Control of the RFI Monitor Station on the GBT Feed Arm

Rick Fisher, NRAO, Charlottesville, VA

Contents

Introduction

Five RFI monitoring antennas are mounted on a one-axis gimbal near the top of the GBT feed arm. The antenna layout is shown in Figure 1. The two directional antennas are mounted on two-axis positioners to control their azimuth and linear polarization orientation. The antenna signals are routed through pre-amplifiers on the gimbal arm and then through an electronics box that contains further amplification, band-limiting filters, RF switches, and two analog fiber modems that send the signals to the GBT equipment room. This electronics box also contains a microprocessor that controls the positioners and RF switches on command from the GBT equipment room via an RS-232 fiber link. This web page describes the GUI's used that control the functions of the RFI monitor station, including spectrum data acquisition.

The Monitor Station is controlled by the Windows-based controller in the National Instruments PXIe chassis near the RFI rack in the GBT Equipment Room.

Figure 1. RFI monitor antennas and gimbal that are mounted atop the GBT feed arm. This view is in the direction AWAY from the GBT main reflector. From left to right the antennas are the 105-1300 MHz log-periodic dipole array (LPDA), a 1.3-2.5 GHz discone, a 960-1215 MHz four-dipole vertical co-linear array, a 25-1300 MHz discone, and a 1.2-1.7 GHz horn.

Quick Start

To start Monitor Station control do the following:

1. Check with colleagues to be sure no one else is using the Monitor Station. Only one user can control the station at a time.

2. Log into the Windows workstation 'fisherda' on the 'AD' network either at its monitor in the GBT Equipment Room or by 'Remote Desktop'.

3. Open a Command Prompt window and execute one if the following commands, dpending on whether you want control of the LPDA, horn, or omnidirectional antennas. The optional arguments, 'sa' or 'ad' allow you to add control of spectrum analyzer or A/D-FFT data acquisition to the control panel. (As of this writing only spectrum analyzer control is implemented and only for lpda. The 'x' in lpdax means that an experimental version of lpda is is being run until it is reasonably well debugged. The old lpda is still available in the meantime.)

lpdax [sa|ad]
horn [sa|ad]
omni [sa|ad]
Note that you cannot run the lpda and horn GUI's at the same time since only one positioner can be controlled at a time, and they share the same RF fiber. Also, no other program may be accessing the serial port while either of these GUI's is running because it will interfere with the RS-232 communications. The omni GUI may be run at any time.

Antenna Control

LPDA and L-Band Horn

Figure 2 shows the GUI for the LPDA antenna, without data acquisition control. The horn antenna display is essentially the same. The antenna's azimuth is shown both relative to true north (Absolute) and relative to the plane of symmetry of the GBT in the direction away from the dish (Encoder). The current azimuth of the GBT is also shown. The RFI antenna may be commanded in either absolute or encoder coordinates by clicking the cursor in the appropriate Command box, typing the azimuth value, and hitting the 'Enter' key. (Response to key strokes may seem a bit slow because of the cadence of display timer.) The most recent of the two command entries has control. If the absolute azimuth has been commanded and the "Track" box is checked, the antenna azimuth will be updated continuously to compensate for GBT azimuth motion. To avoid frequent small positioner motions the position is updated only when the GBT has moved more than five degrees.

Figure 2. Display for LPDA control

The graphical azimuth display in the top right of Figure 2 is in absolute coordinates. The RFI antenna direction is shown by the large arrow. The GBT azimuth is indicated by the dark tick on the circle, in this case to the NE. The pie-shaped wedge shows the direction of the other antennas on the gimbal to indicate the direction were these antennas may interfere somewhat with the reception by the antenna under control. The pie-shape will always be 90 degrees from the GBT azimuth, counter-clockwize for the LPDA and clockwize for the horn.

The E-plane orientation of the antenna polarization is shown just below the center of the display. This orientation may be changed by typing a new value in the command box and hitting 'Enter'. The LPDA values run from 0 to 180 degrees, and the horn orientation runs from -90 to 90 degrees because of the different ways they are mounted on their positioners. The E-plane orientation is also shown in the graphical display to the right. The RF high-pass filter between the pre-amp and the second amplifier may be switched in and out by toggling the button on the bottom right. This filter has a 3 dB cutoff frequency of 100 MHz.

Omni-Directional Antennas

Figure 3 shows the control panel for the three omni-directional antennas, without data acquisition control. Select which of the three antennas in connected to the second RF fiber channel by clicking on its radio button to the left. The high-pass or band-pass filter in each channel may be switched in or out with the toggle buttons to the right.

Figure 3. Display for control of the three omni-directional antennas.

Data Acquisition

Spectrum Analyzer Control

Figure 4 shows the LPDA antenna control GUI with a panel added for basic control of the Anritsu MS-2651B spectrum analyzer added to the bottom ofv the panel. This display is generated with the 'lpda sa' command and argument in the Command Prompt window. The same spectrum analyzer sub-panel can be attached to the horn and omni antenna control with the commands 'horn sa' and 'omni sa', rexpectively.

