# A 12 dBm B1 dB N-Path Notch Filter for Transmitter Leakage Suppression in Wideband Receiver
## 1 Introduction  
As wireless communication rate technologies advance, the frequency bands supported by wireless devices have progressively expanded. In co-address transmitter (TX) and wideband receiver (RX) systems, TX leakage significantly compromises the dynamic range and sensitivity of the receiver channel. Traditional TX link front-end designs typically employ fixed-frequency filters, such as surface acoustic wave (SAW) and bulk acoustic wave (BAW) filters. However, the non-tunability of these filters necessitates an array of devices for multiband receivers, which increases system complexity. In wireless transceiver systems, TX leakage levels reaching up to +12 dBm within an 800 kHz bandwidth can severely degrade receiver performance, as illustrated in Figure 1. This leakage imposes stringent requirements on receiver linearity and power handling. Furthermore, the reliance on multiple SAW filters in multiband radio systems adversely affects noise performance. Figure 1: A large blocker causes saturation of the RX system and notch filter. To overcome these limitations, tunable filters have emerged as a promising alternative to replace fixed-frequency filters. Filter solutions are categorized into conventional passive filters and active filters. Passive resonators can be implemented in traditional passive filter solutions in other ways, such as film bulk acoustic wave resonators (FBARs), which can achieve high-linearity filtering effects, but the frequency is not adjustable, limiting the range of filter use [1]. Passive filters are combined in the passive filter with other parts of the transceiver front end. The combination of the amplifier and the low-pass filter on the printed circuit board (PCB) achieves broadband filtering with precise impedance matching, resulting in a larger bandwidth and better fallback efficiency [2]. RF switches are combined with on-chip integrated transmission lines to reduce insertion loss, but the out-of-band rejection is not large enough to satisfy multiband communication selectivity requirements [3]. Active filters are in the spotlight because of their tunability and high level of integration. Various on-chip active filter technologies, including Q-enhanced, g\({}_{m}\)-C, and N-path filters, have been explored. Q-enhanced filters offer high-Q narrowband filtering capabilities but face challenges with integration and frequency stability [4]. Active feedforward filtering mitigates TX leakage by downconverting the TX signal, applying low-pass filtering, and upconverting the signal. However, these systems are limited by the stringent requirements for phase and amplitude matching [5; 6]. N-path filters have gained significant attention among tunable active filters due to their high Q-factor, tunability, low power consumption, and compact on-chip size. Despite their advantages, the linearity constraints of N-path filters remain a critical limitation for broader applications [7]. N-path filters provide a viable solution for developing SAW-less broadband multiband transceiver front-end systems. However, the linearity of these filters is fundamentally constrained by their design as switched-capacitor (SC) circuits composed of switches and capacitors [8; 9]. As the amplitude of the input signal increases, strong nonlinear effects emerge in the N-path filter, compromising the sensitivity and potentially disrupting the functionality of the transceiver system. Large input signal voltage effects degrade the filter performance [7; 10]. These effects collectively degrade the filter's ability to maintain high linearity under large signal conditions. Researchers have proposed innovative designs to address these challenges and enhance the linearity of TX rejection band reject filters (BRF) [11]. One approach integrates an eight-phase bandpass filter with a BRF, allowing independent tuning to attenuate TX leakage as high as +10 dBm [12]. While this method significantly improves the BRF linearity, it limits the passband bandwidth, rendering it suitable primarily for narrowband reception applications.