TX Carrier frequency offset

[Frederic]

We are in process to certified a product using the nrf52832. While testing the certification team find an offset greater than 20 ppm, like in the following scree shot:

(2 402 058 [Hz] - 2 402 000 [Hz]) / 2 402 000 [Hz] = 24.1 e-6

12pF capacitors are used on a 32MHz crystal with load capacitance of 10pF. The 32 MHz crystal frequency tolerance is +/-10ppm at +25°C, +/-20ppm between -20°C and +70°C.

Is this level of accuracy is possible with the nrf52832?

Thank you for your help!

[Jay Tyzzer]

Hi Frederic,  Have you tried using either a 32MHz crystal with a CL value of 8pF or.. the current Crystal but with 15pF load caps?   

Load cap value = (CL*2) - (PinCap + Stray Cap)  So figure (10*2) - (4+1) = 15.   

Pin capacitance is 4pF and maybe 1pF for stray capacitance. 

Which ever is easier to change out...

[Frederic]

Problem is that I'm using a third-party mini-PCB so I don't really have access to change part but I think I will have to try. But is such level of accuracy of the carrier is possible?

[Andreas]

Hi,

 

This level of inaccuracy is very possible if the crystal is not correctly loaded. As Jay says, increasing the load capacitors to 15pF should bring it closer to center.

Either way, 24ppm is well inside the 40ppm BLE requirement with 10ppm tolerance from the crystal, 20 ppm will be outside though. nRF52 still works well outside this, but it will not be compliant and there is no guarantee the other end of the link will work with higher offsets.

 

Best regards,

Andreas

[Frederic]

I'm trying to find a link to the 40pm BLE requirement but I find only requirement about the offset with modulation but nothing with single-carrier. Could you share a link?

Thank again!

[Andreas]

Hi,

 

It's in the Bluetooth Core Spec, Vol 6, Part A, but it seems I was slightly incorrect. The BLE spec is 50ppm, 40ppm is the IC spec to have some margin for drift.

 

Best regards,

Andreas

[Frederic]

In the 19.4.2 64 MHz crystal oscillator (HFXO) table in the nrf52832 datasheet what exactly the C0 (7,5,3 pF) in the RS_HFXO_7PF, RS_HFXO_5PF, RS_HFXO_3PF values means? Is it the load capacitance of the crystal?