310 N5511A Phase Noise Test System User’s GuideAdvanced Software FeaturesPhase-Lock-Loop SuppressionPLL suppression parametersThe following measurement parameters are displayed along with the PLLSuppression Curve.PLL gain changeThis is the amount of gain change required to fit the Theoretical LoopSuppression curve to the measured loop suppression. A PLL Gain Change ofgreater than 1 dB creates an accuracy degradation (ACCY. DEGRADED) error. Ifan accuracy degradation is detected, the amount of error is determined fromeither the PLL Gain Change or the Maximum Error, which ever is larger. Thedegradation itself is 1 dB less than the greater of these. The parameters of thetheoretical loop suppression that are modified are Peak Tune Range (basicallyopen loop gain) and Assumed Pole (for example a pole on the VCO tune portthat may cause peaking).Max ErrorMaximum Error is the largest difference between the smoothed measured loopsuppression and the adjusted theoretical loop suppression in the frequencyrange plotted for the smoothed measured loop suppression.The frequency of the assumed pole is normally much greater than the ClosedPLL BW and there is no loop peaking. If the smoothed measured PLLsuppression shows peaking, the assumed pole is shifted down in frequency tosimulate the extra phase shift that caused the peaking. If the peaking is reallydue to a single pole at a frequency near the Closed PLL BW, the adjustedtheoretical loop suppression and smoothed measured loop suppression willshow a good match and the maximum error will be small.The four points on the Loop Suppression graph marked with arrows (rangingfrom the peak down to approximately --8 dB) are the points over which theMaximum Error is determined. An error of greater than 1 dB results in anaccuracy degradation.Closed PLL bandwidthThis is the predicted Phase Lock Loop Bandwidth for the measurement. Thepredicted PLL BW is based on the predicted PTR. The Closed PLL BW will notbe adjusted as a result of an accuracy degradation. If an accuracy degradationis detected, the amount of error is determined from either the PLL Gain Changeor the Maximum Error, which ever is larger. The degradation itself is 1 dB lessthan the greater of these.
