[G0HZU_JMR]

I had my first go with this and things are looking good and the loop locked up first time! I'm still optimising the setup but the first crude attempt showed the resonance of the crystal on its own (without the 30pF loading)was 4.031720MHz.

The same crystal was tested yesterday (similar room temperature?) on the VNA and the exported model shows resonance at 4.031723MHz as per the image below. This does not have the 30pF loading cap in series and this explains why it is not at the expected 4.032MHz as per the crystal frequency stamped on the crystal. But it agrees very closely with the HP app note method. Just a few Hz difference! What a result!

On the simulator I can add a perfect 30pF capacitor and measure the resonance with the recommended loading. I've yet to make the 'real' 30pF test cap for the HP8405A and I realise now that this cap needs to be decent in terms of accuracy. It looks like I'm chasing down to tiny frequency differences between the methods now...

I'll have more of a play and then try testing all of the 4.032MHz crystals on the HP8405A and I'll post up the results on here. This method is the fastest/easiest because I just look at the readout on my old Anritsu frequency counter to see the resonant frequency. This is a seriously cool test method!