Unavailability of reference design crstal

RE: Unavailability of reference design crstal

bjorn-spockeli — Mon, 07 May 2018 15:21:54 GMT

AmbystomaLabs You are absolutly correct, I only looked at the standard tolerance @+25degC which was +/-30ppm. It is the total tolerance you need to look at, i.e. Frequency Tolerance (at 25 ℃) + Frequency Stability Over Operating Temperature Range + Aging x Lifetime of the device

In the case of 8Y-32.000MAAV-T this sums up to:

(+/- 30ppm) + (+/-30ppm) + N years x +/- 3ppm = > +/-40ppm

So the crystal you suggested cannot be used with the nRF52832 if you intend on using it for BLE.

RE: Unavailability of reference design crstal

AmbystomaLabs — Mon, 07 May 2018 14:46:16 GMT

I'm sorry, but what bjorn said is incorrect. The BLE spec calls out +/- 50kHz drift and total drift and total erro of +/- 150kHz. At the top end of the band (worst case) this equates to +/- 20.1 ppm drift and +/- 60.4ppm total tolerance.

While your crystal would barely satisfy the total tolerance spec it does not satisfy the drift spec. Also, it would be a poor choice to choose a crystal where the total tolerance spec were barely met, as manufacturing variation would cause numerous out of spec devices due to load cap and pcb trace tolerances.

Also if you choose to use 802.15.4, I should point out that 802.15.4 has an even tighter tolerance for total error of only +/- 40 ppm.

For these reasons you should only choose crystals with 20ppm or better temp tolerance, and 20ppm or better initial offset.

RE: Unavailability of reference design crstal

bjorn-spockeli — Mon, 07 May 2018 13:01:26 GMT

Navdeep The crystal you have suggested has an accuracy of +/- 30ppm, which is within the +/-40ppm requirement for BLE stated in the nRF52832 Product Specification, here is the link to the Infocenter page. The shunt capacitance(C0) of the suggested crystal is 3pF and the ESR is 80Ohms, which also is within the 100Ohm Equivalent series resistance limit with C0 = 3 pF.

In short, this crystal looks good.

Bjørn