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Hi Jeff

My humble opinion is like your indications that L and C coefficient are not relevant for the calibration kit delivered with the NanoVNA and in particular as we have no idea if the kit are the same for all deliveries.

However the NanoVNA has already a build in correction for the open in the form of 50fF which is pretty much correct for the CH0 Female SMA left open, so to use the supplied open standard is wrong and is adding further delay.
Basicly the NanoVNA is for me the "engine" and for using other calibration kits the way is to use the NanoVNA-saver where you can enter delays and L and C coefficient IF YOU HAVE THEM and that is not the case for the majority of NanoVNA users for whatever homemade kit they want to use. One must remember to subtract the 50fF from the Open as the NanoVNA is internally pre-compensated by 50fF equal to a one way delay of 2.5ps. This can be verified by standalone calibrating the NanoVNA, using no open adaptor and run a phase s11 track with 1degree/division and then enable the Scale/Electrical delay to twice the one way delay, as we are dealing with a reflection, so 2.5x2=5ps, and the phase trace is horizontal as proof. This is true for frequencies up to 300MHz and above for a fresh calibration, else it is drifting over time above 300MHz du to temperature changes. Remember to set the Electrical delay back to 0ps ?
Until there is a full blown calibration kit definition embedded in the NanoVNA this is the way forward to use the NanoVNA-saver. By a full blown calibration kit definition I mean that also 6/12 term error correction implemented and again my opinion is that would be an overkill for the majority of NanoVNA users. It is far better to focus on how and with simple means to find the needed delays for a homemade kit or e.g. a BNC kit bought from SDR kits where all these data are supplied with the kit.

David is giving a comment the a short always has a longer delay than open, and that can be misunderstood. That is not the caser for the supplied kit for the NanoVNA. I have made a comment on this on this reflector as it is anticipated to be 0ps by design but it has a very small negative value. I did measure the supplied kit based on calibration by my HP 3.5mm kit on another VNA and I will repeat and publish the result for those values to be entered in NanoVNA saver. It would be nice if David had done that instead of lecturing about the way he seem everything.

Long live a pragmatic approach

Kind regard

Kurt





-----Oprindelig meddelelse-----
Fra: [email protected] <[email protected]> P? vegne af Jeff Anderson
Sendt: 29. september 2019 03:47
Til: [email protected]
Emne: Re: [nanovna-users] Cal-Kit Standards' Definitions



On Sat, Sep 28, 2019 at 03:49 PM, Dr. David Kirkby from Kirkby Microwave Ltd wrote:

I don’t think your simple model is really suitable for the following

reasons

1) The variation of C with homemade standards is likely to exceed that

of commercial standards - this is from experience measuring them.

2) The inductance of shorts is likely to be more with homemade

standards than commercial ones - again this is based on experience measuring them.

3) People may well want to make measurements in a 75 ohm system.

4) it is possible to improve upon the accuracy of loads at low

frequencies by using a DC resistance measurement.

5) In the case of a female N, a simple standard can be made by just

leaving the connector open. This will create a higher impedance

transmission line than 50 ohms as the centre conductor sits in a

cylindrical section with a greater diameter than when its mated.

6) The loss of homemade standards is likely to be greater than

commercial ones from Keysight - again this is based on actual

measurements I have performed.

Dave, thanks for taking the time to reply. I appreciate your comments, and I agree with you on all these points -- but I wasn't really concerning myself with homemade standards, which I assume are almost always uncharacterized.



Instead (and I should have made this clearer), I was wondering what the impact was of the Capacitance and Inductance terms of characterized Open and Short standards. Ditto for their Delay, Loss and Offset Zo terms.



For the HP Open and Short standards I looked at, Delay has the largest impact on Gamma (which was the reason I never set this term to 0 in my calculations), followed by the Open's C0 term. The other terms have an effect, but that effect is much smaller than the effect of Delay or C0.



As for homemade standards, probably the best one can do to characterize them is verify that the load is as close to 50 ohms as possible (using a 4-terminal ohms measurement) and determine the Delays of the Short and Open using an *already-calibrated* VNA (although I did come across a web page where the author actually derived C0-C3 (and perhaps L0-L3? I don't recall any longer).

I see your notes that the

phase variation up to 1500 MHz is smaller than the uncertainty in the

calibration standards. I can’t square that circle.

I was quite surprised to see this result, which is one reason why I originally posted the results here with my question wondering if there were a math error.



Running through the numbers again, they look good to me. Here's an example of the effect of the capacitance terms of an 85032F Male-N Open at 900 MHz. Note that C0-C3 are spec'd as follows (all values in Farads):



C0 = 89.939e-15;

C1 = 2536.8e-27;

C2 = -264.99e-36;

C3 = 13.4e-45;



First, calculating the Open's Gamma using C0-C4. The equations are:



divisor = (2*pi*f * (C0 + C1*f + C2*f^2 + C3*f^3))

Zopen = -j / divisor

GammaOpen = ((Zopen/50) - 1) / ((Zopen/50) + 1)



If the frequency (f) is 900MHz, then GammaOpen equals 0.9962 - j0.0877 (i.e. magnitude of 1, angle of -5.0292 degrees).



Let's now calculate Gamma for the C0-only model. The equations now are:



divisor = (2*pi*f * C0)

Zopen = -j / divisor

GammaOpen = ((Zopen/50) - 1) / ((Zopen/50) + 1)



Gamma now is 0.9964 - j0.08461 (i.e. magnitude of 1, angle of -4.8538 degrees).



From this I draw a couple of conclusions:



1. At 900 MHz this standard's C0 has a significant impact on the angle of Gamma (about -4.9 degrees).



2. Adding in the additional C1-C3 terms only changes the Open's angle of Gamma by about 0.18 degrees, which is significantly smaller than this Standard's spec'd "Deviation from Nominal Phase" of +/- 0.65 degrees (from DC to 3 GHz).



(By the way -- should any one else like to verify the results, the equations and C0-C3 terms are above.)



Thanks again for your comments and insights,



- Jeff, k6jca