How can I plot in dB in Matlab?

Hello:

I am working on app designer. My app is about communications and I need to plot the frequency response of a mixer. However, the levels of signal power I'm using are lower than a reference, so I need to plot some negative amplitude levels in dB.

In the real life I need to obtain this:

but in Matlab I don´t know how to plot those negative values.

The code I'm using is this:

function spectrum(app)

%For sin signal and AM

if app.SinButton.Value == true && app.AmplitudeModulationButton.Value == true

[fc,A,Ap,fm,Fs,Ts,N,t,f,mi,Vcdol,Vcdif] = getdata(app);

x = Vcdif+A*sin(2*pi*fm*t); % IF

p= Vcdol+Ap*sin(2*pi*fc*t); % carrier signal

y=((1+mi*x).*p); %modulated signal

plot(app.UIAxes,f,10*log10(abs(fftshift(fft(y))));

Answer 1

[Peter O]

Decibels are a ratiometric measurement. For power, as shown in your plot, the reference is 1 milliwatt (denoted from the dBm). 0dBm means that the ratio is 1.0. If you've only got 0.5 mW output, then the ratio is 0.5e-3/1e-3 =0.5.

PdBm = 10*log10(0.5) = -3dBm.