LAUSR

This letter presents an ultra-low loss cryogenically-coolable lumped element high pass filter and the measurement method of its S parameters and noise temperature at cryogenic temperature. This filter is going to be used in RFI mitigation of FAST wideband single pixel receiver. It adopts 5 poles Chebyshev high pass structure, and ultra-low loss lumped elements are employed. At cryogenic temperature, its 3 dB cutoff frequency is 248 MHz and the high-end of passband with ultra-low insertion loss is up to 3.5 GHz. Specially, the novel mechanical and microwave design ensure effectively cryogenic cooling to all components of the filter. This filter can be operated at temperatures of 300-4 K. S parameters are measured at room and cryogenic temperatures. At 26 K, its insertion loss in passband is smaller than 0.1 dB. A direct measurement to noise temperature at 24 K physical temperature is performed in cryogenic Testing Dewar, and noise temperature smaller than 0.55 K is achieved. The stopband rejection at 95 MHz is larger than 43 dB. In passband, the return loss of 2 ports is larger than 23 dB.