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STW81200T

Question asked by bendelac.uri on Aug 6, 2015
Hi,

I am trying to operate the STW81200T RF PLL on my proprietary PCB. I had a first problem that I eventually managed to overcome: SPI communication with the board was not reliable. I traced it down to register 9, which according to the datasheet needs to be programmed with 0, while the power up default is 0x04000007. Since this register is used for testing I simply avoid programming it, and now I can communicate reliably with the PLL.

Now I have a new problem: the PLL won't lock, and when reading register 10, there are two related bits that indicate an error: bit 13 (REG_VCO_4V5_STARTUP) is off, which indicates a voltage ramp up problem, and bit 8 (REG_VCO_OCP) is on, indicating an over current event.
Any guidance regarding the cause of those problems?
My design uses the same values as the reference board, except that the input voltage is a regulated 3.3V (from NCP700BSN33T1G) and the LDOs are set to 2.6V, and I use a 10MHz TCXO with the frequency doubler on.

Uri

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