[G0HZU_JMR]

To do a better comparison I spent a bit of time last night comparing the VNA against the HP8405A method and when tested side by side using the same test fixture with no body heating effects and the same ATC800B 30pF loading cap the two methods usually agreed within about 1Hz. This was measured with several crystals several times going back and forth. I did have to zoom in to a narrow span on the VNA to get this resolution and I used lots of points on the display and a narrow RBW.

Just touching the fixture near the crystal with my fingers for a few seconds would cause a slow drift in the crystal so I had to use gloves and insulated tools to swap between the two setups to do a decent comparison. As the manufacturer's initial calibration spec for a typical crystal is 10-20ppm then these differences are negligible I think.