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For a sine wave modulation, the modulation index is seen to be the ratio of the peak frequency deviation of the carrier wave to the frequency of the modulating sine wave. If h ≪ 1 {\displaystyle h\ll 1} , the modulation is called narrowband FM (NFM), and its bandwidth is approximately 2 f m {\displaystyle 2f_{m}\,} .
The modulation index (or modulation depth) of a modulation scheme describes by how much the modulated variable of the carrier signal varies around its unmodulated level. It is defined differently in each modulation scheme. Amplitude modulation index; Frequency modulation index; Phase modulation index
The frequency spectrum of a typical radio signal from an AM or FM radio transmitter. The horizontal axis is frequency; the vertical axis is signal amplitude or power. It consists of a signal (C) at the carrier wave frequency f C, with the modulation contained in narrow frequency bands called sidebands (SB) just above and below the carrier.
However, many modulation schemes make this simple approach impractical because most signal power is devoted to modulation—where the information is present—and not to the carrier frequency. Reducing the carrier power results in greater transmitter efficiency. Different methods must be employed to recover the carrier in these conditions.
A 220 Hz carrier tone f c modulated by a 440 Hz modulating tone f m, with various choices of frequency modulation index, β. The time domain signals are illustrated above, and the corresponding spectra are shown below (spectrum amplitudes in dB). Waveforms for each β. Spectra for each β
Note that since this resultant wave is continuously phase shifting at a steady rate, effectively the frequency has been changed: it has been frequency modulated. And if the IQ data itself has some frequency (e.g. a phasor) then the carrier also can be frequency modulated. So I/Q data is a complete representation of how a carrier is modulated ...
The carrier frequency varies by roughly 5 kHz due to the Doppler effect when the receiver is stationary; if the receiver moves, the variation is higher. The code frequency deviation is 1/1,540 times the carrier frequency deviation for L1 because the code frequency is 1/1,540 of the carrier frequency (see § Frequencies used by GPS). The down ...
For example, a system with a 3 GHz carrier frequency and a pulse width of 1 μs will have a carrier period of approximately 333 ps. Each transmitted pulse will contain about 3000 carrier cycles and the velocity and range ambiguity values for such a system would be: