We are trying to develp a RP (reverse polarity) TNC kit which is not commercially available in the market. We want to have some verification method in our internal laboratory.
I’m wondering why admittance G+jB (inverse smith chart)needs to be plotted. Not the normal impedance Z plot (R+jX, smith chart) where reactance is reflected? i thought normal smith chart reflects the imaginary part of the impedance. doesn't it apply to this case?
The R+jX plot will convert the reflection coefficient to an equivalent series R and series C. The G+jB will convert it to a shunt R and shunt C. I would guess that they should be the same in your case. I just always use G+jB when looking a capacitance to ground, out of habit.
i'm a little bit confused. yes, the R+jX plot will convert reflection coefficient to an equivalent series R and C, however the G+jB will convert to a shunt conductance G (in siemens) and susceptance B (also in siemens). PNA's plot of G+jB shows unit in fF? why is that?
i just did trial and added marks both of R+jX and G+jB. the results as you said are the same. this looks weird to me. are they supposed to be different? also what do the numbers after them mean? for example -1.02 ohms, -12.16ns and 109.16us.
A capacitor to ground from a 1 port measurement can be thought of as a shunt element or a series element to ground.
If a shunt element, the values are in 1/ohms = siemens, and if in series, they are in ohms. Thus, you have the effective shunt admittance, which is conductance and susceptance (g+jb) or the series impedance, which is resistance and reactance (r+jx). In both cases, the imaginary value is converted to the equivalent capacitance. In the case of only a capacitor, it will give the same answer. In the case where something, anything else, is in series or shunt, the answer will vary and looking at it to get a constant capacitance you must choose to view it a series or shunt.
i guess i understand it now. as you sai, in both cases, the imaginary value is converted to the equivalent capacitance.
i tried a short today, and i'm getting different result by using R+jX and G+jB mark, though the variation is not very much. which one do you think i should use as the inductance coefficient caculation? what is the reason for the difference?
look at the numbers: In series, it looks like a 1.35 ohm series resistance and a 1.21 ohm reactance, so the series resistance is bigger than the inductor effect; As a pair of shunt elements, the same reflection would have a 2 ohm resistance in parallel with a 24 ph inductor. Throw the values in a simulator and test for yourself.
when i try to check why there is high resistance, i found a mistake where when i generated the s1p file, i did not set the material of outer contact to perfect conductor, instead i set it to nickel. this is why there are difference between X and B generated fringe parameters because of higher resistivity of nickel.
i changed the material setting and recalled it in PNA and generated the plot as attached. both X and B plot has the same calculated inductance value now. i also used excel to solve L0 to L3 coefficients and it is showed in the attached.
If by "normal" you mean "so small that they can be completely ignored with no effect on the measurement results" then yes, I agree. Look at you scale, the entire error is less the 0.05 degrees. If you tweeked your delay slightly, the error would reduce to less than 0.01 degrees.
Also, because of the ripple, I wonder if you have the impedance of the line set to exactly Z0? Did you include the skin-depth effect when computing the diameters? The ripple, while small, indicates the impedance may be very slightly off.
And, to finish this off ( I doubt there is anyone else viewing this thread anymore) I ran an ADS simultation of a shorted line with your inductance values, then I ran the same simulation, set the inductance to 0! and set the coax impedanc eto 49.99 ohms, and got EXACTLY your ripple (how about that!). Here's the plot; check you Z0. Purple red trace is your inductance values, purple trace is 0 inductance, 49.99 ohm, blue trace is computation of your inductance.
I used the approximation equation of Z0=138/(e)^1/2*log(d1/d2) to get 50 ohms impedance. When plugging in the numbers, I got 49.987 ohms. I also set both outer and inner contacts with perfect conductor in Ansoft HFSS. So there should be no problem of skin depth of R in the impedance equation. Also the conductivity between the outer and inner conductor is negligible. I thought the 49.987 ohms is good enough because in reality, you can never get precisely 50 ohms considering the physical tolerances.
How much does this ripple affect the calibration? As you said the total phase shift is very tiny, less than 0.05 degrees. The peak to peak ripple is extremely small. I guess this would not have much impact on the fringe coefficients determination. Do you agree?
Also when I checked the Agilent 75 ohms type N cal kit .ckt file, I noticed the delay time of the short was set three digits (17.544ps) after zero in the unit of picoseconds, why is that? Does it require that much precision?
My understanding is that it can be set at a rounded number of 17.5 ps and this will adjust the fringe inductance coefficients, and this will be as good as it is according to my understanding of what said in the thread. Is this the correct interpretation?
As I said in one of the earliest postings, the standards are overdetermined, and changing the delay or changing the Inductance generates the same result. So, it simply depends upon the person doing the fitting. In your case, I think you have an error in impedance. I think it is more likely that your impedance is off by 0.01 ohms than the inductance has such an obscure curve. But in the end, as you surmise, it doesn't matter at all as long as Gamma_short model fits the actual response of the the short.