| 摘要 |
Digital transmission using existing AM transmitters leads to the problem of the occurrence of unwanted out-of-band and spurious emissions. The reasons for this are that the two signals A-signal and RF-P signal, which are generated from the digital modulation signal (I/Q signal) and which are required for controlling the AM transmitter, have considerably larger bandwidths than are available in the separate branches A branch and RF branch and that the two signals arrive at the multiplier of the transmitter output stage with a delay. Using the known methods for delay compensation to below 0.3 microseconds and bandwidth reduction, for example, by a hole in the vector diagram of the I/Q signal around the 0/0 point, it is not possible to minimize the out-of-band and spurious emissions to a sufficient degree. The present method starts from the assumption that the distortions which arise because of the band limitation can be minimized using a suitable filter for the phase-modulated RF-P signal, thus also reducing the out-of-band and spurious emissions. A Gaussian filter having a linear phase response is suitable for this purpose. However, the filtered RF-P signal does not lead to success when directly fed to the transmitter output stage because the amplitude variations would be cut off again due to the switched operation. Therefore, the amplitude variations are detected from the tapped-off RF-P signal after passage through the filter using an envelope comparator and are brought together as a correction signal with the A signal in a multiplier additionally implemented in the A branch. Then, the corrected A-signal is multiplied by the RF-P signal in the transmitter output stage.
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