Could you please talk about the relationship between the phase noise of the reference and that of the in-band, close-loop PLL output?

Could you please talk about the relationship between the phase noise of the reference and that of the in-band, close-loop PLL output?

  • On behalf of Jeff Keip.

    The loop filter frequency is the quantity that establishes whether the input or noise from the loop elements is dominant over specific output ranges.  The loop filter essentially acts as a low pass filter for reference noise, and a high pass filter for everything in the loop.  If you have a very clean reference, you will generally want to take advantage of that by utilizing a high loop filter frequency, but if you have a noisy reference you want to clean up, you’ll want a more narrow filter.