N5511A Phase Noise Test System User’s Guide 303Evaluating Your Measurement ResultsProblem SolvingHigher noise levelThe noise level measured by the test system reflects the sum of all of the noisesources affecting the system. This includes noise sources within the system aswell as external noise sources. If the general noise level measured for yourdevice is much higher than you expected, begin evaluating each of thepotential noise sources. The following table will help you identify and evaluatemany of the potential causes of a high noise floor.Break at the upper edge of thesegment below PLL Bandwidth ≥ 4.Accuracy degradation of more than 1 or 2dB can result in a break in the graph at theinternal changeover frequency between thephase detector portion of the measurementand the voltage controlled oscillator tuneline measurement. The accuracydegradation can be caused by: Aninaccurate Tuning or Phase DetectorConstant Injection locking, or Noise near orabove the small angle line at an offset equalto the PLL Bandwidth for the measurement.Check the Parameter Summary listprovided for your results graph tosee if any accuracy degradation wasnoted. If the Tuning constant andPhase Detector constant were notmeasured by the phase detectorsystem, verify their accuracy byselecting the Measured calibrationmethod and then initiating a NewMeasurement. If you suspectinjection locking or noise above thesmall angle line, refer to theProblem Solving section of Chapter3 for specific actions.Small Break at 100 kHz,10 kHz, or 1 kHzTable 14-2 Potential causes of discontinuity in the graphCircumstance Description Recommended Action
