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LMK61E2 - Schematic assistance

Waqas Haque
Intellectual 1185 points
Other Parts Discussed in Thread: LMK61E2, CDCLVP1204

Hi,

I am using the programmable LMK61E2 oscillator to clock a device on my board. I have been looking at the EVM schematic as a guide and I wanted to know if the shunt resistors R26 and R29, and the series capacitances C24 and C25 are necessary for operation. What exactly is their purpose. Will I need them in my application?

In my application (schematic attached), the LMK61E2 clocks the onboard device. In addition, I also have a pair of SMA connectors (through 0 ohm resistors) in case I need to share the clock with another device on a second board. rx_REFP and rx_REFN are the clock inputs of my device.

Another mode of operation would be to disable the LMK61E2 and provide the clock through some test equipment.

Thanks,

  • 0
    TI__Expert 4415 points
    Hi Waqas,

    The resistors are necessary for terminating the oscillator's LVPECL output by biasing the internal transistors and cannot be omitted unless the receiver features internal termination at its input. The capacitors are used to AC couple the clock signal so that it can be interfaced with a receiver with a different common mode voltage. These can be omitted if the receiver has the same common mode voltage as the oscillator. See Table 4 on page 10 of the EVM user's manual for the different termination schemes required for the different output signalling formats (www.ti.com/.../snau188.pdf).

    Also, it's typically not recommended to connect multiple receivers to the same clock output for signal integrity reasons. It's possible but if you choose this route I recommend you simulate the design so you don't run into problems with a degraded signal or reflections later. Another approach would be to use a clock buffer with multiple outputs/inputs (for example, CDCLVP1204 has 2 universal inputs and 4 LVPECL outputs).

    Regards,
    -Tim