14.4.8.4 Calibration Loops

Due to variation of temperature, supply voltage and part-to-part variations of the radio transceiver the VCO characteristics may vary.

To ensure a stable operation, two automated control loops are implemented:

Center Frequency (CF) tuning
Delay Cell (DCU) calibration

Both calibration loops are initiated automatically when the PLL is enabled during state transition from TRX_OFF to PLL_ON or RX_ON state. Additionally, both calibration loops are initiated when the PLL changes to a different frequency setting.

If the PLL operates for a long time on the same channel, for example more than five minutes, or the operating temperature changes significantly, it is recommended to initiate the calibration loops manually.

Both calibration loops can be initiated manually by SPI command. To start the calibration, the device should be in state PLL_ON. The center frequency calibration can be initiated by setting the PLL_CF_START bit in the PLL_CF register to '1' (PLL_CF.PLL_CF_START=1). The calibration loop is completed when the IRQ_0 (PLL_LOCK) occurs, if enabled. The duration of the center frequency calibration loop depends on the difference between the current CF value and the final CF value. During the calibration, the CF value is incremented or decremented. Each step takes t_{PLL_CF} = 8µs. The minimum time is 8µs; the maximum time is 270µs. The recommended procedure to start the center frequency calibration is to read the register 0x1A (PLL_CF), to set the PLL_CF_START register bit to one, and to write the value back to the register.

The delay cell calibration can be initiated by setting the PLL_DCU_START bit in the PLL_DCU register (PLL_DCU.PLL_DCU_START) to '1'. The delay time of the programmable delay unit is adjusted to the correct value. The calibration works as successive approximation and is independent of the values in the PLL_DCU register. The duration of the calibration is t_{PLL_DCU} = 10µs.

During both calibration processes, no correct receive or transmit operation is possible. The recommended state for the calibration is therefore PLL_ON, but calibration is not blocked at receive or transmit states.

Both calibrations can be executed concurrently.