subroutine twkfreq_xy(c4aa,c4bb,n5,a)

  complex c4aa(n5)
  complex c4bb(n5)
  real a(5)
  complex w,wstep
  data twopi/6.283185307/

! Apply AFC corrections to the c4aa and c4bb data
  w=1.0
  wstep=1.0
  x0=0.5*(n5+1)
  s=2.0/n5
  do i=1,n5
     x=s*(i-x0)
     if(mod(i,1000).eq.1) then
        p2=1.5*x*x - 0.5
!            p3=2.5*(x**3) - 1.5*x
!            p4=4.375*(x**4) - 3.75*(x**2) + 0.375
        dphi=(a(1) + x*a(2) + p2*a(3)) * (twopi/1378.125)
        wstep=cmplx(cos(dphi),sin(dphi))
     endif
     w=w*wstep
     c4aa(i)=w*c4aa(i)
     c4bb(i)=w*c4bb(i)
  enddo

  return
end subroutine twkfreq_xy