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LCR Impedance Viewer for Picoscope+Keysight+R&S Bode Plot Data (open source)

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_Wim_:

--- Quote from: Hans Polak on May 15, 2021, 08:49:39 am ---As I understand your proposal,
Make cal. file 1 with only the scope probe attached.
Measure a complex load and correct it with cal. file 1.
Make a new cal. file 2, now with an added cap over the probe.
Measure the same complex load again but now with the added cap and correct the result with cal. file 2.
Corrected result for the complex load should be the same in both cases.

--- End quote ---
Yes, indeed.


--- Quote from: Hans Polak on May 15, 2021, 08:49:39 am ---1) Is there a reason why you attached an image of a probe, or do you suggest this to be used as a complex load

--- End quote ---
=> was just to give an idea on how an probe and input looks to give an idea about the values to make a modified "bad" input


--- Quote from: Hans Polak on May 15, 2021, 08:49:39 am ---2) what capacity do you suggest to place in par. to the probe for cal. file 2 ?

--- End quote ---
=> I would take something like 47pf or maybe up to a 100pf. This will already limit HF response quite severely. For the optional series resistance I would think something like 100k and and 100pf.

Hans Polak:
Hi Wim

O.k. I'll place a 75pF cap par to the probe to produce cal. file 2.

The optional 100K + 100pF that you mention, is that meant to be placed par. to the probe (instead of the above mentioned 75pF ?) to generate a cal. file 3 and if so, the two in series or in parallel.

And still open is the question: how should the DUT look like, or is that the 100K + 100pF ?

Hans

_Wim_:

--- Quote from: Hans Polak on May 15, 2021, 12:47:18 pm ---Hi Wim

O.k. I'll place a 75pF cap par to the probe to produce cal. file 2.

The optional 100K + 100pF that you mention, is that meant to be placed par. to the probe (instead of the above mentioned 75pF ?) to generate a cal. file 3 and if so, the two in series or in parallel.

And still open is the question: how should the DUT look like, or is that the 100K + 100pF ?

Hans

--- End quote ---

The optional 100K + 100pf is to create a high pass characteristic. Together with the parallel cap, this should create a response with:
- some loss of gain due to the 100K series resistor (acts as a voltage divider with the 1M input impedance to the scope)
- for higher frequencies, the 100K is bypassed by the parallel cap, increasing the gain again
- for very high frequencies, we have again a loss in gain due to the parallel input cap.

This should create a non flat response to challenge the calibration logic. I would start however with only the parallel input cap.

For a load I would just a parallel LC network with a series resistor, just to have both some inductive & capacitive behavior. Values are not very critical, I would just ensure that you have some non-flat parts in the zone where the calibration correction is the largest (to challenge the correction).

Hans Polak:
Hi Wim,

I'm extremely sorry, but I have some other bad news.
So far I only concentrated on the capacitance calculations of FRA Imp Viewer.
But now being busy with calibration and so on, I found more that's not O.K.

Still with a 100R reference resistor, and as load 100R+10nF in series, I took FRA's inductance image, having on the right side the ESR.
At 100Hz, the 10nF still has a large impedance, so most of Resr comes from the probe, but the FRA image shows -50R at 100Hz.
To my opinion, Resr can never become negative and a value of 50R seems impossible altogether.
In the Excel Image below, Re is as expected, positive and 45K at 100Hz.

Then the Inductance shows 250H in FRA at 100Hz, where Excel using the same .csv file shows 22H at 100Hz.
As from 1kHz and upwards, FRA and Excel are the same within 10%.

At least ESR has some miscalculation, which could be the cause of the wrong inductance value, but maybe there's more.

Hans


_Wim_:

--- Quote from: Hans Polak on May 15, 2021, 05:43:12 pm ---Hi Wim,

I'm extremely sorry, but I have some other bad news.
So far I only concentrated on the capacitance calculations of FRA Imp Viewer.
But now being busy with calibration and so on, I found more that's not O.K.

Still with a 100R reference resistor, and as load 100R+10nF in series, I took FRA's inductance image, having on the right side the ESR.
At 100Hz, the 10nF still has a large impedance, so most of Resr comes from the probe, but the FRA image shows -50R at 100Hz.
To my opinion, Resr can never become negative and a value of 50R seems impossible altogether.
In the Excel Image below, Re is as expected, positive and 45K at 100Hz.

Then the Inductance shows 250H in FRA at 100Hz, where Excel using the same .csv file shows 22H at 100Hz.
As from 1kHz and upwards, FRA and Excel are the same within 10%.

At least ESR has some miscalculation, which could be the cause of the wrong inductance value, but maybe there's more.

Hans

--- End quote ---

If you put a 10nF capacitor in series with 100 ohm, you create a high pass filter with a 159kHz -3db point. Each octave lower will be another -6db.  You loose another 7-8 db by the voltage divider created by the two 100 ohm resistors and the 50 ohm resistor in the signal generator. Counting all of these losses up you end up in the noise floor of your scope (in other words, for the scope your DUT is an open circuit at these low frequencies).

As your scope channels are not perfectly identical and have some bias offset, the current "measured" through the reference resistor can be negative (as this current is measured by the difference between channel 1 and channel 2), and all sort of strange things can also happen in the noise. Your csv file show a positive gain at lower frequencies (+0.018 db, which means the signal measured on the DUT is LARGER then the signal put into the reference resistor => this can only be via bias offset differences in an open circuit situation, as this is impossible for a passive device)
You will probably see a similar result by measuring an open circuit.

I have no idea what you have done in your excel to handle negative currents (positive gains), in the case of my app I limited the gain to zero (see "GainCorrectedDB" function). A more clear solution would be to plot no data at all when the gain is positive, but having a smooth continuous curve like in your excel is even more confusing (I presume you somewhere use an "absolute value" function to circumvent this issue).


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