A New Feedforward Interference Cancellation using Bandstop Resonators for In-Band Interferers

High power wideband interferers appearing at 20, 15 and 10 MHz away from the mobile RX band edge can now be suppressed by applying feedforward cancellation in the RF domain where the filtering is carried out using a new bandstop resonator. A study is conducted on the design of high selective resonators using power splitters, short-terminated delay lines and no attenuators. A 4-tap resonator is designed and measured showing a 3 dB bandwidth of 11 MHz only, a maximum insertion loss of 3.9 dB and more than 21 dB rejection. The system is tuned to handle interferers at different locations using a voltage control phase shifter. The cancellation performance is tested using two 10 MHz wide LTE modulated signals representing both the desired received signal (-50 dBm) and the interferer (-20 dBm). The power spectrum measurements show more than 45 dB cancellation for the interferers at 20 and 15 MHz, while 20 dB for the interferer at 10 MHz frequency separation leaving the desired signal with less than 5 dB insertion loss and no significant noise is added.