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LMK04826: Clock Distribution for the ground based application system

Srinadh Moka
Prodigy 185 points
Part Number: LMK04826

Tool/software:

Hi,

Here my system receiving the 10MHz (0-15dBm) reference clock from external source and by using i need to distribute as follows

2No's of 10MHz Output (40 dB isolation from output to the input port)

1No of 56MHz 

4No's 10MHz for internal systems.

My requirement is without degrading the phase noise i need to distribute the signal to the sub systems as  reference clock. And if external clock not present then i need to switch to the internal source. Even if the external signal coming with the over a wide range i need to provide the constant or fixed level to the internal sub module, is that possible. For Example, if the signal coming with low means received with 0dBm then it should accept my sub module bcz it accept only in the range of -2 to 7 dBm even if it coming with 15dBm also it should accept. Can i achieve with the LMK0482.

And if the VCO frequency of the LMK0482 is within the range of using frequency( For example, if the LMK0482 VCO frequency is 2.4G and i'm dealing with the s-band frequency does it effects the phase noise/ any spurious related problem.

  • 0
    TI__Genius 9110 points

    Hi Srinadh,

    This sounds like something the LMK04826 can achieve. Would you be able to share a clock tree or block diagram of your system?

    Thanks,

    Michael

  • 0 in reply to Michael Srinivasan
    Prodigy 185 points

    Hi Michael,

    I think LMK04826 outputs are square wave forms but i have a 4 number of sine waves. I don't have any particular block diagram but the actual thing is i have 2 inputs when ever one input is not present then i need to switch to the the other input. And the corresponding digital outputs i need to feed to the some other sub systems as sampling clock. I want to know during switching from the 2 inputs what is the state of the output does it effects the during switching.

    Suggest a suitable scheme for the clock distribution section.

  • 0 in reply to Srinadh Moka
    TI__Genius 9110 points

    Hi Srinadh,

    The switching time has not been characterized, as it is frequency and load dependent. It takes a number of VCO cycles for the signal to switch (typically around 3-5), so there will be transient effects during the switching period.

    You can enable automatic input switching whenever the signal on one input is lost. Additionally, you can also switch inputs manually, either using pin select or the registers. See section 9.3.5 for more information. 

    Something of minor concern is your sine waves, as the recommended minimum slew rate for all input clock signals is 0.5 V/ns. Could you share the driving format of your sine wave inputs (LVDS, LVCMOS, etc - or just share if they are differential or single-ended)? You can use an additional buffer to square up your inputs so that they are suitable for the inputs to the LMK0826.

    Thanks,

    Michael

  • 0 in reply to Michael Srinivasan
    Prodigy 185 points

    Hi Michael,

    Forgot about whether the LMK04826 is support for my scheme or not. My requirement is total 8 outputs required.

    Input Signal is 10MHz sine wave with (0-15dBm)

    First 2 outputs should be 3dB down to the input signal (0-15dBm) and the output signal type is Sine Wave.

    Next 2 outputs are also sine wave, one is fed for reference to synthesizer which generates 56MHz for ADC sampling clock. And another signal is given to the dual driver to convert sine wave as LVDS.

    Last 4 Outputs are given to the reference clock for synthesizer. sine wave input accepts (-2dBm to 7dBm)

  • 0 in reply to Srinadh Moka
    TI__Genius 9110 points

    Hi Srinadh,

    The LMK04826 has a 0.5 V/ns minimum requirement for input slew rate. While all of these frequencies can be buffered in and out, this device typically anticipates a set of digital inputs. I fear that your sine waves will be too slow for the device, and will lead to considerable phase noise degradation. I would suggest using an LMK1C on the input in order to square up your input sine wave. All of your outputs will also be digital signals, so you will not quite be able to output sine waves.

    One last thing I am a bit confused on. You mentioned your input is a 10 MHz sine wave with (0-15 dBm). What is the typical peak to peak voltage of this signal? 0-15 dBm implies that the amplitude varies.

    Thanks,

    Michael