  This paper presents an ultra-low noise L-band radio astronomical cryogenic
receiver for FAST telescope. The development of key low noise microwave parts
of Coupling-LNA and conical quad-ridge OMT and reasonable system integration
achieve outstanding performance of receiver.It covers the frequency range of
1.2 GHz to 1.8 GHz. Novel cryogenic Coupling-LNAs with low noise, large return
loss, high dynamic range and the function of coupling calibration signals are
developed for the proposed receiver.Amplification and coupling function
circuits are integrated as a single Coupling-LNA with full noise temperature of
4 K at the physical temperature of 15 K. And its return loss is more than 18
dB, and output 1 dB compression power is +5 dBm. A cryogenic dewar is
fabricated to provide 55 K and 15 K cryogenic environment for OMT and
Coupling-LNAs, respectively. The receiver's system noise temperature is below 9
K referred to feed aperture plane. Benefiting from optimal design and precise
mechanical treatment, good scattering performance of OMT and equalized
radiation patterns of horn are achieved with an antenna efficiency above 75%.
