# FM demodulation I have a frequency modulated signal. My problem is the following:

1. The sidebands are asymmetric. Meaning that the amplitudes of the sidebands left and right are not equal. Does anyone know what this means for my signal?

2. Sometimes the sidebands have a larger amplitude than my main frequency. In this case, I haven't found a way to detect the true frequency. The sidebands are asymmetric, some of them could be even missing and my main frequency is not the one with the largest amplitude.

Is there a way to solve this problem or even demodulate the signal? I have attached an FFT of my signal. I know that the true frequency is at 0.31

• do you have any figures, plots, code, mathematical equations, attempts...? – Fat32 Jan 26 '18 at 16:54
• is that plot a PSD, or an estimate based on a finite observation? – Marcus Müller Jan 26 '18 at 18:41
• it is based on a finite observation – user31381 Jan 26 '18 at 18:48

## 1 Answer

You do not need to determine the “true” or exact carrier frequency to demodulate a simple FM modulated signal. It does help if your bandpass filter is roughly centered enough to cover the entire bandwidth of the modulated signal, and exclude interfering frequency channels above and below. If you can determine the rate of change of signal phase with reference to any nearby carrier frequency, the result will merely potentially have a DC offset which can be removed from the demodulated audio by a DC blocker or high pass filter.

However, if you are asking about broadcast stereo FM, the may be a narrowband pilot signal you can use for a frequency lock.

• I actually want to retrieve the carrier frequency – user31381 Jan 27 '18 at 19:53