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LMX2571: Is there any guideline for configure R, N and num, den when use LMX2571&2572?

Harry Ma
Intellectual 2700 points
Part Number: LMX2571
Other Parts Discussed in Thread: LMX2572

Hi Team

1. 

when we wanna get a output frequency under a constant reference frequency, there are many configuration options of R, N, num, den.

Example: Ref=20M, Output=400M. we can set R=2, N=10 or R=1, N=20 and so on.  and for Num and Den, if we wanna get 0.1, we can configure them as 1/10 or 10/100 or 20/200  and so on.

So Is there any guideline for configure R, N and num, den when customer use LMX2571&2572?  which is the best choice?

  • 0
    TI__Guru** 110880 points

    Hi Harry,

    It depends, general speaking, we may want to keep N-divider smaller so as to reduce PLL noise. Fractional denominator may affect spurs performance and frequency resolution. In general, a larger DEN might return smaller spurs but higher phase noise. 

    A good design needs to compromise phase noise, switching time and spurs. Suggest use PLL Sim to do the design and simulation.

  • 0 in reply to Noel Fung
    TI__Intellectual 2700 points

    Hi Noel

    Thanks for your support. there are two question hope you can support when we customer evaluate LMX2572.

    1. for the fraction configuration. what are these functions of 'use large fraction' and  'simplify fraction'? which one customer should choose when configure fraction?

    2. the estimated current is below than the tested result when customer test the consumption current, so customer wanna know the precision of the this estimated result?

  • 0 in reply to Harry Ma
    TI__Guru** 110880 points

    Hi Harry,

    The two buttons are used to either simplify the fraction (e.g. 500/1000 --> 1/2) or use the largest possible fraction (e.g. 500/1000 --> 1048576000/2097152001) for the same VCO frequency. These buttons provide a quick way for us to check spurs performance with two very different fraction DEN. They are not used to suggest the optimum DEN.

    The estimated current precision is low, it is just for reference. 

  • 0 in reply to Noel Fung
    TI__Intellectual 2700 points

    Hi Noel 

    Thanks!