It is my understanding that you want to plot STFT for custom frequency range. You would also like to apply the same scale for colorbar to visualize plots for different data.
You can use the output arguments of stft function to visualize custom frequency range. To apply the same scale of colorbar, set colormap limits using caxis function.
The following example illustrates one way to achieve these objectives:
fs = 10e3; % Sampling frequency
LowerFreq = 100; % Lower frequency
UpperFreq = 2000; % Upper frequency
FFTLen = 512; % FFT length
% Generate a signal for two seconds
t = 0:1/fs:2;
x = vco(sin(2*pi*t),[500 1400],fs);
% Comute STFT
[S,F,T] = stft(x,fs,'FFTLength',FFTLen,'FrequencyRange','onesided');
% Compute FFT bins for lower and higher frequencies
LowerFreqIdx = ceil(LowerFreq*FFTLen/fs);
UpperFreqIdx = ceil(UpperFreq*FFTLen/fs)+1;
customFreqRange = LowerFreqIdx:UpperFreqIdx;
% Plot STFT
smag = mag2db(abs(S));
pcolor(T,F(customFreqRange),smag(customFreqRange,:))
xlabel('Time (s)')
ylabel('Frequency (Hz)')
shading flat
colorbar
caxis([-100 100]) % set colormap limits
Hope this helps!
