raja.cedt
|
1. I modulate the refclk phase at various offset frequencies (e.g. 50kHz to 20MHz) and measure the height of the resulting spurs on the spectrum analyzer. I define the height of the spur at the lowest modulation frequency as the 0dB point on the closed-loop transfer function H(s) and then generate the rest of H(s) by subtracting the height of the other spurs from this base value. This yields accurate measurement of the bandwidth (-3dB) but sometimes does not allow us to accurately measure peaking for well-overdamped PLLs (e.g. peaking < 0.5dB). The problem with peaking is that it can be difficult to accurately determine the 0dB point to within a few tenths of a dB. Note that you want to enable averaging on the spectrum analyzer to lower its noise floor(this was written by Dennis from AMD in a personal conversation with me). 2. Simple bot not accurate: measure VCO open loop phase noise and PLL closed loop noise and see where they crossing each other.
Thanks, Raj.
|