PLL lock detect pin low on touching Clock pin of synthesizer by prob

[ben.locas]

Hi
I have 2 board with same configration except first uses SPI1 of STM32F103 and second uses SPI2 and I also use USART3, I connect first board to a PLL circut (ADM4106) and it configures it and PLL is locked and so well. But with second one Nothing happen what so ever except when I touch clock pin of SPI by prob it clear its Lock detect pin but it does not locked actually. It simply clear its lock detect pin.
Any body can give me a clue of what could it go wrong?

Thanks

 

[biff44]

A lot RF chips have a very poor implementation of SPI bus on them! I had a board that I needed to put a set of buffer gates on each line right at the RF chip. SPI enable was ringing (at a 2 ns rate) and double-loading jibberish into the registers (I could only see it with a very fast MSO4104 scope). Try pull down resistors, maybe some shunt 10 pf caps, shorter SPI cable, etc. Worst case, try buffering the RF chip right next to the board.