Direct calibration of microwave amplification chain on an axion cavity haloscope.

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In an axion haloscope, the weak photon signal, theoretically converted from axions, is captured by a detection cavity. The signal from the cavity is too weak to be acquired by a signal receiver. The amplification chain assists the signal acquisition by amplifying the signal and requires accurate gain calibration. Typically, the readout line is calibrated using the Y-factor method, involving a switch that directs either the detection line or the calibration line to the amplification chain. The detection and calibration lines may have different transmissions, which leads to the calibration results being less accurate. In response, we propose a calibration method that eliminates the need for a switch. In this approach, the cavity temperature is decoupled from its incoming noise source and can be controlled, resulting in excess or deficiency of the noise spectrum near its resonance frequency. The experimental result shows that the gain of the amplification chain can be calibrated directly using the temperature-varied cavity radiation.