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LMK01801 LVPECL CLOCK OUT TERMINATION CLARIFICATION

Rajesh
Expert 1010 points
Other Parts Discussed in Thread: LMK01801

Hi,

I am using LMK01801 in my design for the clock distribution purpose. I am driving the clock from LMK01801 to SY58040UMY micral 4x4 clock distribution switch.

The selected voltage level is LVPECL, in the datasheet of LMK01801 has recommended some LPECL o/p termination schemes at the same time SY58040UMY has recommended some LVPECL clock input termination schemes. Kindly suggest that which one should I follow.

Regards,

RAJESH

  • 0
    TI__Mastermind 28550 points

    Hello Rajesh,

    So the SY58040U has internal 50 ohm resistors to from each input pin, to a single Vt pin.

    (1) This allows simple implementation of a LVPECL Y-termination as shown in figure 3a.  I would recommend this termination for simplicity and minimum number of parts.

    (2) The LVPECL AC coupled termination is what we use on our evaluation boards.  This allows the use of RF test equipment (because AC coupled) and proper termination of the LVPECL output.  This termination scheme is also acceptable, but as you can see, requires more parts.  If AC coupling is not required, go with option #1.  Some systems require AC coupling to backplanes.

      - If AC coupling is used.  I would choose 120 to 240 ohm emitter resistors with our LVPECL driver (SY58040U shows 100 ohm).  120 ohms can provide higher swings but increases current consumption of the output.  240 ohm may result in a small decrease in output swing, but give good power savings.   I recommend 240 ohm emitter resistors and that is what is default stuffed on LMK01801 eval board.  Phase noise performance is the same for 120/240 except at lower (sub 100-300 MHz frequencies).

    73,

    Timothy

  • 0 in reply to Timothy T
    Expert 1010 points

    HI,

    Thanks for the support.

    Regards,

    Rajesh