It is possible to read back the measured frequency offset between the transmitted packet and the receivers own center frequency on each received packet on EZRadio devices (Si4455, Si4355). This measurement gives insight into the RF frequency offset of the Tx node with regards to the Rx node.

This measured number is only accessible if AFC (Automatic Frequency Control) compensation is enabled through the PLL synthesizer. Now, even though this feature cannot be configured explicitly from WDS it will still get enabled behind the scenes if „Std. 1010 pattern (>= 40 bits)” is selected as the preamble pattern the receiver is expecting. See below WDS screenshot with the help text popped open on preamble pattern selection.

The measured frequency offset gets frozen at preamble detection and will get latched at sync word detection on each packet. This practically means the error values should be read back after sync word detection for a valid result.

The frequency offset can be read back with the GET_MODEM_STATUS API command whose 7th (MSB) and 8th (LSB) returned bytes contain the value in twos complement format. Refer to the API document for more details on the GET_MODEM_STATUS command.

The unit of the value is the frequency resolution of the PLL synthesizer. This resolution changes with different frequency bands. Below table summarizes the resolution in all the possible bands for the various IC revisions. 

The resolution numbers are valid for a 30 MHz reference source. If other frequency reference sources are used the resolution numbers must be scaled with them. I.e., if a 26 MHz reference source is used all the numbers must be multiplied by 26/30.

*Note that at the time of writing this article these resolution numbers are wrongly stated as 2x worse in the data sheets.

For the same feature on EZRadioPro (Si4460/61/63/64/67/68) devices you can read in AN734.