm »
Here we have the SDS800X HD Bode Function showing the LM358 Op-amp Power Supply Rejection. Bode signal is "injected" by means of a coupling capacitor (1000uF) into the measurement supply rail (+-15V) using an "isolation" resistor (100 ohms). LM358 is configured as unity gain with output load of 10K, with scope CH1 connected to LM358 output and CH2 to injected voltage at supply rail. This configuration is swapped from normal Bode use so the display is in +dB rather than -dB to agree with TI Data Sheet as shown.

1st plot is of the + Supply Rejection, 2nd of - Supply Rejection and result from TI Data Sheet for LM358.

Note the fall off with frequency on both + and - Supply Rejection,  weaker - Supply Rejection common to most Op-amps, and the fair agreement with the TI Data Sheet Plot indicating this is likely a "real" LM358 and not counterfeit  :-+

The Power Supply Rails are a common means for coupling noise and unwanted signals (SMPS) into op-amp based circuits, especially the higher frequency Supply Rail components due to the deteriorating nature of the Op-amp Supply Rejection with frequency.

So do you know for sure your LM358 is a real TI Op-amp or a counterfeit, this measurement is a useful means for verifying such ;)

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« Last Edit: May 24, 2024, 02:51:58 pm by mawyatt »
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