Demodulate using FM method
Communications Toolbox / Modulation / Analog Baseband Modulation
The FM Demodulator Baseband block demodulates a complex input signal and returns a real output signal.
In— Input data signal
Input signal, specified as a real scalar, vector, or matrix.
Out— Output data signal
Output signal, returned as a real scalar, vector, or matrix. The data at this port has the same data type and size as the input signal.
Frequency deviation (Hz)— Frequency deviation of demodulator
Frequency deviation of the demodulator, in Hz, specified as a positive scalar. The system bandwidth is equal to twice the sum of the frequency deviation and the message bandwidth.
Simulate using— Type of simulation to run
Code generation(default) |
Type of simulation to run, specified as
Code generation or
Code generation –– Simulate the model by using
generated C code. The first time you run a simulation, Simulink® generates C code for the block. The C code is reused for
subsequent simulations unless the model changes. This option requires
additional startup time, but the speed of the subsequent simulations is
Interpreted execution –– Simulate the model by
using the MATLAB® interpreter. This option requires less startup time than the
Code generation method, but the speed of
subsequent simulations is slower. In this mode, you can debug the source
code of the block.
A frequency-modulated passband signal, Y(t), is given as
A is the carrier amplitude.
fc is the carrier frequency.
x(τ) is the baseband input signal.
fΔ is the frequency deviation in Hz.
The frequency deviation is the maximum shift from fc in one direction, assuming |x(τ)| ≤ 1.
A baseband FM signal can be derived from the passband representation by downconverting the passband signal by fc such that
Removing the component at -2fc from yS(t) leaves the baseband signal representation, y(t), which is given as
The expression for y(t) can be rewritten as , where . Expressing y(t) this way implies that the input signal is a scaled version of the derivative of the phase, ϕ(t).
To recover the input signal from y(t), use a baseband delay demodulator, as this figure shows.
Subtracting a delayed and conjugated copy of the received signal from the signal itself results in this equation.
where T is the sample period. In discrete terms,
The signal vn is the approximate derivative of ϕn such that vn ≈ xn.
 Hatai, I., and I. Chakrabarti. “A New High-Performance Digital FM Modulator and Demodulator for Software-Defined Radio and Its FPGA Implementation.” International Journal of Reconfigurable Computing (December 25, 2011): 1-10. https://doi.org/10.1155/2011/342532.
 Taub, H., and D. Schilling. Principles of Communication Systems. McGraw-Hill Series in Electrical Engineering. New York: McGraw-Hill, 1971, pp. 142–155.