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CC1120: RX Filter Shape: What is the true bandwidth?

cc1120user
Prodigy 100 points
Part Number: CC1120

We are having a problem with a radio system where we am experiencing link dropouts. After troubleshooting, we suspect that our signal is too close to the RX filter bandwidth.

My question: How close to the edge of the bandwidth of the RX filter do we start to experience attenuation?

Also, am I interpreting Figure 13 from the CC1120 user guide correctly? My current interpretation is that when you activate feedback to the PLL, the RX filter bandwidth takes a more rectangular shape as opposed to a more rounded shape when its turned off.

Thanks!

  • 0
    TI__Genius 10255 points

    Hi,

    This will in part depend on your crystal frequency error. SWRA122D (CC11xx Sensitivity Versus Frequency Offset and Crystal Accuracy): https://www.ti.com/lit/an/swra122d/swra122d.pdf explains how to find the needed RX BW for your application by accounting for crystal accuracy.

    Regarding how enabling feedback to the PLL increases the effective RX BW:

    • Without it enabled, increasing the RX BW will result in an increase in the noise floor and decreasing the RX BW will lower it, degrading or improving the senstivity respectively.
    • If feedback to the PLL is enabled, the RX BW can be increased whilst the noise floor will stay at the same level.
    • The result is that, for a given RX BW, the noise floor will be lower, resulting in the "wider and flatter" right hand side plot in Figure 13 (improved frequency error tolerance) and increasing the sensitivity for that RX BW setting.

    You would need to perform a PER vs Input Power Level vs Frequency Offset measurement for your given settings to determine exactly how close you could be to the edge of your RX bandwidth before the PER is too high, but as you can see from Figure 13 the PER increase can be very steep (how steep depends on your PHY).

    Regards,

    Zack