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I use a crystal test fixture that I built for the Poor Ham Scalar Network Analyzer (PHSNA) and I used it very much with good success with the nanoVNA also. It consist on two 50 to 12,5 Ohm broadband transformers with resistive attenuators at both ends to insure correct loading and adequate drive level for the crystal. It could be useful to have the capability of replacing the crystal with a variable resistor, either with a switch or a plug in resistor, to find the equivalent series resistance, but it could be calculated with the appropriate functions included in the the software / firmware without that resistor. I have been talking with the nanoVNA Saver author about this and he plans to include specific crystal measurement functions in a future version.
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The PHSNA group is plenty of these test fixtures. An example is here: /g/PHSNA/files/Crystal%20Test%20Fixture/Crystal%20Test%20Fixture%20Overview.pdf I built mine without the optional amplifier and without the variable resistor also, so I did it without a PCB, just the input and output connectors, a socket for the crystal, the transformers and the two Tee pads. If anyone is interested I can upload info and pictures to the group. Regards, Ignacio? EB4APL El 10/05/2020 a las 2:20, alan victor escribi¨®:
It appears that this fixture is specific to the crystal under investigation. The fixture is providing a total load R of 25 ohms series from each side. Is possible if you transform the shunt C to its series equivalent from each side of the fixture, the total C load is 19 pF. A check on this requires knowing the test frequency. Seems like a stretch but you need to check if that is the case by transforming the network from its parallel to series form. --
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