252 N9030B PXA Signal Analyzer Service GuideLO Synthesizer/Reference TroubleshootingA14 LO Synthesizer Theory of Operationsynthesizer output drives the LO port of a mixer. The 1st LO from the A13A1Front End drives the RF port of the mixer. The signal at the IF port, whichshould be between 33 and 55 MHz (or between 78 and 88 MHz), drives thephase/frequency detector.The fractional-N divider drives the other input of the phase/frequency detectorwhich then feeds the loop integrator which then drives the YTO Main and FMCoil Drivers.Similar to the single-loop mode, there are two types of dual-loop modes;dual-loop narrowband and dual-loop wideband. The dual-loop narrowbandmode provides lower phase noise at offsets greater than approximately 160kHz.To maintain the best span accuracy, dual loop spans between 2.5 MHz and 10MHz are actually comprised of two to four individual spans that are then“cut-and-pasted” together to yield the desired span.Manual Selection of Single versus Dual Loop OperationThe table below shows how the selections of Phase Noise Optimization, sweeptype, and span affects the selection of single-loop versus dual-loop operation.Note that this table assumes the RBW is auto coupled. If the span is <= 10 MHz(or zero span) and the RBW is manually set to >1.9 MHz, the synthesizer will beset to single-loop operation.Refer to the Phase Noise Optimization topic in the Spectrum Analyzer ModeUser's and Programmer's Reference for details on the autocoupled selections.Table 8-1Sweep Type Span(with autocoupled RBW)Phase Noise Optimization SettingBest Close-In PhaseNoise(offset < 140 kHz)Best Wide-OffsetPhase Noise(offset > 160 kHz)Fast TuningFFT All Dual-Loop,Wideband aDual-Loop,NarrowbandaSingle-Loop,WidebandSwept 0 Hz≤ 10 MHz>10 to ≤ 100 MHz Single-Loop, Narrowband> 100 MHz Single-Loop, Widebanda. Single-Loop, Narrowband if RBW is manually set to > 1.9 MHz
