A 20 ns to 320 ns group delay controller and its application in long time delay self-interference cancellation system

A frequency tunable group delay controller (GDC) based on second-order reflective tunable resonant network and circulator is proposed. The GDC has continuous tunable group delay with allpass magnitude response. Tradeoff between delay time and loss of general one-port lossy resonant circuit is studied, and a one-port second-order coupled-resonator structure is optimized. This proposed second-order reflective tunable resonant network is realized by microstrip resonators with optimized coupling location. A circulator is utilized to convert the reflection-type circuit to transmission type. The influence of the circulator isolation leakage upon the delay time is also analyzed. The GDC is designed, simulated and fabricated. Group delay can be tuned from 20 ns to 320 ns, and central frequency of the group delay response can be tuned from 940 MHz to 980 MHz. The insertion loss is less than 10 dB, 15 dB, 25 dB and 27 dB for 50 ns, 100 ns, 200 ns and 320 ns group delay, respectively. The figure-of-merit that the insertion loss per 1 ns delay of the GDC cell is the lowest among compared delay controllers. Two proposed GDC cells are then cascaded. The measurement demonstrates the bandwidth extension ability and the potential usage in full-duplex communication self-interference cancellation.