Subharmonic Aliasing and Its Effects on the AFTI/F-16 Digital Flight Control System

摘要

The purpose of this research is threefold. First, determine the cause of subharmonic aliasing, described by the AFTI/F-16 engineers as 'the creation of uncorrelated low frequencies whenever a subharmonic of the sample frequency is input into the system'. Second, model the subharmonic aliasing effect, so that, by knowing only input frequency and the system sample rate the output characteristics can be calculated. And third, demonstrate by simulation the effect of input and output filters on the subharmonic alias, and the effect of signals in the subharmonic range (omega(N)/10 < omega(0) < omega(S)/2) on the interchannel difference and the software rate limiter. The model determined that subharmonic aliasing is the result of imposter frequencies (much like aliasing) being introduced into the output signal by the sampling process. We defined subharmonic aliases occur due to: imposter frequencies and a phenomena known as apparent low frequency surge, which occurs when the input frequency is nearly an integer multiple (>1) of the imposter frequency.