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LMX2531 and "Fo/LD"

Frank Dannull
Prodigy 10 points

Other Parts Discussed in Thread: LMX2531

we are programming the bits "FoLD[3:0]" of R3 of the LMX2531 with 3(dec) to get a digital lock detect.
But when looking at the functionality of the "Ftest/LD" pin, it seems that it works like an analog output that can't be used as an input signal for a microcontroller.

We are using the LMX2531LQ1515E with a 10MHz input signal to generate a 1540MHz output. These are the register contents we are programming:
R5 (INIT1)    0x840005
R5 (INIT2)    0x800005
R5    0x8007F5
R12    0x01048C
R9    0x000BA9
R8    0x028008
R7    0x000107
R6    0x08A256
R4    0x000004
R3    0x34CF43
R2    0x490012
R1    0x3E0001
R0    0x9A0000

 

generated frequency is correct.

the "LE" signal is low...

  • 0
    TI__Mastermind 28031 points

    Frank,

    I looked at your registers, but see nothing wrong and I woud expect to see a logic level high or low.   This pin can not drive much current, or turn on an LED and the logic level of this is lower than the supply voltage, around 2.7 V.   I'm wondering if something is going on with the drive level or voltage level.

    Also be aware that if there is a signal present when the device is programmed, the PLL will lock to the correct frequency and the VCO frequency will be restricted to ne of 128 different frequency bands.  If it turns out that the input was to go away, the VCO would not go to the minimum tuning range as specified in the datasheet, but rather the lowest tuning range for this band.   In this case it might be a few MHz off.   In other words, when you say this is locked on the spectrum analyzer, be sure that your span is less than 1 MHz

    Regards,

    Dean