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RLC circuit test using impedance analyzer Agilent 4294a

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R_ZH:
Hello everyone,

I 'm using a impedance analyzer to test a RLC series circuit and want to extract the values. Finally I got a plot as shown below and the impedance analyzer gave me a set of parameters calculated by itself based on curve fitting. My question is in magnitude plot, is the magnitude axis linear or logarithmic? I need to find the self-resonant point of this RLC circuit like shown in the picture, if so, I have to set the magnitude to logarithmic on impedance analyzer. And if I set it to linear, it was not like this. I guess this results from impedance analyzer is a bode plot. I also use matlab to plot a bode plot, but it shows a linear axis of magnitude shown attached. I have been confused for long. Could anybody please explain this to me?

Best regards and thanks in advance!
Runze

RoGeorge:
The magnitude scale is in dB, which means that's a logarithmic scale.  By definition, the Bel is the logarithm of a ratio:
https://en.wikipedia.org/wiki/Decibel

R_ZH:
Do you mean that in the second picture, the magnitude is automatically set to logarithmic if the unit is dB, even though it looks like linear?

RoGeorge:
Not really.

To be pedantic, the first picture is wrong.  The text on the Y axis reads "Magnitude (dB)", which means the numbers on the axis are decibels.  However, the gradations on the Y axis are shown as a logarithmic ratio.  The upper limit, for example, reads 108, so 100 000 000 (a hundred million).  That hundred million is the ratio, not a hundred million dB!  :scared:

First picture is wrong because the units of the magnitude axis is not dB, the unit measure there is adimensional (a ratio, has no units).

The second picture is correct.  When the values are represented as decibels, the axis is usually linear.  You can choose the axis of dB to be drawn in the chart in a logarithmic representation, but that would be rather unusual.

Either way, logarithmic axis with adimensional ratio, or linear axis in dB, the RLC values or the resonance of the circuit is not affected by the way the results are drawn in the chart.

R_ZH:
Yes, the unit in first picture is not dB but probably Ohm. Since it is extracted from impedance analyzer and plot in matlab, there should be some difference from real bode plot. I can show you a screenshot directly from the impedance analyzer.

Yes, it doesn't make any difference to the results. But if I use linear axis for magnitude, it is shown in the picture below and I can't find the resonant point.

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