In the quest for ever-increasing data rates and network reliability, Software-Defined Radios (SDRs) and massive Multi-User Multiple-Input Multiple Output (MU-MIMO) architectures have been explored. With the push towards millimeter-wave (mmWave) frequencies, a wide swath of bandwidth can be made available. A long-term goal of frequency-flexible SDRs is to create RF/mmWave receivers capable of operating in any band over a wide frequency range. Often, this goal is thwarted by the need to withstand large blockers at various frequencies within that same wide turning range, necessitating band-select filters, and thus limiting which band can be accessed. N-path passive mixer-first receivers have been explored to meet this need based on their easily tuned center frequency and bandwidth, combined with extremely high linearity with respect to Out-Of-Band (OOB) blockers. In addition, many radio systems today require access not only to arbitrary bands in a wide frequency range but also to multiple concurrent bands in that range with high selectivity. In contrast, massive MU-MIMO systems alleviate traffic congestion present in dense urban environments by providing spatial multiplexing and filtering, resulting in huge improvements in throughput and radiated energy efficiency. Furthermore, jammer equalization techniques can be implemented in MIMO architectures using synthesized spatial notch filters in the direction of the estimated blockers, thereby relaxing the dynamic range requirements for downstream circuitry.