A wireless system (104, 502, 600) comprising: a local oscillator, LO, signal generation circuit, arranged to generate an LO signal according to a reference clock, wherein the LO signal generation circuit comprises: an active oscillator, arranged to generate internal reference clock, wherein the active oscillator comprises at least one active component, and does not include an electromechanical resonator; a receiver, RX, circuit, arranged to generate a down-converted RX signal by performing down-conversion upon an RX input signal according to the LO signal; a calibration circuit, arranged to generate a frequency calibration control output according to a Z calibration signal characteristic of the down-converted RX signal, and output the frequency calibration control output to the LO signal generation circuit, wherein the LO signal generation circuit is adapted to adjust an LO frequency of the LO signal according to the frequency calibration control output; wherein the wireless system (104, 502, 600) is arranged to couple with an external reference oscillator comprising an electromechanical resonator, Z for generating an external reference clock (CT_EXT), and the wireless system (104, 502, 600) further comprises: a switch circuit, coupled to the external reference oscillator and the active oscillator, wherein the switch circuit is arranged to select the external reference clock (CK_EXT) as the reference clock when the wireless system (104, 502, 600) is used as a calibration reference signal source and serves as a golden sample, and to select the internal reference clock (CK_REF) otherwise; and a transmitter, TX, circuit, arranged to transmit the calibration reference signal. a local oscillator, LO, signal generation circuit, arranged to generate an LO signal according to a reference clock, wherein the LO signal generation circuit comprises: an active oscillator, arranged to generate internal reference clock, wherein the active oscillator comprises at least one active component, and does not include an electromechanical resonator; a receiver, RX, circuit, arranged to generate a down-converted RX signal by performing down-conversion upon an RX input signal according to the LO signal; a calibration circuit, arranged to generate a frequency calibration control output according to a Z calibration signal characteristic of the down-converted RX signal, and output the frequency calibration control output to the LO signal generation circuit, wherein the LO signal generation circuit is adapted to adjust an LO frequency of the LO signal according to the frequency calibration control output; wherein the wireless system (104, 502, 600) is arranged to couple with an external reference oscillator comprising an electromechanical resonator, Z for generating an external reference clock (CT_EXT), an active oscillator, arranged to generate internal reference clock, wherein the active oscillator comprises at least one active component, and does not include an electromechanical resonator; a receiver, RX, circuit, arranged to generate a down-converted RX signal by performing down-conversion upon an RX input signal according to the LO signal; a calibration circuit, arranged to generate a frequency calibration control output according to a Z calibration signal characteristic of the down-converted RX signal, and output the frequency calibration control output to the LO signal generation circuit, wherein the LO signal generation circuit is adapted to adjust an LO frequency of the LO signal according to the frequency calibration control output; wherein the wireless system (104, 502, 600) is arranged to couple with an external reference oscillator comprising an electromechanical resonator, Z for generating an external reference clock (CT_EXT), and the wireless system (104, 502, 600) further comprises: a switch circuit, coupled to the external reference oscillator and the active oscillator, wherein the switch circuit is arranged to select the external reference clock (CK_EXT) as the reference clock when the wireless system (104, 502, 600) is used as a calibration reference signal source and serves as a golden sample, and to select the internal reference clock (CK_REF) otherwise; and a transmitter, TX, circuit, arranged to transmit the calibration reference signal. a switch circuit, coupled to the external reference oscillator and the active oscillator, wherein the switch circuit is arranged to select the external reference clock (CK_EXT) as the reference clock when the wireless system (104, 502, 600) is used as a calibration reference signal source and serves as a golden sample, and to select the internal reference clock (CK_REF) otherwise; and a transmitter, TX, circuit, arranged to transmit the calibration reference signal.