Control of the RFI Monitor Station on the GBT Feed Arm

Rick Fisher, NRAO, Charlottesville, VA



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 spectrum analyzer and A/D-FFT control are implemented only for lpda and horn. The 'x' in lpdax and hornx means that an experimental version of lpda and horn is is being run until it is reasonably well debugged. The old lpda is still available in the meantime.)

lpdax [sa|ad]
hornx [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 sweeps 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, 'E:\Data\RFI_FITS'. 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.

The spectra can be displayed by opening another Command Prompt window and executing the command rfiplot.

Control of Analog-to-Digital Sampling and FFT

In addition to the spectrum analyzer, the signals from the RFI monitor station can be sampled with a high-speed analog-to-digital converter and a spectra computed with a Discrete Fast Fourier Transform (FFT) of the sample stream. The signal from a selected antenna is converted to the 10-20 MHz frequency range with a tunable receiver whose input frequency can be anywhere between 100 and 2500 MHz. The 10-20 MHz signal band is sampled at 20 mega-samples per second with a National Instruments PXI-5105 12-bit A/D convertor module. The FFT is computed on successive subsets of sampled data with lengths between 512 and 65536 samples, depending on the selected frequency resolution, to produced power spectra which are accumulated for the selected integration time. The power spectra are 10 MHz wide, as determined by the 20 MS/s sample rate, but the final RF filter before the sampler is only 6 MHz wide at its 3-dB attenuation points so only slightly more than 6 MHz of spectrum is useful. The relatively narrow filter for this sample rate assures that the alias frequencies below 10 MHz and above 20 MHz are well attenuated by the RF filter.

Figure 7 shows the LPDA control GUI with the A/D-FFT control sub-panel attached.

Figure 7. LPDA control GUI panel with A/D-FFT data acquisition sub-panel attached.

The antenna frequency that is converted to the center of the output spectrum is set by typing this frequency into the "Center Freq" box followed by the Enter key. The frequency resolution of the output spectrum is set by selecting one of the values in the pull-down menu shown in Figure 8.

The integration time and number of spectra with this integration time are set by typing the values into the "Integration" and "# Integ" boxes. The longest available integration time is 6.7 seconds set by the sample buffer size in the A/D module. Longer integration times can be effected by specifying more than one integration and displaying the "Average" spectrum in the plotter display. There will be a time gap of a couple of seconds between integrations while the buffer contents are transferred to memory accessible to the CPU. More than one integration shorter than 3.35 seconds will be assigned such that as many whole integration intervals as will fit in one buffer will be done before each buffer transfer, and as many transfers as necessary will be done to complete the requested number. The last buffer will contain as many integrations as the first. For example, if you request 3 integrations of 3 seconds each, you will get 4, 3-second spectra with a time gap between the second and third.

An integration set is initiated with the "Start" button. The "Stop" button can be used to interrupt the integration set, but the interruption takes place only at a buffer transfer. After all integration sample sets are completed and transferred to memory the FFT and accumulation computations are executed, and a FITS file containing all spectra is created with the file name "last_scans.fits" in the directory, 'E:\Data\RFI_FITS'.

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.

The spectra can be displayed by opening another Command Prompt window and executing the command rfiplotx.

Figure 8. FFT spectrum resolution selection menu.

The other two A/D-FFT data acquisition parameters that may be set are shown in Figures 9 and 10. The "A/D P-P" value sets the input peak-to-peak full scale voltage range of the A/D convertor. This is normally set to the most sensitive value of 0.05 volts unless the the antenna signal is quite strong. The "RF atten" selection sets the attenuation at the input to the frequency conversion receiver. If more than enough gain or signal power is available before to receiver input than is necessary, the attenuator value can be set to avoid receiver overload. Otherwise, zero attenuation is typically set.

Figure 9. Selection menu for the peak-to-peak input voltage range of the A/D convertor.

Figure 10. Selection menu for the RF attenuator setting at the input of the RF receiver that converts the antenna signal to the frequency range sampled by the A/D convertor.

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
This will open a display similar to the one shown in Figure 11. The initial display will be the average of all sweeps in the most recently recorded FITS file.

Figure 11. 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 11 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 12. The right mouse button will also 'unzoom', but it tends to under-fill the horizontal plot range by auto-scaling.

Figure 12. 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 13. 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 13. 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 14. This opens a standard Windows directory and file selection window.

Figure 14. 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 13. An example of the text panel with header information is shown in Figure 15.

Figure 15. FITS header contents for the currently open file.

This Web page last modified February 10, 2009

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