[G0HZU_JMR]

If anyone does read this and tries to repeat the procedure in the HP app note then it's worth emphasising that in section 4 of the 'series resonance' procedure you do have to manually adjust the sig gen mod/freq carefully to keep the phase meter near zero when on the fine (+/- 6deg) range on the HP8405A. Otherwise errors can creep in. I spent some time exploring these errors and I now think it's worth me pointing this out.

Back in those days I suspect that HP were used to testing crystals for use in their classic $$$ OCXOs and they were probably interested in measuring very subtle differences between crystals. Maybe they used a setup like this or maybe they had an even better one but having tried the first procedure in the HP app note I suspect that most users would be better/safer just doing the manual approach without the FM loop connection. It's slower and so you would get less throughput if testing loads of crystals but most of us would only be measuring a handful of crystals at a time. You wouldn't need a sig gen with a decent external FM/DC input either.

However, if you had a boxful of crystals like G6Tanuki had then the FM/loop method would be the best, but care would be needed and the phase meter would need to be monitored (and the sig gen adjusted as per section 4) to keep the phase meter close to zero if you wanted to measure things without introducing some subtle errors. You can then test the crystals very quickly once the setup is proven with a manual cross check.