Some Topics on Limiters and FM Demodulators
Department of Electrical Engineering, Stanford University., 1965 - Radio frequency modulation - 124 pages
An analysis is made of wide-deviation FM systems having low nominal-carrier-frequency to information-bandwidth ratios. The effects of hard limiting of many signals in random noise are analyzed. Expressions are given for the output signal-to-signal ratios (SSR), the inpt signal-to-noise ratios (SNR), and the power in any cross-product. The effect of the power in each output component as a function of the input SNR is investigated. It is found that for all values of input SNR's greater than 10, the strengths of the various output components are relatively constant. For the case of more than two input signals, a weak signal-boosting effect manifests itself when the input SNR's are less than 1 and the input SSR's are greater than 1-10. The signal-suppression effect and the signal-to-signal power sharing, together with their dependence on input signal noise, are presented for various cases. Expressions are presented for the autocorrelation function and the spectral power density of hard-limited FM pulse trains, which allow the computation of the intermodulation products in the information bandwidth or in any other frequency interval. The effect of the nominal-carrier frequency on the interference ratio is exhibited graphically for various constant deviations. A partial interaction of the positive and negative spectra of the modulated wave causes extraneous outputs. For all such spurious components to be filterable, a relation is derived between the carrier frequency, the maximum frequency of information, and the spectral bandwidth of the modulated wave. (Author).
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OUTPUT SPECTRA FOR WIDEBAND FM PULSE TRAINS GENERATED FROM
Evaluation of yt yt T
Figure Page 14 Typical bandwidth of channel using vestigial frequency modulations
Modulated spectrum when fn fvmm
Modulated spectrum when f f shows aliasing
Lowpass filtered output
Block diagram and spectra of the proposed system using the filtering technique
Block diagrams of the phasing technique
APPENDIX A Useful Integrals of Bessel Functions