Circuit and method improving linearity, and reducing distortion, in microwave RF bandpass filters, especially superconducting filters

摘要

In a bandpass filter circuit usable at the front end of a cellular microwave radio receiver, and particularly suitable for implementation with high temperature superconductor transmission lines, an rf input signal is split in a first coupler into a major first portion and a minor second portion. A first bandpass filter of inevitable non-linearity receives the first signal portion and produces therefrom a first-bandpass-filtered signal having distortion products collectively of a first power. A second bandpass filter having substantially identical passband and noise characteristics to, but with a non-linearity much greater than, the first bandpass filter receives the second signal portion of the input signal and produces therefrom a second-bandpass-filtered signal which has distortion products substantially collectively equal to the first power. A phase reverser reverses the phase of the second-bandpass-filtered signal relative to the first-bandpass-filtered signal, and the signals are coupled in a second coupler to produce a bandpass-filtered output signal in which the distortion products are substantially canceled. ;The first-bandpass-filtered is preferably amplified in first low noise amplifier, and the second-bandpass-filtered amplified in a second low noise amplifier of variable gain as well as being phase reversed in a phase reverser of variable phase, both so as to (i) “fine tune” the circuit, and (ii) overcome a slight trade-off that is made in the sensitivity of the bandpass filter circuit to the input signal.