The VLA W8 Monitor

The W8 monitor consists of a HP 71210C spectrum analyzer connected to the RCP output of the cryogenically 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 selected 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 (approximately 0.5 dB) noise level. For the P-band plots, the average noise level is in the 150 deg K (approximately 1.8 dB) range.

A HP 71210C spectrum analyzer is under PC control via an IEEE-488 bus link. Every 1 or 5 minutes, on the 1 or 5 minute mark, the PC reads the stored peak value registers from the 71210C and stores them to disk using a custom "C" based data logging programming running under the Windows XP OS. The purchased commercial software, "PyrobatchFTP", executing on the datalogging PC, generates a remote file transfer every 20 minutes to send all of that day's data files over the NRAO LAN, & store them on a UNIX server disk. A custom IDL program executing on the server then plots the data in grayscale format, showing frequency (typically 1-2 GHz) vs time (24 hours per graph). The grayscale intensity of the plot represents signal amplitude. These PDF, grayscale plots are then made available for viewing on the NRAO world wide web page.