| 摘要 |
<p>DIGITAL RADIO SYSTEMS For tropospheric-scatter and short-wave transmission links for digital data signals employing frequency modulation, frequency diversity is employed to counter any selective fading resulting from multi-path propagation, the digital signal being supported by redundancy using a compound frequency changer stage comprising at least three radio frequencies, the digital signal, together with at least one additional oscillation, in fact an additional fundamental oscillation which determines the frequency spacing in the compound frequency mixer stage, is fed to the input of a frequency modulator (M). At the receiving end, the radio-frequency carriers from the frequency mixer stage each modulated with the useful signal, are all converted by coherent mixing into the same frequency level and fed to a combiner (K) to obtain a sum signal which has an optimum signal-to-noise ratio.</p> |
