Ceramic resonators and ultrasonic vibrations
After investigating a lot of options, it seems very likely that the drive strength of our ASIC is too little to cope with external disturbance.

I tried a similar hardware setup (our old product) with the resonator in question and it worked fine there being driven by a PIC microcontroller.

Another try was with an ultra low power quartz, which is rated for a 1 µW drive power maximum. This one worked well again. This quartz costs around 4 € a piece, so not really a solution, and it requires a new layout as well.

I sent a sample of our circuit to Murata, which they analyzed (well not exactly for my problem) and found that under extreme values of the resonator (maximum impedance) the resonator will not start oscillation. So this I take as another hint, that the drive strength is not enough.

As the damping resistor is integrated into the ASIC there is no way to reduce it to increase the drive strength.

The solutions in our case are rather limited and all not really optimal. Best solution would probably be to redesign the ASIC, but we have so many in our storage that it takes 2 years until the change would be integrated in all our products - suboptimal.

Using the microcontroller in its specification will double the current requirements, which would render the product non-functional.

So we'll redesign the hardware and use an additional inverter to build our own resonator circuit and provide the clock from there to the ASIC and the microcontroller. This means doing quite a few certifications again.



I tried soldering only one third of the pads to allow some movement of the resonator. It changed the behavior (less disturbance) but not enough to rectify the problem completely.

I also tried putting some electronic glue over the resonator to maybe shift the resonance or stop some movement, but that made it worse.

Using different capacitors in parallel didn't change anything.

An old resonator in a round housing worked also fine (no built-in caps). But those aren't in production anymore as far as I can tell.

I wanted to try a resonator without built in capacitors as I thought they might add an additional mechanical resonance, but up to today I never received the samples I was promised. (I can post some pictures if someone is interested about the internal build up of a ceramic resonator)