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diff --git a/testsuite/vests/vhdl-ams/ashenden/compliant/frequency-modeling/lowpass-4.vhd b/testsuite/vests/vhdl-ams/ashenden/compliant/frequency-modeling/lowpass-4.vhd
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+
+-- Copyright (C) 2002 Morgan Kaufmann Publishers, Inc
+
+-- This file is part of VESTs (Vhdl tESTs).
+
+-- VESTs is free software; you can redistribute it and/or modify it
+-- under the terms of the GNU General Public License as published by the
+-- Free Software Foundation; either version 2 of the License, or (at
+-- your option) any later version. 
+
+-- VESTs is distributed in the hope that it will be useful, but WITHOUT
+-- ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+-- FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+-- for more details. 
+
+-- You should have received a copy of the GNU General Public License
+-- along with VESTs; if not, write to the Free Software Foundation,
+-- Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA 
+
+library ieee;  use ieee.math_real.all;
+library ieee_proposed;  use ieee_proposed.electrical_systems.all;
+
+entity lowpass is
+  generic ( fp : real := 10.0;	      -- pole in Hz for 'zoh, 'delayed
+            Fsmp : real := 10.0e3 );  -- sample frequency for 'zoh, 'delayed
+  port ( terminal input : electrical;
+         terminal output: electrical );
+end entity lowpass;
+
+----------------------------------------------------------------
+
+architecture z_minus_1 of lowpass is
+
+  quantity vin across input to electrical_ref;
+  quantity vout across iout through output to electrical_ref;
+  quantity vin_sampled : real;		  -- discrete sample of input quantity
+  quantity vin_zm1, vout_zm1 : real; 	  -- z**-1 
+  constant Tsmp : real := 1.0 / Fsmp;	  -- sample period
+  constant wp : real := fp * math_2_pi;   -- pole in rad/s
+  constant n0 : real := Tsmp * wp;        -- z0 numerator coefficient
+  constant n1 : real := Tsmp * wp;        -- z-1 numerator coefficient
+  constant d0 : real := Tsmp * wp + 2.0;  -- z0 denominator coefficient
+  constant d1 : real := Tsmp * wp - 2.0;  -- z-1 denominator coefficient
+
+begin
+
+  vin_sampled  == vin'zoh(Tsmp);
+
+  vin_zm1  == vin_sampled'delayed(Tsmp);
+
+  vout_zm1 == vout'delayed(Tsmp);
+
+  vout == vin_sampled * n0 / d0 + n1 * vin_zm1 / d0 - d1 * vout_zm1 / d0;
+
+end z_minus_1;