Reducing Phase-Measurement Errors due to RF-Source Band Breaks

A signal source can introduce phase-measurement errors when its output crosses through internal frequency-band breaks. The source phase lock circuits in this band-break region sometimes report approximate phase lock before complete phase lock occurs. The result of this approximate phase lock is a minor error in the output frequency, which can lead to phase-measurement errors at the system level. The magnitude of the phase errors depends on the amount of frequency offset and the difference in electrical lengths between the measurement system's signal and phase-reference paths. If this behavior were deterministic, then the resulting phase errors might be tolerable. Unfortunately, it was found that the final settling time (measured in many hundreds of milliseconds) was not consistent, depended in part on the two specific frequencies surrounding the band break, became more confused if a second sweep encountered the band break before the first break had settled, and of course changed behavior if the frequencies were sequenced in reverse order or measured one at a time. The design approach described herein reduced to negligible the phase-measurement errors due to frequency errors in two large multioctave test systems. The approach relies on managing range transmission line lengths so that propagation time is sufficiently equal among the various signal and reference paths. Measured data are presented that show the advantage of the optimized system design.