| 摘要 |
An agile frequency converter and method employs an IF/RF exchange process to reduce the system noise generated by the frequency conversion process in generating a broadband composite signal. The converter employs a well-known two-stage frequency conversion process, but amplifies the intermediate output signal generated by the first conversion stage to boost the input to the second conversion stage, achieving a commensurate increase in the level of the output signal from the second conversion stage. This increased output level results in an increased signal-to-noise ratio between the RF channel carrier component and the system noise floor. The converted output signal is then attenuated by an amount substantially equal to the amount of amplification of the intermediate signal prior to the second stage of conversion. The result of this attenuation is an RF signal having frequency components with amplitudes approximately equal to that which they would have had had the intermediate signal not been amplified, and results in a noise floor which is reduced by that same amount. Thus, the increased signal-to-noise ratio is maintained. The converter also employs two tunable notch filters in series for purposes of filtering two second-order distortion components that find their way back into the range of channel frequencies of the broadband signal. These tunable notch filters are employed because the IF/RF exchange process actually decreases the signal-to-distortion ratio for one of these components. For those RF channel frequencies that fall in the middle of the channel frequency range of the broadband signal, no IF/RF exchange is employed because one of the distortion components cannot be filtered because it is too close in proximity to the RF signal components. In this case, the second stage of the frequency conversion process is operated to produce an optimal signal-to-distortion ratio in the output of the converted signal, including no amplification of the intermediate signal. The tunable notch filters employ two varactors in a back to back push-pull configuration to eliminate first order distortion in the transfer function of the filters. |
