The local oscillator synthesizes the local frequency signal (500-1000
MHz : 10 kHz step) for the video converter. The measured phase noise
is better than 3 deg, which is calculated according to the value of the
measured Allan variance. Coherence loss caused by this phase
instability is less than 0.04%. The local oscillator has now been fully
developed and is in commercial use. The video converter converts
windows in the IF signal input (500-1000 MHz) to video signals (0 - 32
MHz). The input interface unit samples the 16-channel (max.) video
signal, and sends the sampled data to the data recorder following a time
data block, and produces a data train of up to 256 Mbps. The format
does not include a time code but only a digitized signal. It was
designed to make the best use of the K-4 recorder's abilities at levels
of up to 256 Mbps. The anti-aliasing filtering is made in analog (32
MHz), and sampling is carried out by an 8 bit A/D converter of 64 Msps.
The quantization threshold level of the A/D converter is adjustable.
After sampling, the 16 MHz, 8 MHz, etc. filtering and bit
discriminations are accomplished by digital filter. The digital filter
is good for producing excellent phase characteristics, and for reducing
coherence loss. It is possible to select suitable coefficients for the
digital filter, for example, the digital filter is used as a band pass
filter during line observations. The configuration for one- and two-bit
sampling is shown in the table. It is possible to support the Mark-III,
the VLBA, and the VSOP data acquisition modes. This Input interface
unit is adopted to the VSOP data acquisition sub-system.
