1.软件版本

matlab2013b,quartusii121.

2.本算法理论知识

       带通滤波器在数字幅频均衡功率放大器中一个重要的组成部分，在介绍带通滤波器之前，我们首先来详细介绍一下数字幅频均衡功率放大器。

     本系统要求的指标为：

      本题要求在输入电压有效值为5mV的条件下，放大倍数达到400倍。而且20Hz到20kHz衰减不能超过1dB。-1 dB转化为信号幅值变化为11%，可以说指标要求很高。我们可以选择使用PGA或AD620实现这一指标。

     整个系统的基本结构如下所示：

图1 系统总提结构框图

       根据以上分析，系统的整体框图如图1所示。输入信号首先通过前置放大电路放大到一定幅度，经过带阻网络后，信号的幅频特性发生变化。由于AD输入幅度限制，信号先经过衰减网络衰减两倍，再经过抗混叠滤波并使用AD对输入信号进行采样，将采样结果送入FPGA做幅频均衡。最后通过DA输出并滤波，经过D类功放后即可得到大功率信号。

·前置放大电路的设计

       输入信号电压放大倍数不小于400倍，单级运放的放大倍数难以做到这么高，所以采用两级运放的形式来做前级放大。由于系统频率为20Hz~20kHz，所以选用OP27运放完成电路。

·抗混叠滤波器电路设计

       根据采样定理，为了使采样信号不发生频域混叠，必须在A/D采样电路的前端加入抗混叠滤波器电路，滤波器截止频率为采样频率的一半。由于本系统主要处理20kHz以内的信号，所以选用开关电容滤波器LTC1068—25设计一个八阶椭圆滤波器，其截至频率为25kHz。

·A/D采样电路的设计

       根据题目的指标及系统频率的要求，我们需要一款采样率超过40KHz的采样芯片。AD9223是一款12bits、最高采样频率10MHz的性能优良的AD采样器件，由于以前使用过该芯片，为了更快的完成题目，所以选用AD9223作为采样芯片。

·数字幅频均衡模块设计

       数字幅频均衡模块的原理图如图4-3所示，如果要实现对带阻网络的完全补偿，那么FIR滤波器应与带阻网络互为逆系统.带阻网络的系统函数可以通过点频法测得，然后使用MATLAB求出加窗后FIR滤波器应该具有的单位脉冲响应。因为FIR系统具有线性相位特性，所以由其幅频响应就可以求得其系统函数。

图2 数字幅频均衡模块的原理图

·D/A输出电路设计

      根据题目的指标及系统频率的要求，我们需要一款频率超过40KHz的模数输出芯片。DAC904是一款14bits、最高采样频率165MHz的的DA器件，由于以前使用过该芯片，所以仍选用DAC904作为数模输出芯片。

·功率放大电路设计

       D类功放第一部分为调制器，输入信号接比较器的正输入端，与三角波相比较。当正端上的电位高于负端三角波电位时，比较器输出为高电平，反之则输出低电平。这样，比较器输出的波形就是一个脉冲宽度被音频信号幅度调制后的波形，称为SPWM波。D类功放后级输出电路是一个脉冲控制的大电流开关放大器，正半周期比较器输出高电平，MOSFET晶体管Q1导通，且Q2截止，负半周期比较器输出高电平Q2导通，且Q1截止，这样它就把比较器输出的PWM信号变成高电压、大电流的大功率PWM信号，最后只需要通过一个二阶低通滤波器就可以把声音信息还原出来。

 

 

3.部分源码

library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
use std.textio.all;
entity tb_firs is
--START MEGAWIZARD INSERT CONSTANTS
  constant FIR_INPUT_FILE_c  : string := "firs_input.txt";
  constant FIR_OUTPUT_FILE_c : string := "firs_output.txt";
  constant NUM_OF_CHANNELS_c        : natural := 1;
  constant DATA_WIDTH_c             : natural := 16;
  constant CHANNEL_OUT_WIDTH_c      : natural := 0;
  constant OUT_WIDTH_c              : natural := 35;
  constant COEF_SET_ADDRESS_WIDTH_c : natural := 0;
  constant COEF_RELOAD_BIT_WIDTH_c  : natural := 35;
 
--END MEGAWIZARD INSERT CONSTANTS
end entity tb_firs;
 
 
--library work;
--library auk_dspip_lib;
 
