| 摘要 |
A direct QAM modulator for generating a high power QAM modulated signal at microwave frequencies comprises a hybrid to create the required RF signals for I and Q paths. In each path, a rat-race coupler is used to get the in phase and out of phase RF signal. The subsequent circuit blocks in each path consist of a single pole double throw switch (SPDT) switched by the polarity of the incoming data, a reflection attenuator, implemented by using forward biased pin diode terminations and controlled by the absolute values of the baseband pulse amplitude modulation (PAM) signal, and a variable reflection phase shifter to adjust for the phase error. To realize modulated signal filtering and to compensate for the nonlinearities, a combined prefiltering and predistortion block is used to shape the baseband PAM signal in each path. Finally, a power combiner is used to combine signals from the two paths into one high power QAM modulated and filtered output signal at microwave frequency.
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