DDC.rar_NCO_SINE_chirp_ddc_数字下变频

clear; clc; close all; %% Parameters f0=125e6; % IF signal frequency Tp=50e-6; % Time duration B=20e6; % Bandwidth K=B/Tp; % Chirp slope fs=100e6; % Sampling frequency f_nco=25e6; % NCO Frequency N=5120; % Sampling points t=0:1/fs:N/fs; f=(0:N-1)/N*fs; %% Input signal s=rectpuls(t-Tp/2,Tp).*exp(1i*2*pi*f0*(t-Tp/2)+1i*pi*K*(t-Tp/2).^2); s_fft=fft(s,N); figure(1); subplot(121);plot(t,real(s)); % Time domain xlabel('t/s');title('Input Sampled Signal'); subplot(122);plot(f/1e6,abs(s_fft)); % Spectrum of the input sampled signal xlabel('f/MHz');ylabel('Amplitude'); title('Spectrum'); % Frequency domain %% NCO signal x_i=2*cos(2*pi*f_nco*t); x_q=2*cos(2*pi*f_nco*t-pi/2); x_ifft=fft(x_i,N); x_qfft=fft(x_q,N); figure(2); subplot(221);plot(t,x_i); xlabel('t/s');title('In-Phase NCO Signal'); subplot(222);plot(f(1:N/2)/1e6,abs(x_ifft(1:N/2))); xlabel('f/MHz');ylabel('Amplitude'); title('In-Phase Signal Spectrum'); subplot(223);plot(t,x_q); xlabel('t/s');title('Quadrature NCO Signal'); subplot(224);plot(f(1:N/2)/1e6,abs(x_qfft(1:N/2))); xlabel('f/MHz');ylabel('Amplitude'); title('Quadrature Signal Spectrum'); %% Design a 32-th low-pass filter, whose stop frequency is 20 MHz b=fir1(32,0.4); [H,w]=freqz(b,1,N); figure(3); subplot(121); plot(f(1:N/2)/1e6,db(abs(H(1:N/2)))); xlabel('f/MHz');ylabel('Magnitude/dB');title('Amplitude Response'); subplot(122); plot(f(1:N/2)/1e6,angle(H(1:N/2))); xlabel('f/MHz');ylabel('Magnitude/degrees');title('Phase Response'); %% Down-conversion s_i=s.*x_i; s_q=s.*x_q; si=filter(b,1,s_i); si_fft=fft(si,N); sq=filter(b,1,s_q); sq_fft=fft(sq,N); figure(4); subplot(221);plot(t,real(si)); xlabel('t/s');title('Baseband In-Phase Signal'); subplot(222);plot(f(1:N/2)/10^6,abs(si_fft(1:N/2))); xlabel('f/MHz');ylabel('Amplitude'); title('In-Phase Signal Spectrum'); subplot(223);plot(t,real(sq)); xlabel('t/s');title('Baseband Quadrature Signal'); subplot(224);plot(f(1:N/2)/10^6,abs(sq_fft(1:N/2))); xlabel('f/MHz');ylabel('Amplitude'); title('Quadrature Signal Spectrum'); figure(5); base=si-1i*sq; base_fft=fft(base,N); subplot(121);plot(t,real(base)); xlabel('t/s');title('Baseband Signal'); subplot(122);plot(f(1:N/2)/10^6,abs(base_fft(1:N/2))); xlabel('f/MHz');ylabel('Amplitude'); title('Spectrum')