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The W8 monitor consists of a Tektronix 2712 spectrum analyzer connected
to the RCP output of the cooled, L-band and uncooled P-band receivers
of whichever VLA antenna is currently on antenna pad west 8 (W8).
L-band is the default, although P-band is occasionally sellected for
special events. After going through a -10 dB directional coupler and a
custom 36 dB amplifier module, the signal is routed through 500+ feet
of buried 1/2 inch Heliax and into the VLA Control Building. Since the
monitor operates continuously, but we only set the antenna up for
L-band around 40% of the time (and P-band even less frequently, the
pointing is undefined, and we assume most signals are coming in the
average 0 dBi side-lobes of the antenna. The monitor system has excess
gain on the spectrum analyzer, so the background noise level of the
L-band plots is approximately equal to our L-band receiver's average
35 deg K noise level. For the P-band plots, the average noise level
is in the 150 deg K range.
The Tektronix 2712 spectrum analyzer is under PC control via a IEEE-488
bus link. Every 5 minutes, on the 5 minute mark, the PC reads the
stored peak & average value registers from the 2712 and stores them to
disk using a custom "C" based data logging programming running under
the Linux 2.0 OS. A SUN engineering workstation located at the Array
Operations Center in Socorro generates a remote file transfer every 5
minutes to grab all of that day's data files over the NRAO LAN, & store
them on it's local disk. A custom IDL program then plots the data in
grayscale format, showing frequency (typically 1250 - 1750 MHz) vs time
(24 hours per graph). The grayscale intensity of the plot represents
signal amplitude. These postscript, grayscale plots are then made
available for viewing on the NRAO world wide web page.
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