subroutine peakdf9(c0,npts8,nsps8,istart,foffset,idfpk)

  complex c0(0:npts8-1)
  complex zsum
  include 'jt9sync.f90'

  twopi=8.0*atan(1.0)
  df=1500.0/nsps8
  smax=0.
  do idf=-5,5
     f0=foffset + 0.1*df*idf
     dphi=twopi*f0/1500.0
     sum=0.
     do j=1,16
        i1=(ii(j)-1)*nsps8 + istart
        phi=0.
        zsum=0.
        do i=i1,i1+nsps8-1
           if(i.lt.0 .or. i.gt.npts8-1) cycle
           phi=phi + dphi
           zsum=zsum + c0(i) * cmplx(cos(phi),-sin(phi))
        enddo
        sum=sum + real(zsum)**2 + aimag(zsum)**2
     enddo
     if(sum.gt.smax) then
        idfpk=idf
        smax=sum
     endif
  enddo

  return
end subroutine peakdf9