program signal_receiver
      r=1.0
      x=5.0
      xmin=0.0;xmax=10.0;ymin=0.0;ymax=10.0;
      i_dummy=pgopen('/xwin')
      call pgenv(xmin,xmax,ymin,ymax,1,-1)
      call pgsfs(2)
 1000 open(unit=10,file='signal.txt',status='old')
      read(10,*)sig
      close(10)
      call pgbbuf
      call pgsci(0)
      call pgcirc(x,sig_old,r)
      call pgsci(1)
      call pgcirc(x,sig,r)
      call pgebuf
      sig_old=sig
      goto 1000
      end