基于FPGA的多级CIC滤波器实现四倍抽取三
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基于FPGA的多级CIC滤波器实现四倍抽取三
在《基于FPGA的多级CIC滤波器实现四倍抽取一》和《基于FPGA的多级CIC滤波器实现四倍抽取二》中我们先来了解滑动平均滤波器、微分器、积分器以及梳状滤波器原理以及它们的幅频响应。在三中我们将用verilog实现基于FPGA的多级CIC滤波器实现四倍抽取。
1 CIC滤波器的基本概述
CIC（积分梳状）滤波器是无线通信中的常用模块，一般用于数字下变频（DDC）和数字上变频（DUC）系统。CIC滤波器结构简单，只有加法器、积分器和寄存器，适合于工作在搞采样率条件下，而且CIC滤波器是一种基于零点相消的FIR滤波器，已经被证明是在高速抽取或插值系统中非常有效的单元。
2 matlab实现CIC滤波器的四倍抽取
设计目标：将载波频率352.8khz的1khz sine降采样率到88.2khz。
close all
clear all
clc
%set system parameter
fs = 1000; %The frequency of the local oscillator signal
Fs = 352800; %sampling frequency
Fs1 = 88200;
N = 24; %Quantitative bits
L = 81920;
%Generating an input signal
t =0:1/Fs:(1/Fs)*(L-1); %Generating the time series of sampling frequencies
sc1 =sin(2*pi*fs*t); %a sinusoidal input signal that produces a random starting phase
sc2 =sin(2*pi*Fs*t); %a sinusoidal input signal that produces a random starting phase
sc = sc1 +sc2;
%滑动平均滤波器
b =[1,-1];%comb
a =[1,-1];%integerator
i1 =filter(1,a,sc);
i2 =filter(1,a,i1);
i3 =filter(1,a,i2);
y = downsample(i3,4);%decimate
c1=filter(b,1,y);
c2=filter(b,1,c1);
c3=filter(b,1,c2);
sf = c3./64;
f_osc =fft(sc,L);
f_osc=20*log(abs(f_osc))/log(10); %换算成dBW单位
ft1=[0:(Fs/L):Fs/16]; %转换横坐标以Hz为单位
f_osc=f_osc(1:length(ft1));
f_o =fft(sf,L);
f_o=20*log(abs(f_o))/log(10); %换算成dBW单位
ft2=[0:(Fs1/L):Fs1/4]; %转换横坐标以Hz为单位
f_o=f_o(1:length(ft2));
figure(1),
subplot(211),stem(t(1:512),sc(1:512));
xlabel('时间(t)','fontsize',8);
ylabel('幅度(dB)','fontsize',8);
title('sin_osc','fontsize',8);
subplot(212),stem(t(1:128),sf(1:128));
xlabel('时间(t)','fontsize',8);
ylabel('幅度(dB)','fontsize',8);
title('sf','fontsize',8);
figure(2),
subplot(211),plot(ft1,f_osc);
xlabel('频率(Hz)','fontsize',8); ylabel('功率(dBW)','fontsize',8);
title('原始信号信号频谱图','fontsize',8);legend('sinosc');
subplot(212),plot(ft2,f_o);
xlabel('频率(Hz)','fontsize',8); ylabel('功率(dBW)','fontsize',8);
title('滤波后信号频谱图','fontsize',8);legend('sinosc');
3 FPGA实现CIC滤波器的四倍抽取
FPGA设计：FPGA由i2s输入352.8khz的1khz sine（当然也可以是歌曲352.8khz采样率），经过i2s串转并后经过mult_cic模块进行采样率下降处理（变成88.2khz 1khz sine或者歌曲），再通过i2s_tx_master并转串送到DAC 。
多级CIC滤波器的结构主要由积分器+抽值+梳状滤波器构成。
FPGA代码：
