a transmitter comprising a modulator configured to up-convert a first baseband signal into a first RF (radio frequency) signal in accordance with a first LO (local oscillator) signal of a LO frequency, and a power amplifier configured to receive the first RF signal and output a second RF signal to be emitted by an antenna and a third RF signal to be looped back, wherein the third RF signal is magnetically coupled from the second RF signal; and

a loopback network comprising a shielded serial inductor configured to receive the third RF signal and output a fourth RF signal, and a demodulator configured to down-convert the fourth RF signal into a second baseband signal in accordance with a second LO signal of the LO frequency, wherein: the shielded serial inductor is laid out in a multi-layer structure including an upper metal layer and a lower metal layer, and comprises a serial inductor of spiral topology comprising a coil laid out on the lower metal layer shielded by a shield that is laid out on the upper metal layer and connected to a ground node.