Phase locked loop control of turboprop aircraft engines for cancellation of interior or exterior noise

Interior and exterior noise generated by multi-engine, turboprop aircraft remains a serious problem. In this paper, two phase locked loop schemes are used for engine synchro-phasing to effect noise cancellation in a twin engine OV-10A aircraft. For interior noise, a sampled microphone technique serves as the phase detector. For exterior noise, a digital timer/counter board monitoring engine pulse tachometers is used. Both schemes use high bandwidth, inner frequency lock loops to improve capture range.