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Yeah, you just drop a resistor into your fixture in place of the crystal to give you a phase=0 reference. Fiddle with the resistor value to get the same voltage ratio on the two VVM channels and your resistor value equals the crystal ESR, and you have the best zero phase reference.
The Cathodeon fixture with its low-Z environment between its minimum loss pads is rather good in this area. Find the SER this way, and find the series res frequencies with and without the 30pf load C in circuit and you have enough to extract motional parameters.
There are some superb apps notes by Cathodeon, but I've never seen them on the web.
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Thanks. I need to do a bit more research to see what can be achieved with this method. I'll probably stick with the VNA method when it comes to obtaining an accurate LCR model of the motional parameters (you can see how good my model is in the earlier plots!) but I think the HP8405A method wins for measuring the resonant frequency the easiest and with the best resolution.