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CDCE62005 clock outputs

maril
Intellectual 560 points
Other Parts Discussed in Thread: CDCE62005, LMK03806, CDCM6208

Dear All ,

I am using CDCE62005 chip in my custom board. i would liek to generate the following clock outputs . All are LVDS differential signals . I am using a 25 MHz crystal. The PRI_REF and SEC_REF signals are unconnected.

Output 0 - 100 MHz

Output 1 - 312.5 MHz

Output 2 - 312.5 MHz

Output 3- 125 MHz

Output 4 - 312.5 MHz

Can CDCE62005 generate the above set of outputs ? I tried to use the CDCE62005 EVM controller software and figured out that this is possible . However , the VCO is going OFF limits with this combination (shown in the fig below). Can anyone suggest an alternative scheme of generating the above set of clocks ?

 -Anil

  • 0
    TI__Expert 4150 points

    Hi Anil,

    please use the Frequency planner tool under the GUI tools menu. It will tell you if a certain frequency plan is possible or not.

    The least common multiplier for 312.5MHz, 125MHz, and 100MHz is 2500MHz, which is outside the CDCE62005 VCO frequency range. Practically, even 2500MHz would not work, because the pre-divider is limited to 2, 3, 4, and 5. Therefore, it is not possible to generate all 3 frequencies from the CDCE62005 simultaneously. In your solution found above, you basically show the VCO running at 5000MHz, which would overcome the pre-divider limitation but at the expense of exceeding the VCO maximum frequency.

    You could consider the following alternative approaches:

    1. can you change your 100MHz target frequency to 25MHz, or 156.25MHz? (sometimes processors allow different configurations) and your 125MHz to 62.5MHz?

    2. Use an alternative device, either the 9x9mm LMK03806 (released on the TI web) or if you need a lower power and smaller footprint solution, use the future CDCM6208 (product not on TI web yet) in 7x7mm package, samples in February, RTP 2Q12.

    By the way, to generate 312.5MHz, the best possible oscillator frequency for the CDCE62005 would be 26.04166666666MHz as in this case the PFD frequency can run ~4x faster than with a 25MHz XTAL.

  • 0 in reply to Fritz
    Intellectual 560 points

    HI Fritz ,

    Thanks for the info.

    -Anil