Convert 3.3V clock into 1.2V for DSP clock

Hello Jun,

You have a couple of options. You can either buy a dedicated device to do the level shifting for you or use an external passive network to divide the voltage. If you want an easy, robust solution, I recomment to use an external level shifter:

http://focus.ti.com/paramsearch/docs/parametricsearch.tsp?family=analog&familyId=705&uiTemplateId=NODE_STRY_PGE_T

or: http://focus.ti.com/download/aap/pdf/VLT_PF_Overview.pdf

If however the jitter performance matters a lot, you might be better of using a resistor divider. If you can get the oscillator very close to the DSP, you can simply use a couple of resistor in the order of 300Ω and 700Ω minus the output impedance of the driver (which is probably between 5Ω and 50Ω). If however you have a longer transmission line, you may need to also add a series resistor right after the oscillator output to match the output to 50Ω. Let's say the output impedance was 17Ω. In this case yοur signal chain would look like the following:

XTAL out --> 33Ω --> pcb transmission line --> 650Ω --> DSP input --> 300Ω --> GND

You can of course fine-tune your resistors once you receive the boards with XTAL and DSP assembled.

Best regards, Fritz