Up Chirp and Down Chirp Generation in a Single plot

I am trying to generate a chirp signal with both up chirp and down chirp for one of my project, upchirp - which has start frequency of 57 GHz and Bandwidth of 150MHz then after 2microseconds and again have to generate down chirp with the same bandwidth and frequency

[Mathieu NOE]

hello

so what is the problem ? do you have a code to share ?

[Sri Janani]

Is there any reference example for this case?

Answer 1

[Mathieu NOE]

hello

well there is the documentation related to the chirp funtion, have you looked here ? chirp - Swept-frequency cosine - MATLAB

I can suggest this code :

NB : the first plot is just to show the ending portion of the up chirp signal (in blue) with the first samples of the down chirp signal (in red) - which is simply the time mirrored copy of the up chirp. I also aplied a sign inversion because then the transition is smooth between the two. It's a very simple trick that works here but may fail with other input parameters - you may improve this by checking if the up chirp signal stops with a positive or negative end point (and slope) and truncate accordingly to have the smooth transition)

I notice also that the frequency variation is very low (can be visualized in the spectrogram too) so I wonder if the starting freq is 57 or 5.7 GHz.

code :

BW = 150e6; % Hz
f_low = 57e9; % Hz
f_up = f_low + BW; % Hz
t_up = 2e-6; % up chirp duration

% choose a sampling rate in power of 2
n = nextpow2(f_up)+3;
Fs = 2^n;
dt = 1/Fs;

% up chirp
t = 0:dt:t_up; % Time vector
phi = 90;                 % Initial phase in degrees
y = chirp(t, f_low, t_up, f_up,"linear",phi); % Generate chirp signal

% figure,plot(y(1:25)); % Plot the chirp signal beginning to actually check
% it starts with 0


% add time flipped version of y (downchirp)
% the neg sign is a trick to ensure a continous and smooth transition
% between the up and down chirps
y_down = -fliplr(y);
nn = 25;
xx = 1:nn;

figure,plot(xx,y(end-nn+1:end),'b*-',xx+nn,y_down(1:nn),'r*-') % Plot the chirp signal ending portion 

% combine up and down chirps
 y = [y y_down]; 

% spectrogram 
Ndft = 1024*16;
overlap = 0.75;
L = round(overlap*Ndft);
g = bartlett(Ndft);

[s,f,t] = spectrogram(y,g,L,Ndft,Fs);
s_dB = 20*log10(abs(s));

% display the spectrogram, defined as the magnitude squared of the STFT.
figure
imagesc(t,f,s_dB)
set(gca,YDir="normal");
colorbar
clim([max(s_dB(:))-60 max(s_dB(:))])
ylim([0.99*f_low 1.01*f_up])
shading interp

[Sri Janani]

Thank you