The major parts of the receiver system are designed and developed for the VLBI projects (Ma et al. 1999). We made a slight modification and added a second mixer, which has a reference frequency input from a frequency synthesizer in order to compensate for the Doppler shift due to the motions of source. This generates a fixed output at the 529 MHz central frequency. The diagram of this system is shown in Fig. 1. The HEMT front end is packaged in a dewar, in which a silver plated, oxygen free copper radiation shield was attached to the 70 K stage to reduce the thermal load on the 15 K stage. Cooled dual polarization waveguide transitions will be integrated into the dewar to minimize the added noise due to loss between the feed and HEMT amplifier. The receiver has two filters: One is a special waveguide filter, a combination of band pass filter (BPF) and band stop filter (BSF) to reject the image frequency ahead of the first mixer; the PLO uses a 2.2 GHz phase locked oscillator and two multipliers to provide the final 22 GHz output frequency. Another BSF after the mixer is designed to prevent leakage from the PLO oscillator. For any phase instability mostly in the synthesized band-width of the back end (e.g. MKIII), the phase calibration in the system is inserted at the IF port like most other systems. An average noise temperature of 50 K and a phase stability of 10-14 (the overlapping estimate of the Allan variance) were measured for the receiver.
|Figure 1: Diagram of the 22 GHz receiving system in which there are two special features: 1) the IF was made with the 2 GHz wide, in order to suit several different kinds of back ends; and 2) two frequency conversions are used for reducing the cost|
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