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LMX2820: LMX2820 RFin external feedback mode

?? ?
Prodigy 40 points
Part Number: LMX2820

Tool/software:

HELLO

When using the LMX2820 RFin external feedback mode, if the same frequency is repeatedly sent, the hardware remains unchanged, the data sent by the software is consistent and the charge pump current is not altered, the loop bandwidth will change. Could you please tell me what might be the possible reasons for this situation?

Also, apart from the charge pump current and the loop filter, what other conditions might affect the loop bandwidth of the LMX2820?

使用LMX2820 RFin外部反馈模式时，重复发同一个频率，硬件没改变，软件发的数据一致且未改变电荷泵电流的情况下，环路带宽会发生变化。请问一下造成这种情况可能的原因是什么呢？
还有LMX2820的环路带宽除了电荷泵电流，环路滤波器之外，还有哪些情况可能会影响环路带宽呢？

  • 0
    TI__Guru** 107660 points

    Hi There,

    When you re-program the device, did you also program R0 (with FCAL_EN = 1) to calibrate the VCO? If this is the case, can you try again without programming R0 or program R0 with FCAL_EN = 0?

  • 0 in reply to Noel Fung
    Prodigy 40 points

    Hello, our experiment has shown that changing the status of Loopback_EN (i.e., when Loopback_EN changes from 0 to 1) will cause a change in the loopback bandwidth. In fact, the loopback bandwidth will randomly enter one of several states. Is there any way to avoid this phenomenon?

  • 0 in reply to ?? ?
    TI__Guru** 107660 points

    Hi There,

    Loopback and normal mode use different N-divider value, loop bandwidth is therefore different. 

    You mentioned there are several loop bandwidths, how many different loop bandwidth you've got? Do you have a band pass filter in front of RFin?

  • 0 in reply to Noel Fung
    Prodigy 40 points

    Hello,
       There are approximately four or five different states for the loop bandwidth. There was an acoustic filter before the RFin.
    In the experimental process, taking 8431M as an example: LMX2820 will first use the normal mode to lock onto 8431M, then switch to the LOOPBACK_EN mode, and again lock onto 8431M. During this process, the N value will change.


        Repeat the above steps: LMX2820 will switch from the LOOPBACK mode to the normal mode, and then switch back to the LOOPBACK mode. At this time, the loop bandwidth is inconsistent with the previous state.


        The following figure shows the black curve and the blue curve, which represent the loop bandwidth when sending 8431M twice. From the figure, it can be seen that the loop bandwidth has changed.


        According to what you said, does the change in the N value affect the loop bandwidth? Is this effect uncertain? Will it cause the loop bandwidth to enter a random state? If only using the normal mode, the N value will also change when switching frequencies, and the loop bandwidth will change, but this change is certain.

        If the N value changes but the output frequency remains unchanged, will the loop bandwidth also change at this time?