# Check FM RDS tone before demodulation.

• posted

Hi All,

I want to check RDS tone before demodulation.

Can it be done?

Ex:

Best regards, Boki.

• posted

It doesn't work that way with FM. To see the tone that originally modulated your FM signal, you have to do a Fourier transform or a reverse Fourier transform, or something, which equates to demodulation.

What will show up on your spectrum is a whole bunch of little sidebands, spaced at the fundamental of the modulating frequency, and with a kind of unpredictable pattern, depending on modulation index, deviation, and things like that.

With AM, you get sidebands which are just an image of the modulating spectrum. With FM, it's all over the place - theoretically, all the way fro 0 to infinity, but with generally smaller amplitudes as they get farther from the carrier.

But, I think, if you have a narrow enough bandwidth on your spectrum analyzer, and you're just modulating with a sine wave, you should see sidebands equally spaced at the modulation frequency.

(i.e., 100 MHz + 57 KHz, 100 MHz - 57 KHz, 100 MHz + 2 * 57 KHz, and so on.)

Good Luck! Rich

• posted

Rich Grise =E5=AF=AB=E9=81=93=EF=BC=9A

Hi Rich,

I am newbie.

The music freq is 100MHz, I can get tone at 100MHz and

200MHz...300MHz..400MHz..

no problem.

I can understand 100MHz + 57KHz, but why also 100MHz - 57KHz...

Best regards, Boki

• posted

...

You need to study frequency modulation.