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!

[Frederic]

in fact it's (2 402 058 [KHz] - 2 402 000 [KHz]) / 2 402 000 [KHz] = 24.1 e-6

[Andreas]

Hi,

 

The total offset is 24.1ppm, contributing to this is both the component variation and a base offset from incorrect capacitive loading. If we assume that your specific crystal is quite accurate, near the center of the normal distribution of this part, the majority of your 24.1ppm comes from incorrect loading. You should then expect up to ~20ppm on top of this, from crystal variation, temperature, aging etc. It's impossible to get the crystal loaded such that it is spot on (for a perfect crystal), but closer than 10ppm is generally possible with E12 capacitors and gives good enough overall accuracy for most crystals.

 

Best regards,

Andreas