Outputting the crystal oscillator directly in the Si5351

I’m using a OCXO/Si5351A kit as an external 27MHz reference for my LNBF-based 10GHz receiver. At first, I intended to use a buffer amplifier to take out directly the 27MHz cyrstal oscillator in the kit. However, I finally configured the Si5351A to generate 27MHz, as that was simpler.

Taking a look today at the documentation for the Si5351, I’ve realised that it is possible to configure the Si5351 to connect some of its outputs directly to the crystal oscillator input, acting as a buffer and bypassing all the frequency synthesis stages. To do this, XO_FANOUT_EN, which is bit 6 in register 187 “Fanout enable”, must be set to 1. The selector CLKn_SRC, which is bits 3 and 2 of clock control register (registers 16-23), is set to 00 (XTAL source) on reset, so this is already set correctly. It is probably a good idea to set CLKn_IDRV to 11 to get the highest drive strength on the output pin.


  1. Estimado Daniel, estoy tratando de sacar la frecuencia del cristal por CLOCK1 y he seguidos sus instrucciones al pié de la letra sin obtener resultados. Debo decir que también he tratado de cambiar la capacidad de carga del cristal (reg 183) y también he tenido problemas. Parecería que al tratar de escribir en los registros más altos hay dificultades. Alguna idea? Gracias!
    PD he revisado la IIC y funciona bien.

    1. Hola Hugo, no sé qué decirte. Hace bastante tiempo que no uso el Si5331A. ¿Al menos eres capaz de leer de nuevo los valores que has escrito en los registros?

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