CC1175 PLL Unlock

Dear Sir

We are trying to set the CC1175 with the setting below.

-       Crystal = 32MHz

-       Tx frequency = 915MHz (need to +/- 5ppm)

-       Tx Power = -6dBm

-       High Performance Mode

-       No Ramping

-       Continue Tx without modulation

This is the register values generated by SmartRF Studio7.

Our MCU program flow is show below.

1. Power up;

2. MCU send out Command Strobes "SRES";

3. MCU send out the register values above;

4. MCU send out Command Strobes "STX";

The result is the TX power is correct but the carrier frequency  is not 915MHz, it is about 768MHz and drifting.

We tried other Fs value setting but the output carrier frequency still hold on about 768MHz and drifting.

The PLL is totally unlock.

The PLL is locked until the register value of SETTLING_CFG change from 03H to 0BH.

Now the carrier frequency is 915.00020MHz with SETTLING_CFG = 0BH.

My questions are:

1. Why the PLL is locked with SETTLING_CFG = 03H on the SmartRF EVB and unlocked on our PCB?

2. Why the PLL is locked with SETTLING_CFG = 0BH on our PCB?

3. Should we keep to use SETTLING_CFG = 0BH in our application?

4. Do we need to do the VCO Calibration according to the errata?

5. What is the draw back if we do not do the VCO Calibration according to the errata? Because the carrier frequency is so accuracy now.

Thanks!

Zuy Ho