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LMK04832: Sine wave input power/vppk requirement for clk inputs

Part Number: LMK04832

Hi TI,

In our design we will be using the 10MHz or 100MHz external sine wave clock as reference clock for LMK04832.

Power for this 10MHz or 100MHz clock will be -1.53dBm or 0.528 vppk at input of SMA connector on LMK PCB.

Now we are planning to use a balun to convert this single ended input clock to differential signal for connecting to LMK inputs.

We have selected mini-circuits balun part number : TC2-1TX+  https://www.minicircuits.com/pdfs/TC2-1TX+.pdf

We have also selected this balun because it is 1:2 balun so it will increase the voltage at LMK clock inputs.

Insertion loss for this balun is approx 0.5dBm @ 100MHz frequency.

So power available at secondary of balun(LMK clock input side) will be -2.03dBm (-1.53-0.5= -2.03)

And voltage available at secondary of balun(LMK clock input side) will be 1.33*0.528 = 0.807vppk.

So my question is:

1. Will this scheme of using 1:2 balun for increasing the peak to peak voltage at LMK clock inputs work?

2. Although power is decreasing at the secondary of balun because of insertion loss but voltage will be increased by 1.33 times of voltage at primary.

   So I am assuming current drawn by LMK clock inputs will be very less otherwise voltage will drop at secondary of balun since balun is increasing voltage by decreasing current at secondary?

   Is my assumption correct?

An early response will be highly appreciated.

Thanks,

Lalit

  • Hi Lalit,

    Ideally, if the input to a 1:2 balun is X dBm, then the power on each of the differential output pin will be X-3 dBm. (We cannot get more energy from a passive device!)

    In this case, you can simply configure CLKin Single-ended, 0.528Vpp meets the min. Vin requirement.

  • Hi Noel,

    As per my understanding, balun is a type of transformer and transformer decrease or increase the voltage or current but ideally power will be same at input and output of balun/transformer.

    I think X-3 dBm at output is the case for a 2 way splitter and not for balun.

    So I am assuming since clk inputs of LMK does not sink much current, the voltage will be 1.33 times of primary at the secondary of balun.

    So, Noel please correct me if I am wrong.

    Thanks,

    Lalit

  • Hi Noel,

    Please update on this.

    Thanks,

    Lalit

  • Hello Lalit,

    If you are using Balun, I would recommend AC couple to the input.  you are correct, there is some loss in balun, but you will also gain by going differential.

    You may also want to drive single ended and AC couple N to GND and compare performance. This could reduce complexity.

    Depending on the input frequency, 100MHz may have sufficient edge rate at the 500mVpp.

    regards,