DESIGN PARAMETERS FOR A FM/FM SYSTEM

Design parameters for a FM/FM telemetry system are determined in terms of the IRIG specifications for proportional bandwidth channels. Three mathematical models used by designers of the above processes are extended and compared. That is, FM multitone models are used to establish the relationship between frequency deviations, modulation indices, signal-to-noise and IF bandwidth for the IRIG channels. Since spectral efficiency and signal quality are of major importance, a goal of the design is to have a minimum IF bandwidth, while fixing as large as possible the values of the modulation indices for the subcarriers modulating the carrier in order to achieve as large as needed output signal-to-noise ratio. INTRODUCTION Since the noise spectral density after the carrier demodulator is parabolic, it is necessary to vary system parameters such that the signal quality in each individual channel is the same. In these channels the frequency deviation of the subcarrier oscillator by the signal is a constant percentage of the frequency of the subcarrier and the deviation ratio is nominally set to 5. Since the only remaining unspecified parameters are the frequency deviation of the carrier by the individual subcarriers they must be used in order to achieve equal signal to noise ratios in all the channels. The frequency deviations by the individual subcarriers on the carrier is related to maximum specified peak frequency deviation of the carrier. The FM/FM spectrum is complex and is basically multi-tone. Three mathematical models used by designers of the above processes are extended and compared. That is, FM multi-tone models are used to establish the relationship between frequency deviations, modulation indices, signal-to-noise ratios and IF bandwidth for the IRIG channels. These models are then used to determine parameters such that specifications are met. The output signal-to-noise ratio, numerically, in the i subcarrier channels is given by th [1]