Avoiding RF Isolators: Reflectionless Microwave Bandpass Filtering Components for Advanced RF Front Ends

RF/microwave bandpass filters (BPFs) are fundamental components in high-frequency RF transceivers. On the transmitter side, they limit the transmitted RF signal bandwidth by suppressing out-of-band spurious signals and intermodulation products, which are primarily generated by the nonlinear active stages and can affect other RF systems. On the other hand, in the receiver, they reject out-of-band interfering/jamming signals and noise. Whereas a large variety of microwave BPFs have been proposed in the technical literature for multiple technologies (e.g., [1] and [2]), most of these BPF devices achieve their filtering functionality by means of the frequency-selective power reflection processing of the RF input signal. This means that those RF signals allocated within the operational bandwidth of the BPF are transmitted to the output terminal (with some tolerable added in-band loss and reflective-type filter, hence decreasing the mixer conversion gain.