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Original question:

LMX2820: phase synchronization issue between two PLL

LMX2820: LMX2820: about phase synchronization between two PLL

APPLO ZHANG
Intellectual 515 points
Part Number: LMX2820

Tool/software:

Hi, 

As shown in the topology below, the LMX2820 works in Cat.3 synchronization mode. 
Whether to support the phase alignment of the output clocks of two chips, the error phase between the two outputs is fixed. 
There is no bimodal distribution like that described in the data sheet. Because the chip works in a network analyzer-like mode, 
different LMX2820s are used for reception and transmission to generate LO, which is very sensitive to phase changes.

Thanks!

  
 
  • 0
    TI__Guru** 107780 points

    Hi Applo,

    I am afraid I don't understand your question, what help do you need?

  • 0 in reply to Noel Fung
    Intellectual 515 points 

    As shown in the topology below, the LMX2820 works in Cat.3 synchronization mode.
    The output of two different LMX2820 is expressed as：

    out1 = A1*cos(w*t + θ1) =  A1*cos(w*t + Δt1*w)

    out2 = A2*cos(w*t + θ2)  = A1*cos(w*t + Δt2*w)

    It is hoped that （Δt1 - Δt2） is a fixed value with a small variance, such as a variance of 1~2ps.

    Thanks！

  • 0 in reply to APPLO ZHANG
    TI__Guru** 107780 points

    Hi Applo,

    After synchronization, the phase between the LMX2820 devices will not be exactly the same as the propagation delay is not exactly the same on every chip. 

    In addition, this delay will change with temperature. 

    The purpose of phase synchronization is to maintain a deterministic phase difference between devices, but not try to create a zero phase difference.