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| LCR Impedance Viewer for Picoscope+Keysight+R&S Bode Plot Data (open source) |
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| _Wim_:
If you look at the gain of the CSV you posted, you can see that somewhere just before 10-15kHz the scope runs out of resolution. To measure your DUT in these lower frequencies, the reference resistor should be much larger to give the system again the change to see variations in gain. |
| Hans Polak:
Wim, I appreciate very much that you are still communicating after so many points I brought forward with results that deviate from what was to be expected. Your program has a wonderful graphical interface, that's the reason I like to use it. In Excel I have all the formulas to calculate the same, but with very simple graphical options. But at the end no matter where the calculations are taking place, results should be 100% identical, which they are not. When leaving the capacity calculation as a separate issue, I would have expected no difference in Resr, Inductance and impedance. No offense, but if I may say so, the used way of calculating is rather complicated with additional current and a second phase that I haven't checked because calculation can be done much simpler with less chance of making mistakes, see calculation below. The fact that my Excel shows different graphs based on one and the same .CSV file says that one of both must be wrong. So here is the Math that I have checked over and over. |
| _Wim_:
Hi Hans, No offence taken. For me your calculation looks more complex, but that is probably because we both spent more time on our own method or due to different backgrounds. ;) I found this webpage from Tektronix which also uses the I-V method and matches very closely with what I have done: https://uk.tek.com/document/application-note/capacitance-and-inductance-measurements-using-oscilloscope-and-function-ge I especially like the piece posted below ;D As to the difference in results, in your last measurement the DUT acts as an amplifier (positive gain). It is no surprise for me that in that case different calculation strategies lead to different results, as there exist is no passive device combination that can work as an amplifier (when talking about RLC-components). So whatever formula's used to calculate the equivalent set of RLC-components, the only correct result should be "not possible", and any numbers that are spit out should be ignored. Compared these numbers for "correctness" does not make sense. One interesting point of the Tektronix article, if you look at equation 1, this would place the maximum achievable accuracy at 0.5% for a Picoscope 5000 series. Not bad. A top of the line Keysight impedance analyzer (https://www.keysight.com/be/en/product/E4990A/impedance-analyzer-20-hz-10-20-30-50-120-mhz.html) does "only" 10-times better. :) |
| _Wim_:
--- Quote from: Hans Polak on May 15, 2021, 09:40:43 pm ---No offense, but if I may say so, the used way of calculating is rather complicated with additional current and a second phase that I haven't checked because calculation can be done much simpler with less chance of making mistakes, see calculation below. --- End quote --- Looking at your calculation, I think you did oversimply a bit. You consider the measured phase as the phase of the capacitor/inductor, but the measured phase is the phase of the current through the reference resistor only. For purely capacitive or inductive loads the phase of the current will be identical, but the difference gets larger for combined loads. |
| Hans Polak:
Hi Wim, I don't get what you mean with the phase. The only reason there is a phase shift in the first place is because the DUT (and the probe) are causing this. But I will dive into the Tektronix calculation and see what it brings. No matter what roads are taken to calculate the characteristics of a DUT, the outcome should be 100% the same. It's not the calculation that determines the properties, but the only DUT. When trying to achieve the same result, one of both must be wrong, but I'm sure we will find out. As a control system in the background, we can also use LTSpice, because with 3 simple commands it calculates: L = IM(Rref*(V2/(V1-V2))/w) Resr=Re(Rref*(V2/(V1-V2))) Imp=Mag(Rref*(V2/(V1-V2))) Hans P.S. I saw in the above shown calculation that I forgot to include R* in the imaginary part E. But in my Excel this is correct. |
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