-------------------------------------------------------------------------------
 
architecture rtl of tb_firs is
  
  signal ast_sink_data    : std_logic_vector (DATA_WIDTH_c-1 downto 0) := (others => '0');
  signal ast_source_data  : std_logic_vector (OUT_WIDTH_c-1 downto 0);
  signal ast_sink_error   : std_logic_vector (1 downto 0)              := (others => '0');
  signal ast_source_error : std_logic_vector (1 downto 0);
  signal ast_sink_valid   : std_logic                                  := '0';
  signal ast_source_valid : std_logic;
  signal ast_source_ready : std_logic                                  := '0';
  signal clk            : std_logic := '0';
  signal reset_testbench        : std_logic := '0';
  signal reset_design   : std_logic;
  signal eof            : std_logic;
  signal ast_sink_ready : std_logic;
  signal start : std_logic;
  signal cnt   : natural range 0 to NUM_OF_CHANNELS_c;
  constant tclk           : time := 10 ns;
  constant time_lapse_max : time := 60 us;
  signal time_lapse       : time;
  function div_ceil(a : natural; b : natural) return natural is
    variable res : natural := a/b;
  begin
    if res*b /= a then
      res := res +1;
    end if;
    return res;
  end div_ceil;
 
  function to_hex (value : in signed) return string is
    constant ne     : integer        := (value'length+3)/4;
    constant NUS    : string(2 to 1) := (others => ' ');  
    variable pad    : std_logic_vector(0 to (ne*4 - value'length) - 1);
    variable ivalue : std_logic_vector(0 to ne*4 - 1);
    variable result : string(1 to ne);
    variable quad   : std_logic_vector(0 to 3);
  begin
    if value'length < 1 then
      return NUS;
    else
      if value (value'left) = 'Z' then
        pad := (others => 'Z');
      else
        pad := (others => value(value'high));             
      end if;
      ivalue := pad & std_logic_vector (value);
      for i in 0 to ne-1 loop
        quad := To_X01Z(ivalue(4*i to 4*i+3));
        case quad is
          when x"0"   => result(i+1) := '0';
          when x"1"   => result(i+1) := '1';
          when x"2"   => result(i+1) := '2';
          when x"3"   => result(i+1) := '3';
          when x"4"   => result(i+1) := '4';
          when x"5"   => result(i+1) := '5';
          when x"6"   => result(i+1) := '6';
          when x"7"   => result(i+1) := '7';
          when x"8"   => result(i+1) := '8';
          when x"9"   => result(i+1) := '9';
          when x"A"   => result(i+1) := 'A';
          when x"B"   => result(i+1) := 'B';
          when x"C"   => result(i+1) := 'C';
          when x"D"   => result(i+1) := 'D';
          when x"E"   => result(i+1) := 'E';
          when x"F"   => result(i+1) := 'F';
          when "ZZZZ" => result(i+1) := 'Z';
          when others => result(i+1) := 'X';
        end case;
      end loop;
      return result;
    end if;
  end function to_hex;  
begin
  
  DUT : entity work.firs
    port map (
      clk                => clk,
      reset_n            => reset_design,
      ast_sink_ready     => ast_sink_ready,
      ast_sink_data      => ast_sink_data,
      ast_source_data    => ast_source_data,
      ast_sink_valid     => ast_sink_valid,
      ast_source_valid   => ast_source_valid,
      ast_source_ready   => ast_source_ready,
      ast_sink_error   => ast_sink_error,
      ast_source_error => ast_source_error);
  -- for example purposes, the ready signal is always asserted.
  ast_source_ready <= '1';
 
  -- no input error
  ast_sink_error <= (others => '0');
 
  -- start valid for first cycle to indicate that the file reading should start.
  start_p : process (clk, reset_testbench)
  begin
    if reset_testbench = '0' then
      start <= '1';
    elsif rising_edge(clk) then
      if ast_sink_valid = '1' and ast_sink_ready = '1' then
        start <= '0';
      end if;
    end if;
  end process start_p;
  -----------------------------------------------------------------------------------------------
  -- Read input data from file                                                                 
  -----------------------------------------------------------------------------------------------
  source_model : process(clk) is
    file in_file     : text open read_mode is FIR_INPUT_FILE_c;
    variable data_in : integer;
    variable indata  : line;
  begin
    if rising_edge(clk) then
      if(reset_testbench = '0') then
        ast_sink_data  <= std_logic_vector(to_signed(0, DATA_WIDTH_c)) after tclk/4;
        ast_sink_valid <= '0' after tclk/4;
        eof            <= '0';
      else
        if not endfile(in_file) and (eof = '0') then
          eof <= '0';
          if((ast_sink_valid = '1' and ast_sink_ready = '1') or
             (start = '1'and not (ast_sink_valid = '1' and ast_sink_ready = '0'))) then
            readline(in_file, indata);
            read(indata, data_in);
            ast_sink_valid <= '1' after tclk/4;
            ast_sink_data  <= std_logic_vector(to_signed(data_in, DATA_WIDTH_c)) after tclk/4;
          else
            ast_sink_valid <= '1' after tclk/4;
            ast_sink_data  <= ast_sink_data after tclk/4;
          end if;
        else
          eof            <= '1';
          ast_sink_valid <= '0' after tclk/4;
          ast_sink_data  <= std_logic_vector(to_signed(0, DATA_WIDTH_c)) after tclk/4;
        end if;
      end if;
    end if;
  end process source_model;
  ---------------------------------------------------------------------------------------------
  -- Write FIR output to file                                               
  ---------------------------------------------------------------------------------------------
 
