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LMK04808BEVAL: Using LMK04808 in single-loop mode (PLL2, internal VCO)

HIMA SUNEETH PUTTA
Prodigy 70 points

Part Number: LMK04808BEVAL

Hello there,

We are looking to buy LMK04808B eval board to make use of the PLL2 to generate 750MHz and 1GHz LVPECL outputs. I aiming to use it in a single-loop mode (PLL2, Internal VCO mode).

I've used TICS-Pro to generate a configuration for the same using 500MHz OSCin (based on our application) - see the config file attached. My questions/concerns are following, please if you could help:

1. As I understood - PLL2's VCO output feeds back to PLL1 in Dual-loop mode. Does this still applicable in single-loop mode?LMK04808_500MHz_In_750MHz_1GHz_out_single_loop.tcs

The configuration I did - gives the warning about PLL1's VCO freq and PLL2's reference don't match

Do I need to worry about this warning -  as I want to use it in single-loop mode (PLL2 only) and don't necessarly need PLL1-lock

2. Do we need to buy USB2ANY device seperately, or Is it included in the eval kit?

  • 0
    TI__Genius 10975 points

    Hello Hima,

    HIMA SUNEETH PUTTA said:
    1. As I understood - PLL2's VCO output feeds back to PLL1 in Dual-loop mode. Does this still applicable in single-loop mode?

    That is correct, PLL2's output will loopback, go through the N-divider and be an input to the phase detector. Therefore, you do not need to worry about the warning. I looked over your configuration and it looks correct.

    HIMA SUNEETH PUTTA said:
    2. Do we need to buy USB2ANY device seperately, or Is it included in the eval kit?

    Yes, it does!

    Also, may I ask:

    1) Why you chose to use the LMK04808 and not one of our newer parts?

    2) Why you considered to use a jitter cleaner in single-loop mode and not one of our clock generators?

    3) What is the end-equipment of this part? What is this clock driving?

    Thanks in advance!

    Best,

    Andrea

  • 0 in reply to Andrea Vallenilla
    Prodigy 70 points

    Hello Andrea,

    Thanks for the quick turn-around.

    Please see my responses below:

    - It says LPT cable OR USB2ANY will be present inside the kit - If we know in advance which one exactly will be shipping, we will be able to plan better. Few months ago when we purchased LMK03806BEval boards- we received LPT cable with the kit - and we had to make a seperate purchase for USB2ANY where we lost some time Slight smile

    Reason for choosing LMK04808 is:

      -- our application requires a maximum reference frequency of 500MHz (OSCin) and CLKouts of 750MHz and 1GHz. I used TI's Clock Architect to search for the clock generators with above requirements. 

      -- Attached is the list of results I got from Clock Architect

      -- If you have a better part number that satisfies the above requirements - please do let us know. many thanks.

    Regarding our application:

       -- we are looking to use LMK04808 as a clock, for driving SerDes multi-protocol PHY

    Many thanks for the support Andrea,

    Hima Putta

    TI_parts_Ref_500MHz_Out_1GHz.pdf

  • 0 in reply to HIMA SUNEETH PUTTA
    TI__Genius 10975 points

    Hello Hima,

    HIMA SUNEETH PUTTA said:
    - It says LPT cable OR USB2ANY will be present inside the kit - If we know in advance which one exactly will be shipping, we will be able to plan better.

    It says this because the LMK04803 can be programmed with either an LPT cable or a USB2ANY; therefore, whichever one you get it should not matter. Note that the LMK04832 or the LMK0482x (ending in 21 or 28 from the document), only can use a USB2ANY, so if you choose either of those, then you will for sure get a USB2ANY.

    HIMA SUNEETH PUTTA said:
    -- If you have a better part number that satisfies the above requirements - please do let us know. many thanks.

    In general, the LMK04832 or the LMK04828 are newer most used parts so there's more (updated) information on past posts and online on these parts, so I would recommend those too. They also have better performance because of improved technology.

    HIMA SUNEETH PUTTA said:
     -- we are looking to use LMK04808 as a clock, for driving SerDes multi-protocol PHY

    Thanks!

    Best,

    Andrea

  • 0 in reply to Andrea Vallenilla
    Prodigy 70 points

    Hi Andrea,

    Just one other thing related to LVPECL AC coupling output terminations, please:

    Datasheet suggests that LVPECL receiver typically has a 2V DC bias voltage. But our reciever has ~700mV of common mode voltage (DC bias). WIll this be a problem with the LVPECL driver of the LMK04808 Eval. Could you let me know this for us, please?

    Many thanks,

    Hima

  • 0 in reply to HIMA SUNEETH PUTTA
    TI__Guru* 90900 points

    Hi Hima,

    if you receiver already has internal bias, use AC couple.

    Since your common mode is only 700mV, you should also set LMK04808 LVPECL output to 700mV or 1200mV option.