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Re: NanoVNA-Saver 0.2.0
Hi Herb
Yes you did, and murphy is a bloody optimist. Kind regards Kurt -----Oprindelig meddelelse----- Fra: [email protected] <[email protected]> P? vegne af hwalker Sendt: 16. november 2019 12:58 Til: [email protected] Emne: Re: [nanovna-users] NanoVNA-Saver 0.2.0 #Ferrite_Impedance Kurt Poulsen wrote, "You have a problem as you are calibrating on the output side of the test fixture. " ----------------------------------------------------------------------------------------------------------------------------- Kurt, Great explanation and much thanks for taking the time to do so. Did I mention something about "going down the rabbit hole" ¡ - Herb |
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Re: NanoVNA-Saver 0.2.0
Hi Herb
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You have a problem as you are calibrating on the output side of the test fixture. Then the fixture is part of you test cable going from the NanoVNA to the test jig and including also the "mystery" unknown impedance of the wire thru the testjig, mainly an inductor for low frequencies but for higher frequency you will even experience quarter wave length resonances. It will not work as you do it. You must calibrate at the input of the test jig with a female kit, first a calibration of the NanoVNA stored to C0, and the use NanoVNA-Saver and enter the calibration kit data if you have the using the delay setting for short and open and also the data for load if you got it. Then you measure you fixture without the clamp choke save it as s1p then measure the fixture with clamp choke and save it as s1p. Then you will measure far better than before well knowing you have the testjig wire thru the choke and if you has a shorter wire from input to output of the testjig then you will approach the single wite method like the manufacturers use. To take the process a step further to get material measurements without influence from parasitic element: The big problem comes how to subtract the two s1p from each other. In my case it was very easy using my VNWA software as I simply in a custom trace convert the two s1p files saved to e.g. Mem1 and Mem2 to subtractable impedances, by a simple expression like z2s(s2z(Mem2)-s2z(Mem1)). As you cannot subtract s parameter from each other the trick is to convert the measurement without the choke to and impedance s2z(Mem1) and the subtract it from the s1p with choke s2z(Mem2) and the convert the difference back to s parameter with the expression z2s. Then the custom trace is showing the true s parameter of the choke and the test fixture impedances is thus totally eliminated and the custom trace can be saved as a new s1p file for nice presentation in the NanoVNA-saver by importing the file. I think the free QUCS (Quite Universal Circuit Simulator) has facilities to do this trick as well and suggest you download it and have some fun. Take notice there are two version not compatible a German originated derived version of the original. I fancy the original. It has a lot of display features and is a fantastic piece of software. Regarding the fixture It ought to be completely closed so no field lines escape out in the blue air. Next level is how to calculate the real material parameter which involved all the math shown in the article. I added that in the custom trace also after calculation the chamber parameters in a spreadsheet and I settled for circular chambers like paint can with a fixed shorting rod mating a SMA SMA bulkhead thru adaptor in the center of the lid which then make the calibration better with the VNWA calibration kit configured a female kit, of which I have publish data to enter in the NanoVNA-saver. UPS that was a long story Kind regards Kurt -----Oprindelig meddelelse----- Fra: [email protected] <[email protected]> P? vegne af hwalker Sendt: 16. november 2019 03:35 Til: [email protected] Emne: Re: [nanovna-users] NanoVNA-Saver 0.2.0 #Ferrite_Impedance On Fri, Nov 15, 2019 at 06:33 AM, Kurt Poulsen wrote:
" I have done a lot of such measurements in the past and eliminated the inductance of the wire thru the core. In your setup I think you get even better result by doing a SOL calibration at the end of the BNC adaptor using the Shorting wire thru the core without the core as short and place a leaded 50 ohm resistor as load and nothing for the open. The you get eliminated the "funny" impedances of the adaptors in the calibration process " ------------------------------------------------------------------------------------------------------------------------------- Kurt, The one turn loop gave me a method of sorting my ferrites by visual signature, but I could never get a good SOL calibration, per your suggestion, that extended beyond a few MHz. Since most of my unidentified ferrites are clamp-on style, I decided to "go down the rabbit hole" and try to build a fixture using some of the references you provided and material that I had on hand. Attached is a photo of the fixture I built. I tried to construct a coaxial type fixture where I could easily get access to the center conductor. I performed an OSL calibration from 1 - 500 MHz with the fixture cover in place and no ferrite installed. I made a reference plot with the fixture terminated in 50 ohms. The S11 impedance plot showed a straight 50 ohms with a slight bump at the 300 MHz band switch over point (see red trace in attached plot). I then replaced the 50 ohm load with the short standard used during calibration, installed a Fair-rite 0443164151 ferrite on the inner conductor (see attached photo), replaced the fixture cover and made a test run. The S11 impedance plot showed a rise in impedance with at least 100 ohms of impedance between ~17 MHz and 400 MHz (See black trace in attached plot) . The slight bump at 300 MHz during the reference run turned into huge outlier during the test run. I would typically remove it from my data set. The typical impedance data in the chart provided by Fair-rite does not closely match the data I obtained (see embedded chart data). I discovered that the coaxial fixture could also be used to measure the transfer impedance of an rf current probe that I had constructed from ferrites, so there was a secondary benefit to its construction. - Herb |
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Re: NanoVNA Noise improvements. Hugen79's NanoVNA Github Issue #14
#filtering
#noise
#improvement
#hack
#circuit
On Fri, Nov 15, 2019 at 09:34 PM, Gabriel Tenma White wrote:
" ¡ It's the absolute accuracy and confidence in the measurements that you care about, and if you look at the smith chart that hugen posted, it looks the same as the image erik posted earlier in his "chasing ghosts" thread, namely big unexplained dips in S11 above 600MHz that is unphysical when measuring a stub ¡." ------------------------------------------------------------------------------------------------- Thanks Gabriel. Its much easier for me to read the logmag plots and it takes more of a trained eye, like yours, to read the smith chart. hugen commented in the attached copy of the smith chart that reliable operation is limited to 1300 MHz, I suppose that is related to your comments as values after that become exceedingly noisy and eventually exit the circle of the smith chart (loss of directivity?). Most of my own measurements above 1 GHz are S21 through measurements. I would more than happy with 40 - 50 dB of head room above 1 GHz. When I first purchased the NanoVNA I was only expecting it to be useable to 300 MHz. All this additional measurement capability is icing on the cake for me. Regards, - Herb |
Kurt Poulsen
