Change ylim of spectogram

I'd like to change the `ylim` (frequency) values of my spectogram plot.

fs = 20000;
n = 0:5*fs - 1;                        
N = length(n);
t = n/fs;
fT = 1000;
fM = 3;
delta_phiT = 10;

uFM = 1*cos(2*pi*fT*t + delta_phiT*sin(2*pi*fM*t));
spectrogram(uFM, 1024, 256, 4096, fs, 'yaxis');

What I tried so far:

ylim([800 1200]);

But after doing this the spectogram is empty.

Answer 1

[Simon Chan]

It should divide by 1000.

fs = 20000;
n = 0:5*fs - 1;                        
N = length(n);
t = n/fs;
fT = 1000;
fM = 3;
delta_phiT = 10;
uFM = 1*cos(2*pi*fT*t + delta_phiT*sin(2*pi*fM*t));
spectrogram(uFM, 1024, 256, 4096, fs, 'yaxis');
ylim([800 1200]/1000);      % Divide by 1000

[Dominik Deml]

Thank you! Can you explain me why?

[Simon Chan]

Here is the original code:

fs = 20000;
n = 0:5*fs - 1;                        
N = length(n);
t = n/fs;
fT = 1000;
fM = 3;
delta_phiT = 10;
uFM = 1*cos(2*pi*fT*t + delta_phiT*sin(2*pi*fM*t));
spectrogram(uFM, 1024, 256, 4096, fs, 'yaxis');

After that, you can extract the axis information as follows and it shows that the y-limit is from -0.0024 to 10.0024, but not something from -0.0024 to 10,002.4.

On the other hand, the ylabel in the above figure shows that the unit is in kHz instead of Hz and hence you need to divide the freqency in Hz by 1000 in this case.

ax=gca

ax = 
  Axes with properties:

             XLim: [0.0064 4.9600]
             YLim: [-0.0024 10.0024]
           XScale: 'linear'
           YScale: 'linear'
    GridLineStyle: '-'
         Position: [0.1164 0.1100 0.6937 0.8150]
            Units: 'normalized'

  Show all properties