For a 2:1 impedance transformer, 14:10 turns ratio gives 1.96:1 impedance ratio which may be close enough, or 17:12 turns ratio gives 2.007:1 impedance ratio (<0.35% error). A few minutes playing around with a calculator will probably find even better approximations.
73
John
M0JBA
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On 17 Aug 2020, at 18:19, Jim Lux <jimlux@...> wrote:
On 8/17/20 7:36 AM, Stephen Laurence wrote:
I seem to recall from the distant past, reading instructions for one of the early hobby vna devices about this.
The software with the vna (mandatory pc to use it) could automatically readjust the response. The demonstration was an ssb sideband filter with input and output inpedance of several 100ohms. The initial result seemed to be a poor filter with maximum skirt attenuation of only 20db but after the corrections were applied, was the expected 80+ db. This was at hf of course.
What was going on? Can this sort of correction be applied. How do I assess a filter with several hundred ohm in and out impedance, or do I resort to a spectrum analyser with tracking generator ( do not have one) or noise source, which is what I have done in the past. I still have that kit ( does 0-120mhz using a scope for display) for posterity.
Is this an opportunity for a firmware ( or saver) addition?
There's a couple ways to approach this:
1) make a test fixture and cal kit that has 100 ohm loads - run the cal, and it should automatically fix everything - I would be concerned that various displays wouldn't "know" about 100 ohms, so displays of R and X would be wrong, even if S11, S21 are correctly displayed. The "center" of the Smith chart would be 100 ohms.
2) calibrate with 50 ohms, and post process to change to 100 ohms - one could implement such a thing in a program like NanoVNA-Saver, and that's probably the cleanest way.
Ultimately, though, for both of these the measurement uncertainty will maybe be slightly worse - the "bridge" in the NanoVNA is designed for 50 ohms, so you'll always get reflected power and the transmitted power will always be less - How that would quantitatively affect your results, I can't say.
3) build a test fixture that has a 2:1 impedance transformation (non trivial, since it's a transformer with a 1.414:1 turns ratio).
Now you're back to sort of a #1 scenario.
I note that this is of some interest to people making measurements on twisted pair cables (i.e. Cat 5), since they're 100 ohm nominal impedance balanced. And a TDR is a great thing for finding places where the installer bent the cable too tightly or some similar problem.
On a fancy analyzer, they'll provide software inside to change the reference impedance, so you can do a cal with 50, measure 100, display with 100, or any other combination. Here, I think NanoVNA-Saver is the right kind of place. Keep the inexpensive box with limited firmware space simple. Put the fancy features in software that can run on any platform.
