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Original question:

[ LMX2582 ] Multiplier Setting and Minimum Output Frequency

LMX2582: Input reference section on the datasheet

Randhir Kalsi
Expert 4545 points
Part Number: LMX2582
Other Parts Discussed in Thread: LMX2571, LMX2572

Hello team,

Hope you are doing well and staying safe. When you get a chance please provide some feedback on below customer question:

In the LMX2582 datasheet, it states that input reference can be 5 to 1400 MHz. We used a 10MHz VCTCXO and noticed that this will not work.

There is only a doubler inline before the multiplier and the input for the multiplier is 40 MHz to 70 MHz. So, should the minimum value for input reference be 20 MHz as compared to 5 MHz?

  • 0
    TI__Guru* 81580 points

    Hi Randhir,

    This device can support 5MHz input. If needed, the doubler can be used to make fpd = 2x 5 = 10MHz. Please note when the doubler is engaged, the MULT should not be used.

    MULT = 2x is also not suggested, use the doubler instead.

    TCXO has 1Vpp voltage swing, could you tell us how it is connected to LMX2582? Were you using TICS Pro to program LMX2582 or using your own software?

  • 0 in reply to Noel Fung
    TI__Expert 4545 points

    Hello Noel,

    Thank you for the response and this helped the customer.

    Going through the Datasheet the best Phase noise is met with a 200MHz Input with our low input freq is the phase noise spec going to be greatly decreased? We are currently using Analog's ADF4351 in other products so if we can at least meet what we are getting with the analog part we would be happy. We switched to the LMX2582 for this project due to the lower output frequency, the ADF4351 we would of needed a divider on the output to get our 30MHz.

    Our issue is we are sharing this reference with another part, the most it can handle is 26MHz. So we can supply anything in the 10 to 26Mhz range.

    We are using our own software, We haven't tried anything yet our software engineer was just starting to go through the datasheet getting ready to write some software and we got confused a bit on the input reference.

    We have it pretty much the same as the Eval board with the intention to tweak it once we get it up and running with our software.

    Regards,

    Randhir

  • 0 in reply to Randhir Kalsi
    TI__Guru* 81580 points

    Hi Randhir,

    Right, 10MHz input is not an idea frequency especially when it is given with a TCXO, the slew rate of this 10MHz clock is bad.

    PLL noises are affect by the slew rate of the input clock, a square wave is perfectly good for PLL. Slew rate increases with frequency, if the clock frequency gets higher, the slew is getting higher even if it is a sine wave clock. 

    To get best phase noise from the synthesizer, I suggest use 26 MHz clock because the slew rate is better. On the other hand, we can enable the doubler to make fpd = 52 MHz, this would reduce the N-divider value and therefore returns lower PLL noise.

    Is 30MHz the only output frequency needed? If this is the case, we can stack on using the 10MHz input as this would make integer mode possible. Also suggest LMX2571 or LMX2572 for they are low power series synthesizers.

  • 0 in reply to Noel Fung
    TI__Expert 4545 points

    Noel,

    Thank you very much for the great support.