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lmx2531_

sheetal verma1
Intellectual 600 points
Other Parts Discussed in Thread: LMX2531

  wanted to design "Loop filter using lmx2531 , following are queries

1)Can we calculate the values without design tool?

2)Resistors R3_ADJ, R4_ADJ and capacitors  C3_ADJ,C3_ADJ are not shown in block diagram of LMX2531?

3)What happens in Fastlock? does it grounds any external resistor shown in the block diagram of datasheet of LMX2531?

4. Output of N divider of LMX2531 is a sine wave or square wave?

5. Which crystal oscillator is preferred (sine wave or square wave or clipped sine wave) for connecting at OSCin pin of LMX2531?

 

  • 0
    TI__Mastermind 28001 points

    Verma,

    1.  The calculation on the comonents without the tool is complicated, but the math is fully outlined in my PLL book "PLL Performance, Simulation, and Design" at http://www.ti.com/tool/pll_book

    2.   On the block diagram on page 2, on the Vtune pin, we see 2 resistors and 2 capacitors with arrows.  These are R3_ADJ, R4_ADJ, C3_ADJ, and C4_ADJ.

    3.   During fastlock, the following happens:  a.  Fastlock charge pump current is engaged.   b. Fastlock output pin is grounded, which grounds a resistor in parallel with R2.  c.  R3_ADJ and R4_ADJ get adjusted to the fastlock values specified in the datashee.

    4.   The output of the N divider is a square wave, not 50% duty cycle though.  But this is internal to teh chip.

    5.   Clipped sine wae is preferred.

     

    Regads,

    Dean

  • 0 in reply to Dean Banerjee
    Intellectual 600 points

    Dear Dean,

    thanks for reply.But their are few other queries.

    1.R3_ADJ is in the Charge pump side or VCO side.?

    3. During fastlock, Fastlock charge pump current is engaged. We noticed that value of Fastlock charge pump current and normal charge pump current is same.?

    4 Is it possible to throw some light on reason for preference of clipped sine wave over sine wave or square wave?

  • 0 in reply to sheetal verma1
    TI__Mastermind 28001 points

    Sheetal,

    1.  R3 and the programmable loop filter components are all on the VCO side on the Vtune Pin.

    3.   Fastlock does not come for free.   To make fastlock effective, you need to switch in a higher current during fastlock, which means that you can't use it for steady state.  But the part has the best PLL phase noise for the highest charge pump current.   Therefore, we often default our software not to use fastlock because we want the best PLL noise.  

    4.   For the best PLL noise, we want a fast slew rate, so actually square wave is best for this and sine wave is the worst.   Square wave is probably the better than clipped sine wave.  We say clipped sine wave a lot because a lot of TCXOs are this way.

    Regards,

    Dean