`timescale 1ps/1ps module mult_cic#(parameter DW = 38)( input mclk,//45.1584MHZ input reset_n, input signed[DW-1:0] pcm_in,//352.8khz output signed[31:0] pcm_out //88.2khz ); wire signed [DW-1:0]integrator_temp;wire signed [DW-1:0] decimate_temp;wire signed [DW-1:0] comb_temp; integrator#(.DW(38)) U_integrator( .mclk(mclk), .reset_n(reset_n), .din(pcm_in), .dout(integrator_temp) ); decimate#(.DW(38)) U_decimate( .mclk(mclk), .reset_n(reset_n), .din(integrator_temp), .dout(decimate_temp) ); comb#(.DW(38)) U_comb( .mclk(mclk), .reset_n(reset_n), .din(decimate_temp), .dout(comb_temp) ); //divideassign pcm_out = comb_temp[37:6]; endmodule module integrator#(parameter DW = 38)( input mclk, input reset_n, input signed [DW-1:0] din, output signed [DW-1:0] dout ); localparam LAST_CYCLE = 128;reg [6:0] i; reg signed [DW-1:0] temp_xin1,temp_xin2,temp_xin3;wire signed [DW-1:0] i1_temp,i2_temp,i3_temp; always @(posedge mclk or negedge reset_n) begin if(reset_n == 1'b0) i <= 0; else i<= i+1;end always @(posedge mclk or negedge reset_n) begin //The first level integrator if(reset_n == 1'b0) temp_xin1 <= 0; else if(i == (LAST_CYCLE-1)) temp_xin1 <= i1_temp;end assign i1_temp = (reset_n == 1'b0)?38'b0:( din + temp_xin1); always @(posedge mclk or negedge reset_n) begin //The second level integrator if(reset_n == 1'b0) temp_xin2 <= 0; else if(i == (LAST_CYCLE-1)) temp_xin2 <= i2_temp;end assign i2_temp = (reset_n == 1'b0)?38'b0:( i1_temp + temp_xin2); always @(posedge mclk or negedge reset_n) begin //The third level integrator if(reset_n == 1'b0) temp_xin3 <= 0; else if(i == (LAST_CYCLE-1)) temp_xin3 <= i3_temp;end assign i3_temp = (reset_n == 1'b0)?38'b0:( i2_temp + temp_xin3); assign dout = i3_temp; endmodule module decimate#(parameter DW = 38)( input mclk, input reset_n, input signed [DW-1:0] din, output signed [DW-1:0] dout ); localparam LAST_CYCLE = 512;reg [8:0] i;//88.2 reg signed [DW-1:0] dout_pcm; assign dout = dout_pcm; always @(posedge mclk or negedge reset_n) begin if(reset_n == 1'b0) begin i <= 0;dout_pcm<=0; end else begin i<= i+1;if(i == (LAST_CYCLE-1)) begin dout_pcm<=din; //downsmple(x,n)--n--4end endend endmodule module comb#(parameter DW = 38)( input mclk, input reset_n, input signed [DW-1:0] din, output signed [DW-1:0] dout ); localparam LAST_CYCLE = 512;reg [8:0] i;//88.2 reg signed [DW-1:0] d1,d2,d3,d4;wire signed [DW-1:0] c1,c2; always @(posedge mclk or negedge reset_n) begin if(reset_n == 1'b0) begin i <= 0;d1 <=0;d2 <=0;d3 <=0;d4 <=0; end else begin i<= i+1;if(i == (LAST_CYCLE-1)) begin d1<=din; d2<=d1; d3<=c1; d4<=c2;end endend assign c1 = (reset_n ==1'b0)?38'b0:(d1-d2);//comb1assign c2 = (reset_n ==1'b0)?38'b0:(c1-d3);//comb2assign dout =(reset_n ==1'b0)?38'b0:(c2-d4);//comb3 endmodule
FPGA仿真:
仿真输入1khz sine输出依然为1khz sine，设计成功。
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