Figure 4. Display for LPDA and spectrum analyzer control.

The Center Freq and Span entries are self-explanitory. The Anritsu instrument generates a spectrum with 500 spectrum samples so the sample spacing is the span divided by 500, e.g., a span of 100 MHz will produce 200 kHz sample spacing. For many measurements the RF bandwidth (RBW) and video bandwidth (VBW) may be left on 'Auto' which allows the logic in the spectrum analyzer to choose the optimum values for the span selected. This generally gives a reasonably accurate intensity measurement and best match of RBW to sample spacing with a reasonable sweep time. The Sweep Time that results from the combination of span, RBW, and VBW selected is shown in the grey box below the RBW/VBW selections. One or both of RBW and VBW may be set explicitly with the list selection buttons as illustrated in Figure 5. Keep in mind that an excessively small RF bandwidth will result in undersampling of the spectrum and that a smaller video bandwith will increase the sweep time.

Figure 5. Selection of RF bandwidth with its list menu.

The number of spectrum sweeps to be executed in one data acquisition set may be typed in the '#Sweeps' entry box. Clicking the 'Sweep' button will the execute the number of sweeeps specified with the parameters chosen and store the data with antenna and spectrum analyzer information in one FITS file with the name 'last_scans.fits' in the directory, 'TBD'. The sweep set may be terminated early with the 'Stop' button, but the current sweep must finish before stopping.

To save this data set in a FITS file with a name and directory of your choice use the 'File/Save as...' menu selection, as shown in Figure 6, which brings up a standard Windows directory and file selection dialog screen. It is generally a good idea to save the file in the same directory as 'last_scans.fits' to make finding your data for display easier, as described below. If you don't save the data set before executing the next sweep set, the data will be overwritten.

Figure 6. To save the most recent scan set use the 'Save As' menu selection.

Control of Analog-to-Digital Sampling and FFT

Under construction,

Spectrum Data Display

The data in the FITS file, 'last_scans.fits', just created with the data acquisition part of the RFI monitor station can be displayed with a companion plotter. Open another Command Prompt window and execute the command
rfiplotx
This will open a display similar to the one shown in Figure 7. The initial display will be the average of all sweeps in the most recently recorded FITS file.

Figure 7. Companion plotter for RFI station data acquisition..

In this plot the mouse cursor (a "+" cross not shown) may be placed anywhere in the graph to display the accurate coordinates in the lower left corner (96.58 MHz, -60.00 dBm in this case). At the bottom of the display frame is a message bar with a few hints about useful mouse or keyboard functions. In Figure 7 the message indicates that the left mouse button can be used to draw a rectangle around a selected area on the plot for zooming in on it. To restore the full plot use the "Redraw" selection under the "Plot" menu in the top left corner as shown in Figure 8. The right mouse button will also 'unzoom', but it tends to under-fill the horizontal plot range by auto-scaling.

Figure 8. Pull-down menu for plot modification functions.

If there is more than one sweep in the FITS file the average, maximum, and minimum values at each spectral frequency in the sweep set by making the corresponding selection in the "Plot" menu. These selections are equivalent to 'Average', 'Max Hold', and 'Min Hold' on the spectrum analyzer with the added feature of being able to see all three functions on the same data set. To look at individual sweeps in the data set select "Step" in the "Plot" menu and use the 'N' (next) and 'P' (previous) keys to step forward and backward through the sweep set.

When comparing spectra it is convenient to hold the same vertical power scale nstd of having every spectrum autoscale, which they do by default. Select "Pwr Scale" in the "Plot" menu to diplay the pop-up entry dialog shown in Figure 9. Enter the lower limit followed by upper limit separated by a comma and/or space, and click OK. The plot can be inverted by setting the upper limit more negative (less positive) than the lower limit. To restore auto caling to the verical axis, open the "Pwr Scale" pop-up and click OK with an empty entry field.

Figure 9. Pop-up entry window for setting a fixed vertical axis range, lower limit first followed by upper limit separated by a comma and/or space.

The plotter initially opens the 'last_scans.fits' file, which is the most recently recorded sweep set. To open an older saved FITS file or load data from a more recetly recorded 'last_scans.fits' file, use the "Open" selection of the "File" menu shown in Figure 10. This opens a standard Windows directory and file selection window.

Figure 10. File pull-down menu for opening a new FITS file or displaying the contents of the currently loaded FITS file headers.

The contents of the headers and columns in the currently open FITS file may be display by selecting "Show Header" in the "File" menu as shown in Figure 9. An example of the text panel with header information is shown in Figure 11.

Figure 11. FITS header contents for the currently open file.

This Web page last modified February 10, 2009

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