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LMX2592: LMX2592 for VCO frequency question and the minimum step question

Mickey Zhang
Genius 12080 points
Part Number: LMX2592


Hi team,

The customer has two questions for LMX2592. The customer's input frequency is 100MHZ. 

The output frequency is from 400MHZ to 1500MHZ. 

Q1. The customer would like to get how to select VCO frequency to achieve his requirement. 

Can PLLatinum Sim software be achieved?

Q2. The customer needs to output 400MHZ first, then he would like to increase 1KHZ each time, finally, the output frequency can 

be reached to 1500MHZ. E.g. First, 400MHZ. Second, 400MHZ+1KHZ=400.0001MHZ. Third, 400.001MHZ+1KHZ. And so on.

The customer would like to get the  minimum output frequency step.  

Q3. I note the datasheet show this information on page 15. I shows Minimum output frequency step = Fpd × PLL_N_PRE / PLL_DEN / [Channel

divider value]. Is this formula that the customer's minimum output frequency step? 

If this is true, I would like to get how to determine the Fpd, PLL_N_PRE and  PLL_DEN values?

Best Wishes,
Mickey Zhang
Asia Customer Support Center
Texas Instruments

  • 0
    TI__Expert 5370 points

    Dear Mickey,

    answer #1: The synthesizer works with a calibration for each frequency selected and hence will NOT select a VCO directly. All configuration are selected such N , Fraction(num/den), etc. and then will trigge a calibration by writing into register R0. Pllatinum Sim can be used to simulate the performance of the synthesizer. I would recommend to use TiCS PRO to see and set actual register configuration.

    answer #2: The minimum step is mainly function of the denominator size selected. The minimum frequency step  is the smallest fraction increment of the phase detector frequency at the VCO hence divided by the output divider  when referenced to the output.

    answer#3: the equation in the Datasheet is correct. I would also recommend to use TiCS PRO to explore the automatic calculator included in the "PLL" interface.

    Please find below a register accurate setting to produce the 1 kHz resolution from 400 MHz. As you move up in frequency, you will change the output divider as required and recalculate the required fraction for the 1 kHz step.

    I hope this helps,

    Regards, Simon.