% 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;