2.7.5.4 VCO Tuning
The VCO must be calibrated before it can be used in the fractional-N PLL; otherwise, the analog control voltage may lie outside the operating range of the charge pump and the fractional-N PLL will not lock.
If at least one of the following conditions is true, a VCO tuning must be initiated:
- Changes in the frequency band (Low-Band, High-Band)
- Changes in the RF frequency that change the RF front end main and swallow the control register (FEMS) settings. (For more information, see RF Front-End Register Description.)
- Changes of the MSB of the 18-bit fractional frequency setting FFREQ (see Fractional-N PLL Register Overview).
It is generally recommended to activate the VCO tuning for any TXMode start-up.
As long as no VCO tuning process was performed after the power-on, an initialization value is used from the EEPROM. The value can be configured independently for each service in the eepServices.FEVCT.FEVCT[3:0] variables. The default value is 7.
| Address Service0 | Name | Bit 7 | Bit 6 | Bit 5 | Bit 4 | Bit 3 | Bit 2 | Bit 1 | Bit 0 |
|---|---|---|---|---|---|---|---|---|---|
| 0x00D0 | FEVCT | — | — | — | — | FEVCT[3:0] | |||
An automatic VCO tuning process can be activated at the following time points:
During the start-up of TXMode as the operation mode after the system start if enabled in eepTrxConf.sysStartConfig.VCO_TUNE.
| Address | Name | Bit 7 | Bit 6 | Bit 5 | Bit 4 | Bit 3 | Bit 2 | Bit 1 | Bit 0 |
|---|---|---|---|---|---|---|---|---|---|
| 0x0019 | sysStartConfig | RF_CAL | ANT_TUNE | VCO_TUNE | IDLEModeSelector | — | TMDEN | OPM[1:0] | |
During the start-up of TXMode via the SPI command “Set System Mode”, see Set System Mode. From IDLEMode via the SPI command “Calibrate and Check”, see Calibrate and Check.