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LCR Impedance Viewer for Picoscope+Keysight+R&S Bode Plot Data (open source)
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Hans Polak:
Wim,

1) You already have the results for measuring the DUT with probe, uncompensated and compensated,
Here are the results when adding a 68pF cap over the probe, first image uncompensated, second image compensated and third image both compensated versions on top of each other in the same image.
I suppose the two dips at 5Mhz in the compensated version are because of the inductance in the wires connecting the 68pF, but below 3Mhz, both compensated versions are exactly equal so I did not try to correct this.

2) I didn't understand your comment that you were not surprised,
I did both Tektronix tests for resp. 10uF and 1mF and got exactly the same results with my formulas.
You even got the Excel file that calculated these results.

The simulation with LTSpice also gave the exact same plot as my real life DUT measurement.
What else do you need as a prove that my formulas are correct ?

Hans
Hans Polak:
Hi Wim,

I have the impression that you still find it hard to believe that there's something wrong with your calculations.
That's why I have implemented the full Tektronix formulas from the link that you gave.
I ran two sessions, one with the DUT with 100R/1mH+38R/1nF, first image below.
The other one was with the 100R+10nF in series, where we had the discussion whether Resr could be -50R, see second image below,
I gave both models these two files:  model1 with my math , the other model2 with the Tektronix math.
I overlaid the outcome of both ways of calculation to clearly see the differences between the two.

Conclusion: For Imp, Resr and Induction, result is exactly the same as was to be expected.
Only the Capacity differs because of the reason I mentioned before, that when having j in the numerator and in the denominator, you can not simply put an equation upside down, as was also 100% confirmed in the LTSpice simulation.
But nevertheless, the plot produced using this simplification, seems quite useable so I can live with it.
Where Tektronix differs from my math, I have put a T in red on the plotted line.

One example of what is still not O.K. in your Imp Viewer is visible for the 100+10nF series giving Resr=-50R and the L=250H both at 100Hz, because as you see they have to be resp 43K an 22H, So it's not the math that is wrong, but something in your calculation goes aside.
Hope you can find it.

Hans
_Wim_:

--- Quote from: Hans Polak on May 17, 2021, 12:10:16 pm ---I have the impression that you still find it hard to believe that there's something wrong with your calculations.
That's why I have implemented the full Tektronix formulas from the link that you gave.
I ran two sessions, one with the DUT with 100R/1mH+38R/1nF, first image below.
The other one was with the 100R+10nF in series, where we had the discussion whether Resr could be -50R, see second image below,
I gave both models these two files:  model1 with my math , the other model2 with the Tektronix math.
I overlaid the outcome of both ways of calculation to clearly see the differences between the two.
Conclusion: For Imp, Resr and Induction, result is exactly the same as was to be expected.
Only the Capacity differs because of the reason I mentioned before, that when having j in the numerator and in the denominator, you can not simply put an equation upside down, as was also 100% confirmed in the LTSpice simulation.
But nevertheless, the plot produced using this simplification, seems quite useable so I can live with it.
Where Tektronix differs from my math, I have put a T in red on the plotted line.
 
--- End quote ---
Your impression is indeed correct. Below the Tektronix math vs mine (screenshots from post 32) To express the Tektronix formula’s in "gain", you only have to divide by Va1. When you do that you get EXACTLY the same formula. No mathematical simplification, the same.  If you see it otherwise, please point me to the difference.

The reason for the difference between your graph and the Tektronix one, is by not calculating for current first, as explained above. See the second term in equation 3 of Tektronix, this is the reason for your difference. If the measured phase goes to 90° (pure inductive or capacitive loads), the second term disappears and the data is the same. Also as explained above.


--- Quote from: Hans Polak on May 17, 2021, 12:10:16 pm ---One example of what is still not O.K. in your Imp Viewer is visible for the 100+10nF series giving Resr=-50R and the L=250H both at 100Hz, because as you see they have to be resp 43K an 22H, So it's not the math that is wrong, but something in your calculation goes aside.
 
--- End quote ---

As also  explained above, I limit the gain to zero (so positive gains are removed), but you seem to insist to pick a part of the plot were the DUT was acting as an active device. This was a deliberate choice or mine, because otherwise plots can have a very large difference between min and max, causing the user to have to zoom in every time he wants to see the "interesting part" (because the part were the DUT acts an amplifier does not mean anything). 


--- Quote from: Hans Polak on May 17, 2021, 12:10:16 pm ---That's why I have implemented the full Tektronix formulas from the link that you gave.
I ran two sessions, one with the DUT with 100R/1mH+38R/1nF, first image below.
The other one was with the 100R+10nF in series, where we had the discussion whether Resr could be -50R, see second image below,
I gave both models these two files:  model1 with my math , the other model2 with the Tektronix math.
I overlaid the outcome of both ways of calculation to clearly see the differences between the two.
 
--- End quote ---
Can you post your excel file? I would expect that ESR becomes negative where the DUT becomes an amplifier. The direction inversion in your plot seems strange to me as the measure profile is both smooth for phase and gain.
Hans Polak:
Hi Wim,

I must say, you are rather hard to convince, but anyhow, here is the Excel file. That should finally convince you as it follows exactly each step accordingly.
Used file is the 100R+10nF in series that you already have with Rref=100R
And as said, it's obviously not the math that is wrong, but there must be something going wrong in your calculation giving L=246H and Resr=50R at 100Hz instead of 21.6H and 43.8K, and there is more.

Hans


 
Hans Polak:
You mentioned to limit the gain to one.
That will indeed increase the value of L considerably (and influence the capacity) , but also the value of Resr will increase to ca 47K at 100Hz and higher.
So after all the things I have done so far, it seems fair to say that now it's your turn to tell what exactly it is that you do on top of calculating the formulas , that may help to explain part of the differences.

Hans
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