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Re: testing non-50 ohm filters was Re: [nanovna-users] NanoVNA port renormalization
It wasn't clear to me whether the 10 pF test load that simulates the capacitance of the circuitry the filter feeds should be reversed along with the filter when doing the reverse measurement. Don had left the capacitor in place on his test board when reversing the filter in a socket. He made some new measurements with no capacitor. The plots show the renormalized response with (red) and without (blue) the capacitor. It has some effect in the stopband but very little in the passband.
Brian |
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Re: testing non-50 ohm filters was Re: [nanovna-users] NanoVNA port renormalization
All three plots below are for renormalization from 50 to 330 ohms. Red used the forward and reverse measurements while blue is forward only and green is reverse only. The "only" measurements set S12 = S21 and S22 = S11. The curves are virtually identical in the stopband, but they show differences up to 3 dB in the passband.
The bump at 10.78 MHz is coincident with one of the two pronounced passband peaks in the unnormalized 50 ohm data. Brian |
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Re: testing non-50 ohm filters was Re: [nanovna-users] NanoVNA port renormalization
Hi Brian,
Thanks big time for sending me the .s2p renormalized file. As a reminder to everyone your renormalized curve used my original forward and backward measurements of my ceramic filter using a 10 pf connected load and the measurement was made with a direct connection to my NanoVNA-F (NanoVNA-F calibrated using 50 ohm loads, and no external matching devices used). My 10.7 MHz ceramic filter (SFE10.7MA5-A) has a published input and output impedance of 330 ohms. Attached are 3 files comparing your renormalized curve using my forward and reverse measurement data versus 3 different matching methods that I used to measure the same ceramic filter with the same 10 pf load (all shown connected in the forward connected direction since their impedance matched forward and reverse connected measurements fall almost directly on top of each other). The only difference between the 3 attached files is that I used different scaling on the vertical and horizontal axis to provide some zoomed in and zoomed out views of the frequency response curves. In the attached plots the curves are identified with the following colors: Red = your renormalized curve Blue = 50 to 330 ohm resistive L matching curve Green = 280 ohm series resistor matching curve Orange = 2 turn primary, 5 turn secondary BN-73-202 binocular core matching transformer curve (not a perfect match, provided more like 280 ohms whereas it should have been 330 ohms). I hate to provide a firm opinion but let me make some quick observations to help folks understand what they are looking at as follows: The renormalization process did a great job fixing my direct measurement curves to form a more realistic frequency response curve of my filter but it was not perfect if you zoom in and compare the very top of the renormalized curve with the curves I obtained using two different resistive matching techniques. The top of the renormalized curve has a premature dip and then a bump. Also if you look way down in the rejection area of the curves you will see the renormalized curve does not agree with my 3 other matching technique curves, especially in the lower frequency direction. While my matching transformer provided an impedance that did not exactly match the published input and output impedance of the ceramic filter it did a decent job and it provided much better dynamic range which helped when looking at the filter where it provided a lot of rejection (way down in the mud so to speak). I could have added a half turn on my transformers to provide a slightly better match but had to cut my time consumption on this exercise. Not having a perfect match using the transformer appears to have had some slight impact on the shape of the top of the curve as well as a slight downward shift in the frequency response when you look at the zoomed in curve. At some point it would be interesting to see how your renormalization routine compares with that on the NanoVNAs that have that function but unfortunately my NanoVNA-F does not have that function. Hopefully the work we have done will provide some insight for others regarding what to look for if they want to better evaluate the renormalization routine on the NanoVNAs that have that function. And there also is the big caveat regarding renormalization of nonlinear devices that folks need to be cognizant of. Thanks again for all your help. Don |
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Re: Adapting the LiteVNA for SAR?
All of this begs a simple question.
Use the NanoVNA as a Field Strength Meter. Why - Just built a new antenna using the NanoVNA to adjust it to the operational Frequency and adjusted the impedance for a match now it would be nice to be able to have some form of active display of the radiated signal, without blowing up the front end the NanoVNA While I cause a simple diode detector and a meter using the NanoVNA is another possible use. Any suggestions on how or how not to do it. Thanks |
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Re: testing non-50 ohm filters was Re: [nanovna-users] NanoVNA port renormalization
The plot compares S21 of the forward 50 ohm file with S21 after forward and reverse were renormalized to 330 ohms. Stopband details of the renormalized curve differ from your earlier curve using a transformer, but the passbands look similar. Thanks for all your help, Don. Also thanks to John Gord for explaining why renormalization works even with improper device loading, which I didn't expect.
Brian |
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Re: testing non-50 ohm filters was Re: [nanovna-users] NanoVNA port renormalization
Hi Brian,
I repeated the tests but this time using resistive matching pads to provide 330 ohm impedance to both the input and output of the ceramic filter versus using my very broadband transformers that provided a lower impedance (approximately 280 ohms). I also repeated using 270 ohm series resistors for matching. Since we now are only looking at the upper part of the response curve the loss in measurement dynamic range using the two different resistor methods should not be a problem. Much different looking response using the resistive matching versus the transformers. I again tested with a 10 pf, 33 pf, and 68 pf cap output loads and my reference curve used no capacitor on the load side. Just FYI, Don |
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Re: Adapting the LiteVNA for SAR?
? Use S21 mode with an amplifier.
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People have been using VNAs as radars for years. The NanoVNA just makes it easier. Dealing with the big first reflection is tricky. One way is to use a separate cancellation path that is adjusted to null the first reflection. ?That’s what we did for the FINDER victim detection radar. FINDER: Radar for Locating Disaster Victims ( ) jpl.nasa.gov ( ) ( )
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Re: testing non-50 ohm filters was Re: [nanovna-users] NanoVNA port renormalization
Hi Brian,
I went ahead and tested the filter response using a 10 pF, 33 pF, and 68 pF load capacitor. Attached are 3 plots and the reference plot on each of the attached plots is when I'm not using a load capacitor. It should be noted that the curves shown in the datasheets you attached have the left vertical axis identified as attenuation in dB. In reality it appears that axis has been normalized based on the peak of the response curve. In my attached plots I have not normalized the vertical axis so you can see what the true attenuation is through the filter. Just FYI, Don |
Roger Need
VE6LX
Roger C