  sink_model : process(clk) is
    file ro_file   : text open write_mode is FIR_OUTPUT_FILE_c;
    variable rdata : line;
    variable data_r : string(div_ceil(OUT_WIDTH_c,4) downto 1);
  begin
    if rising_edge(clk) then
      if(ast_source_valid = '1' and ast_source_ready = '1') then
        -- report as hex representation of integer.
        data_r := to_hex(signed(ast_source_data));
        write(rdata, data_r);
        writeline(ro_file, rdata);
      end if;
    end if;
  end process sink_model;
-------------------------------------------------------------------------------
-- clock generator
-------------------------------------------------------------------------------      
  clkgen : process
  begin  -- process clkgen
    if eof = '1' then
      clk <= '0';
      assert FALSE
        report "NOTE: Stimuli ended" severity note;
      wait;
    elsif time_lapse >= time_lapse_max then
      clk <= '0';
      assert FALSE
        report "ERROR: Reached time_lapse_max without activity, probably simulation is stuck!" severity Error;
      wait;      
    else
      clk <= '0';
      wait for tclk/2;
      clk <= '1';
      wait for tclk/2;
    end if;
  end process clkgen;
 
  monitor_toggling_activity : process(clk, reset_testbench,
                                      ast_source_data, ast_source_valid)
  begin
    if reset_testbench = '0' then
      time_lapse <= 0 ns;
    elsif ast_source_data'event or ast_source_valid'event then
      time_lapse <= 0 ns;
    elsif rising_edge(clk) then
      if time_lapse < time_lapse_max then
        time_lapse <= time_lapse + tclk;
      end if;
    end if;
  end process monitor_toggling_activity;
 
 
-------------------------------------------------------------------------------
-- reset generator
-------------------------------------------------------------------------------
  reset_testbench_gen : process
  begin  -- process resetgen
    reset_testbench <= '1';
    wait for tclk/4;
    reset_testbench <= '0';
    wait for tclk*2;
    reset_testbench <= '1';
    wait;
  end process reset_testbench_gen;
  reset_design_gen : process
  begin  -- process resetgen
    reset_design <= '1';
    wait for tclk/4;
    reset_design <= '0'; 
    wait for tclk*2;
    reset_design <= '1';
    wait for tclk*80;
    reset_design <= '1';
    wait for tclk*128*2;
    reset_design <= '1';
    wait;
  end process reset_design_gen;
 
-------------------------------------------------------------------------------
-- control signals
-------------------------------------------------------------------------------
 
end architecture rtl;

4.仿真结论

我们在MATLAB中对该IIR滤波器进行验证。代码如下所示：

b=[1 -1.8986 08991];

a=[2 -3.192 1.193];

[h1,f1]=freqz(b,a,100,1000000);

plot(f1,20*log10(abs(h1)));

其仿真波形如下所示：

图3 IIR滤波器的特性曲线

从上图可以看到，其通带是从0开始的，虽然在整个通带范围20~20k中20hz误差很小，但是IIR由于本身的缺陷，并不能满足要求。

 

·基于FIR的方案验证

    其代码如下所示：

fs=200000;

wn1=[0.02 0.2];

b = fir1(1024,wn1,'DC-0');

freqz(b,1,1024,fs);axis([0,30000,-100,30]);grid;

title('设计的FIR带通滤波器');

其仿真结果如下所示：

图4 带通FIR滤波器仿真图

这里由于20hz的起始带通频率非常低，为了能使仿真效果能够明显点，这里通带频率为2K~20K。在实际使用的时候：

图5 带通FIR滤波器仿真图

由此可见，采用FIR滤波器可以达到设计要求。

其仿真结果如下所示：

5.参考文献

[01]Mark Zwolinski.VHDL数字系统设计[M].电子工业出版社.2004

[02]褚振勇,翁木云.FPGA设计及应用.西安电子科技大学出版社[M].2002

[03]李玉山,来新泉.电子系统集成设计技术[M].电子工业出版社.2002.A25-15