Each VERA antenna has the receivers for 4 bands, which are S (2 GHz), C (6.7 GHz), X (8 GHz), K (22 GHz), and Q (43 GHz) bands. For the common use in 2009, the K band and the Q band are open for observing. The low-noise HEMT amplifiers in the K and Q bands are enclosed in the cryogenic dewar, which is cooled down to 20 K, to reduce the thermal noise. The range of observable frequency and the typical receiver noise temperature () at each band are summarized in the Table 5 and Figure 5 .
|(40-50 for Ulsan)|
|Receiver noise temperature|
After the radio frequency (RF) signals from astronomical objects are amplified by the receivers, the RF signals are mixed with standard frequency signal generated in the first local oscillator to down-convert the RF to an intermediate frequency (IF) of 4.7 GHz - 7 GHz. The first local frequencies are fixed at 16.8 GHz in the K band and at 37.5 GHz in the Q band. The IF signals are then mixed down again to the base band frequency of 0-512 MHz. The frequency of second local oscillator is tunable with a possible frequency range between 4 GHz and 7 GHz.
The correction of the Doppler effect due to the earth rotation is carried out in the correlation process after the observation. Therefore, basically the second local oscillator frequency is kept to be constant during the observation. Figure 6 shows a flow diagram of these signals for VERA.
The KVN quasi-optics are uniquely designed to observe 22, 43, 86 and 129 GHz band simultaneously (Han et al. 2008, 2013). Figure shows the layout of quasi-optics and receivers viewing from sub-reflector side. The quasi-optics system splits one signal from sub-reflector into four using three dichroic low-pass filters marked as LPF1, LPF2 and LPF3 in the Figure The split signals into four different frequency bands are guided to corresponding receivers.
Figures 8 shows a signal flows in KVN system. The 22, 43 and 86 GHz band receivers are cooled HEMT receivers and the 129 GHz band receiver is a SIS mixer receiver. All receivers receive dual-circular-polarization signals.
Among eight signals (four dual-polarization signals), four signals selected by the IF selector are down-converted to the input frequency band of the sampler. The instantaneous bandwidth of the 1st IF of each receiver is limited to 2 GHz by the band-pass filter. The 1st IF signal is down-converted by BBCs to the sampler input frequency (512-1024 MHz) band.
Typical noise temperatures of K and Q bands are presented in Table 5. Since the calibration chopper is located before the quasi-optics as shown in Figure 7, the loss of quasi-optics contributes to receiver noise temperature instead of degrading antenna aperture efficiency. Therefore, the noise temperature in the table includes the contribution due to the quasi-optics losses.
The receiver noise temperatures of three stations are similar to each other except that the noise temperature of the Ulsan 43 GHz because of the different type of thermal isolator, which is used to reduce heat flow from the feed horn in room temperature stage to cryogenic cooled stage more effectively.