An apparatus includes a first circuit transmitting a first signal to a port, a second circuit receiving a second signal from the port, a first peak-notch filter coupled between the port and the first circuit and configured to pass the first signal and block the second signal, a second peak-notch filter coupled between the port and the second circuit and configured to pass the second signal and block the first signal, and a third filter coupled between an input of the first filter and a ground or between an output of the second filter and the ground. The first signal has a first frequency and the second signal has a second frequency that is different from the first frequency.