When I needed to make up a bunch of baluns for HF I did two things, first I made as nonreactive a load as I could, in that case 300 ohms, by careful construction using carbon comp resistors, and then once I was confident of the first balun I constructed I used it to be working at 300 ohms. Perhaps some combination of these would help work at the higher impedance.
Peter
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On Aug 7, 2019, at 3:21 PM, tuckvk3cca <tuckvk3cca@...> wrote:
I am not a fan of calibrating with an impedance load that is too high. This is a limitation of the design. Theoretically we have a wheatstone bridge. 2 legs are 50 ohms and one leg is fixed at 50 ohms. I have a RLB that allows this to be set to any other value which is great. If you do not have this and calibrate with too high a value you eat into the dynamic range of the instrument, no amount of math correction can help you improve that. For accurate work one should really have a good RLB with adjustable standard and a good solid vector phase meter. Everything else are gimmicks.
Call me old fashion.
This business is quite d¨¦j¨¤ vu for me. As a undergraduate student I took a challenge to use a standard Wheatstone bridge with 10 or 12 ohm standards. The exercise was to measure to milliohm accuracy four orders of magnitude down. It was a tough exercise but I won. Nowadays you can buy 1 milliohm resistors with 1 or even 0.1% accuracy, but 5% is available on EBay, I did not have that luxury then.
Then I found an excellent paper on Wheatstone bridges that thought me a lot that I did not know. Still have the article somewhere I think. Drink deep or not taste the Pierian spring!
-----Original Message-----
From: [email protected] <[email protected]> On Behalf Of Peter Gottlieb
Sent: Wednesday, August 7, 2019 9:07 PM
To: [email protected]
Subject: Re: [nanovna-users] Measurement challenge
How about running a cal with a known higher impedance load? Would that give a better chart? I don¡¯t think you would need super accuracy for this application.
Peter
On Aug 7, 2019, at 3:04 PM, alan victor <avictor73@...> wrote:
After you get comfortable with the calibration process you may want to
consider measurement of components whose |Z| is much greater (or
smaller) than 50 ohms. Grab a couple of known parts out of the junk
box. I am working on a couple of tube amplifiers and in need of RF chokes and elements for a PI matching network. The desired chokes are 1-3 mH and the inductor for the PI match, 6 uH.
However any range where the magnitude of Z is several hundred ohms or
more and gamma gets to the edge of the chart will challenge the quality of your cal.
Currently I am measuring these devices in the SHUNT mode, simply as a one port.
Give it a go at HF, confine yourself below 30 MHz. See if you can get numbers that make sense.
Start with something whose value you know!
Alan
