Crystal/crystal oscillator frequency trimming

The IEEE 802.11 specification defines the worst-case frequency offset that can be tolerated, measured in ppm.

The limit is ± 20 ppm in the 5 GHz band, and ± 25 ppm in the 2.4 GHz band. This limit ensures that demodulation is successful for all modulation schemes even when both sender and receiver exhibit worst-case and opposite offsets. This limit needs to hold across the full operating temperature range.

The offset is a combination of the selected crystal and the on-chip crystal oscillator circuit. The nRF70 Series device includes a programmable capacitor bank that can be configured to correct this frequency offset. The configuration of this capacitor bank needs to be determined using calibrated test equipment, with the resulting parameter (trim value) programmed into the One Time Programmable (OTP) memory on the nRF70 Series device.

The following table shows the ppm offsets that need to be managed.

Table 1. Ppm offsets
Offset source Ppm offset Mitigation
Crystal oscillator variation over process ± 24 ppm Production line trim
Crystal oscillator stability over temperature ± 3 ppm None
Crystal tolerance Crystal dependent

Typically ± 20 ppm

Production line trim
Crystal stability over temperature Crystal dependent

Typically ± 10 ppm

Crystal aging Crystal dependent

Typically ± 3.5 ppm

Production line trim accuracy ± 3.5 ppm None

The table shows that there are several sources that cannot be trimmed and therefore must be included in the IEEE 802.11 limit of ± 20 ppm (or ± 25 ppm if only operating in the 2.4 GHz band). These are the crystal oscillator and crystal stability over temperature, the crystal aging, and the resolution or accuracy on the trim function itself. To operate over an extended temperature range, it is necessary to trim the crystal/crystal oscillator combination. The trim function has sufficient range to trim a crystal with ± 20 ppm tolerance in addition to the ± 24 ppm crystal oscillator variation.