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LMX2594: Problem using Channel Divider with division rations greater than 2

Richard Mountain51
Prodigy 70 points
Part Number: LMX2594


I have an custom board with several LMX2594's on it using a 100MHz reference and PFD, in integer mode, N=80, with a VCO frequency of 8000MHz, with RFOUT_A and RFOUT_B outputs configured for an output of 4000MHz,  Register 75 / 0x4B = 0x0800 and register 0x1F = 0x03EC (or 0x43EC). this is stable and works fine!

From the manual, and the TICS Pro programs output, If I want to change the output frequency to 2000MHz, it looks like all I need to do is change the CHDIV value and possibly  set the CHDIV_DIV2 bit?

When I do this, Register 75 / 0x4B = 0x0880, and register 31 / 0x1F = 0x43EC,I expect a 2000MHz output, but the output stays at 4000MHz,

I am using a custom board, so I can’t directly use TICS to talk to the board but my initial register load came straight out of TICS, and I am programming all 113 registers as recommended. (and the Same as TICS outputs) This happens the same on power-up as it does reloading all, or the two registers on a powered part.

Am I missing a register somewhere? To me it looks like the part won’t run with both outputs with a divider order greater than 2?

Respectfully

Rich

  • 0
    TI__Guru** 109830 points
    Hi Rich,

    You should be able to get 2GHz output by simply changing CHDIV to divide by 4. That is, change register R75 to 0x4B 0840. Register 31 is fine.
    Let me try to understand your test procedure:
    1. after initial Vcc power up, you can get it locked and output 4GHz
    2. now you want to change the output to 2GHz
    3. you program R75 (and R31), but the output remain 4GHz?

    If this is the case, I suggest you use a scope to probe the waveforms in Step3. If the board can do register readback, we can read back to confirm whether R75 is in fact updated or not.