CLEANUP - changes to folder structure

projects/ECOC_2025_MPI/estimate_linewidth.m

@@ -0,0 +1,44 @@
+% Parameters
+
+reclen = 133169152;
+Scp_long = ScopeKeysight("model","DSAZ634A",'autoscale',0,"fadc","GSa_160","channel",[0,0,1,0],"recordLen",reclen,"removeDC",1,"extRef",1);
+Scpe_sig_raw = Scp_long.read("channel",[0,0,1,0]);
+Scpe_sig_raw = Scpe_sig_raw{3};
+
+signal = Scpe_sig_raw.signal;
+Fs = Scpe_sig_raw.fs;                 % Sampling rate [Hz]
+N = length(signal);         % Number of samples
+f_center = 20e9;               % Frequency of interest (~20 GHz)
+
+% Create time vector
+t = (0:N-1)' / Fs;
+
+% Mix down to baseband
+signal_mix = signal .* exp(-1j * 2 * pi * f_center * t);
+
+% Low-pass filter (optional but recommended)
+% Design a filter with bandwidth ~ linewidth * safety margin
+BW = 1000e6;  % 100 MHz bandwidth for example
+
+% Decimate (reduce sampling rate)
+decimation_factor = 10;%floor(Fs / (2 * BW));
+signal_decim = downsample(signal_mix, decimation_factor);
+Fs_decim = Fs / decimation_factor;
+
+% FFT of downmixed signal
+Nfft = 2^nextpow2(length(signal_decim) * 4); % Zero-pad
+spectrum = fft(signal_decim .* blackmanharris(length(signal_decim)), Nfft);
+freq = (0:Nfft-1) * (Fs_decim / Nfft);
+
+% Only positive frequencies
+halfIdx = 1:floor(Nfft/2);
+spectrum_half = abs(spectrum(halfIdx));
+freq_half = freq(halfIdx);
+
+% Plot
+figure;
+plot(freq_half / 1e6, 20*log10(spectrum_half));
+xlabel('Frequency (MHz)');
+ylabel('Magnitude (dB)');
+title('Zoomed Spectrum around 20 GHz');
+grid on;