GBT Configuration: Requirements and Algorithms

4.8 Swper: switching cycle period.

Reasonable defaults depend on the observing type:

• Spectroscopy: 1 sec.
• Continuum: 0.2 sec.
• Pulsar: 0.04 sec.
• Anything else: 1 sec.

4.9 Swfreq: switching frequency table.

Some backends have restrictions on the possible range of Tint, as detailed in the following table:

Table 4.10.2. Ranges of Tint
Backend	Range of Tint
Spectrometer/ 1 quadrant	0.5 to 40 sec
Spectrometer/ 2 quadrants	0.7 to 40 sec
Spectrometer/ 3 quadrants	1.0 to 40 sec
Spectrometer/ 4 quadrants	1.2 to 40 sec
SpectralProcessor	1.5 to ∞
DCR	0.01 to 60 sec
Other backends	no restrictions

The system will always be set up for dual polarization, so there will always be at least two signals per spectral window per beam. A signal from each beam of each receiver is split 4 ways to go to 4 pairs of converter modules. Each polarization pair can be mixed with a different LO2 to provide 4 spectral windows. The possibility of 8 spectral windows is provided by splitting the signals from the receiver to go to both converter racks A and B. This splitting is done for Rcvr8_10 and lower frequencies. Eight spectral windows means 16 signals going to the back end. The only back ends that can acquire 16 signals are the DCR_AF and Spectrometer, when using 12.5 or 50 MHz bandwidths.

The possibilities for Nwin are detailed in the following table: The maximum number of windows listed in the table is for one beam. If using more than one beam, divide by the number of beams, except for the DCR_IF case.

Table 4.11. Maximum Nwin
Backend(s)	Receiver	Max Nwin
Spectrometer, 12.5 or 50MHz BW or DCR_AF, 12.5 or 50MHz BW	< 10 GHz	8
Spectrometer, 12.5 or 50MHz BW or DCR_AF, 12.5 or 50MHz BW	> 10 GHz	4
Spectrometer, 200 or 800 MHz BW or DCR_AF, 200 or 800 MHz BW	Any	4
SpectralProcessor	Any	4
DCR_IF	Any	1 (all beams)
BCPM	Any	2
Radar	Any	1
VLBI	< 10 GHz	2
VLBI	> 10 GHz	1

5.0 Keyword Dependencies

6.0 Frequency Calculations.

• V1 = Vhigh
• V2 = Vlow

Table 6.1. Formulas for conversion to local frame
Vdef	Formula
Radio	fvdef(V, Frest) = Frest( 1 - V/c)
Optical	fvdef(V, Frest) = Frest( 1+V/c) -1
Relativistic	fvdef(V, Frest) = Frest { 1 - (V/c)2}1/2/(1+V/c)

Let Fk = Restfreq, and dF = Deltafreq. For N spectral windows, our configuration keywords will specify
Fk[i] and dF[i], the list of rest frequencies and offsets, for each i = 1,N.

Convert the Fk frequencies to the local frame, and add the offsets:

Determine the center frequency and total bandwidth:

How these quantities are used:

Given that the primary rest frequency, Fk[1], will not in general be at the center of the IF band, we need to modify the IF1 frequency in the LO1 manager so that the IF band represented by (Fcent, BWtot) is centered.

Now we can determine the appropriate frequency settings for the LO2 synthesizers:

(where IF3 comes from Table 4 and depends on the chosen back end.)

Table 6.2.

Nominal Center Frequencies (IF3)
Back End	Frequency (MHz)
ACS-12.5MHz	468.75
ACS-50MHz	425
ACS-200MHz	900
ACS-800MHz	1200
Spectral Processor	250
VLBI	750
BCPM	400
Radar	720

7.0 Wiring: pathfinding through the IF system.

The possible paths through the system can be regarded as a tree structure. This structure is defined by the cabling file and knowledge of the internal structure of the IFRack, Converter Racks, and Analog Filter Rack.

The first step is connecting the Receiver beam(s) to the IFRack. This is already implemented in the IFRack manager software, so the only action required is to send the receiver name to the IFRack manager, and its inputs will be set correctly. The user will have specified the Receiver and Beam keywords, so the desired input ports of the IFrack will be known.

Some of the receivers (Rcvr8_10 and lower frequency) have their signals split to go to two pairs of IFRack inputs. In this case the tree has two branches. Any IFRack input can be connected to one of two optical fiber drivers, depending on the IFRack transfer switch. Here are two more branches. The optical receiver outputs split to four converter modules: a four-fold branching.

Each converter module can select between four outputs, but these are determined by the Backend and Bandwidth, so there is no branching here. If using the Spectrometer or DCR_AF, the Analog Filter Rack needs to be considered. The choice of AFR input is determined by which converter modules were chosen, and the filter settings in the AFR are determined by the Bandwidth, so there is again no branching. Each AFR module connects to a particular sampler in the Spectrometer, so there is, again, no choice.

Thus for any beam of the selected receiver, there are at most 2x2x4 or 16 possible paths to a backend port.

The software should consider all possible paths and consult the quality file to eliminate paths that contain a substandard module. If there is no path with all good modules, an error message will be issued. We leave as an open question at this point exactly how to rate modules and how each path is to be ranked based on the quality file. For paths that have equal quality the choice will be to select the lowest numbered modules first, for example, choose converter modules 1&5 first, then 2&6, and so on.

The procedure would be to start with the first beam of the first spectral window, search the tree and decide on the path. Then go to the second beam, if any, and continue through however many beams are required. Next, repeat the process for each remaining spectral window. In this way, all paths are found for the specified observing.

The keywords that drive the pathfinding process are: Receiver, Backend, Bandwidth, Beam, and Nwin.

This process of pathfinding is the first step the configurator must go through. It determines how the IFRack transfer switches are to be set, and how the input switches of the converter modules and analog filter modules are set. It identifies which converter modules go with which spectral windows, and thus which LO2 synthesizers are set to which frequency.