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LMX2581 - VCO Calibration Time

Maxime ATHEE
Prodigy 740 points

Other Parts Discussed in Thread: LMX2581

Dear Team,

One of my Customers is using the LMX2581 and it relates lock time of the PLL.

According to the datasheet, there are two locking time (see table on page 8 and Figure 7 page 10).
First case: "No pre-programming" where lock time is shown with a typical value of 140 microseconds.
Second case: "With pre-programming" where lock time is given with a typical value of 10 microseconds.
In the first case, we measured a good time approximating 140μs.

In the second case, we have not managed to put us in the conditions of the second case.

Is it possible to achieve the second case with the interface of the evaluation board? What is the order of the registers to be programmed?

Thanks for your support,

Maxime

  • 0
    TI__Mastermind 28031 points
    Maxime,
    To do the pre-programming, you need to give the VCO guidance on where to start.
    VCO_SEL is the starting VCO. We have a table in the datasheet applications section that gives the approximate frequency ranges for each of the 4 VCO cores.

    VCO_CAPCODE is teh frequency band within the VCO core. There are 256 bands, where band 256 is the lowest frequency.

    So the idea is that before you want to change freqencies, you program VCO_SEL to a VCO core that covers your frequency and VCO_CAPCODE to a frequency that is close to where you want to go. Then when you actually change frequencies by programming the R0 register, the VCO calibration starts at a frequncy that is closer to the intended value.

    Regards,
    Dean