BLE frequency accuracy

RE: BLE frequency accuracy

Hieu — Tue, 10 Jun 2025 12:20:34 GMT

The reason I asked for the measuring methods and brought up PCB design review is to see whether 2pF is actually giving you exact 32.00000 MHz +- 30ppm, and whether the existing product with 12pF is having 31.99744 MHz.

At 31.99744 MHz, your circuit is at 80ppm, above our specifications.

With that clock, BLE communication will not be reliable. There would be frequent packet losses and disconnection, which will get worse as the crystal ages, or temperature shifts.

RE: BLE frequency accuracy

BOE — Fri, 06 Jun 2025 13:41:57 GMT

The crystal is Abracon ABM12W-32.0000MHZ-4-D1X. Its load capacitance is 4 pF, which is low, but the design seeks minimal power usage, so the crystal is appropriate. The flanking capacitors are both 2 pF. This is appropriate if we assume 3 pF parasitic capacitance, and it yields almost perfect empirical results. This is as measured with a benchtop frequency counter and a high frequency probe. The only downside is that the parasitic capacitance is now the predominant factor.

Please address the actual question. What is the consequence of shipping units with the 31.99744 MHz timebase?

RE: BLE frequency accuracy

Hieu — Fri, 06 Jun 2025 13:19:42 GMT

Hello BOE,

We recommend this formula to calculate the load capacitance:

Ccap = 2*CL – C_pcb – C_pin

Where CL is the capacitance of the crystal, C_pcb is the parasitic capacitance from the PCB, and C_pin is from the clock pin of the nRF SoC.

For the HF crystal on the nRF52840, we usually roughly estimate (– C_pcb – C_pin) to be -4pF. This is assuming negligible parasitic capacitance.

The fact that you measured 2pF to work best suggests that something is unusual with the situation. It could be that the crystal traces produce too much parasitic capacitance but could also be something else.

Could you please share the details of how you measure the oscillator circuit frequency and the part number of the crystal?

It might be helpful to look at the PCB design too. If you want to share this, I can make this case private.

Hieu

RE: BLE frequency accuracy

Hieu — Tue, 03 Jun 2025 16:14:19 GMT

Hello BOE,

My apology for the late follow-up. I had some preliminary investigation done but arrived at two conflicting information sources. I will figure this out and follow up with you in one or two days.

Hieu