This web page is a user's guide to the operation of the BYU CASPER data acquisition system for the NRAO phased array feed (PAF). This data acquisition system consists of the second-generation BYU 40-channel analog receiver, a CASPER 64-channel analog-to-digital converter, a ROACH I CASPER FPGA board, and a Linux host computer with RAID data storage. This system can record high-data-rate complex FFT output spectra or accumulated cross-product spectra from all input channel combinations. At the moment (Feb. 2013) this user interface software supports only the complex FFT recording mode.
The PAF system is designed for use on the GBT and includes the dipole array, cryogenic amplifiers, an L-band fiber optic RF signal transmission system, a frequency conversion and band-limiting analog receiver, and the digital data acquisition back-end. The dipole array, cryogenics and fiber-optic transmitters are in the PAF front-end box. Everything at the other end of optical fibers is in the Jansky Lab "tape room" adjacent to the GBT control room. The front-end box may be at the GBT prime focus, the focus of the 20-meter telescope, or the outdoor test building. There are optical fibers from each of these locations to the "tape room".
The PAF system may be run stand-alone, as when performing tests and calibration in the lab or outdoor test building, or as a GBT receiver/back-end in coordination with telescope control. Figure 1 shows Ethernet socket the connections between the software components of the PAF system. The dashed lines in Figure 1 represent socket messages used to setup the PAF system and initiate data acquisition. The socket messages are described in a separate document (to be attached here). The "BYU Rcvr GUI" is the graphical user interface for stand-alone control and shown in Figure 2. This software module is suplimented by GBT Monitor and Control when used on the telescope.
Two file types are produced by each data acquisition scan: a raw data file on the BYU DAQ system disk and a FITS file that contains all of the hardware setup parameters for that scan. Both data files will use the same date/time string, e.g. 2013_02_04_14:23:45, as the file name so that these two files can be associated for data analysis. The FITS file will be written in the Linux file system. Under GBT control this will always be in the directory /home/gbtdata/[prog name]/RcvrArray1_2/, where [prog name] is the name assigned to the test or observing program by the telescope schedule administrator plus an observing session serial number, e.g., AGBT08B_014_07. In stand-alone mode any accessable directory may be specified in the designated text window on the GUI screen.
As shown in Figure 1, the "BYUServerGBT" server, whose code file is BYUserverGBT.py, accepts messages from the stand-alone GUI manager or GBT Control system and passes them to the computer that hosts the BYU DAQ system. The DAQ setup parameters are also passed to the FITS file server so that they can be archived in the receiver FITS file associated with every scan
The GPIB server translates socket messages into GPIB instructions for the signal generators that act as local oscillators, ADC clock, and phase monitor tone burst. These instruments are connected to the host PC, "papillon", with a USB to GPIB adapter cable. Typically, the instruments are set once at the beginning of an observing run. The default values set when the server is started should be the correct values for the PAF receivers and data acqusition system. The user only needs to change the sky frequency to the required value.
The FITS server collates setup information from the GPIB and ADC servers, assembles it into FITS format, and writes this into a new FITS file at the beginning of each data acquisition scan. The FITS file will be written in the Linux file system so this server must run on a Linux machine. Under GBT control this will always be in the directory /home/gbtdata/[prog name]/RcvrArray1_2/. In stand-alone mode any accessable directory may be specified in the designated text window on the GUI screen.