In this post we are going to review some basic concepts about Frequency Modulation (FM) to set the basis for our next post, which will be a Matlab tutorial, including code and exercises.
We are going to review some properties of this scheme, followed by the description of the Matlab code that we will use to develop the exercises in our next post.
- The transmitted signal is not always a very high quality signal, because the bandwidth is limited.
- It is difficult to detect interference at the receiver.
FM presents improvements to the previous points, as in this case, we change carrier frequency according to the information signal intensity. The expression for an FM signal in the time domain is:
As in previous posts
, we will work on the frequency domain so it will be easier to get all the information from the signals. Therefore, we need to take into account the following points:
- The FM signal spectrum has a carrier component and a finite set of lateral frequencies which are placed symmetrically at both sides of the carrier and spaced in the frequencies fm, 2fm, 3fm… This is different from what we saw in the AM scheme, where the only sinusoidal modulation signal would originate only two lateral frequencies.
- In the special case of β being smaller than 1, just the Bessel coefficients, J0 and J1 will have a significant value and, in this case, the FM signal will be formed by the carrier frequency component and a couple of frequency components in fc ± fm.
- The amplitude of the carrier component changes according to β, as J0(β). Therefore, differently from the AM scheme, the FM carrier amplitude changes according to the modulation index, β. As we will observe, this means that the FM envelope is constant.
FM Matlab Code
In our next post, we will be studying some exercises to understand the FM signals properties. To do that, you need to get familiarized with two Matlab functions: fmmod and fmdemod. As you can appreciate, that we didn’t mention yet: this is thefrequency deviation (Af). Therefore, in our next post by using these functions, we will study 3 cases:
- fc and fm have the same values and Af will change
- fc and Af have the same values and fm will change
- Af and fm have the same values and fc will change
Therefore, we encourage you to understand these concepts and leave a comment in case there is something not clear so you will be ready for our next Matlab Tutorial! 🙂