Motivation. The Green Bank Interferometer (GBI) has been controlled, since about 1966, by a DDP-116 computer. The DDP-116 has a cycle rate of 588 KHz and 16 KBytes of magnetic core memory. This computer and its interfaces to the hardware have become increasingly difficult to maintain and to repair. The programming is all in assembly language, making any modifications very difficult.
Furthermore, interference from the digital delay rack (DDR) may pose a problem for the new 100-meter GBT. Overtones of its 70 MHz clock can be a source of RFI in the 21-cm band. Thus is it important to move the DDR and control system from its location in the Interferometer Control Building near the GBT, to a shielded room in the new Reber Lab Control Building. The old equipment is too bulky to move easily, so the best solution is to replace the hardware with new, more compact equipment.
The control software was based on the "YGOR" system developed for the GBT. This system implements communication using TCP/IP, allowing operator and user interfaces, as well as control programs, to be distributed among any number of computers. Operator interfaces use the "CLEO" screens. The user and array coordination uses the Glish/TK language.
The MCB (or SIB) is a serial bus running at 56KBaud. The serial links run over fibers, allowing the bus to be extended to interfaces located many miles from the control center.
The receiver MCB interfaces are located in the receiver boxes. Each receiver is at the prime focus of the telescope. The interface controls the noise calibration and turns the LOs on and off. A great deal of status information is collected: temperatures, total power, cryogenics system status, power supply voltages, etc.
The antenna MCB interfaces are located at the base of each telescope. The telescope position, focus, box rotation, brakes and limit status are monitored. Also, data on the cryogenic compressor are monitored. The brakes can be turned on and off, and the motors can be commanded to position the telescope.
A reference signal of 500 MHz, locked to a Hydrogen Maser frequency standard, is transmitted over single-mode fiber to the receiver boxes. There it is converted to 4 LO signals that are mixed with the RFs. The IF signals come from the receivers over fiber to the back end. Four channels from each receiver (X-band, S-band, Left and Right circular polarizations) are converted to baseband. Each channel is a dual-sideband 35 MHz band.
Digital Delay Rack
The input signals (4 channels from each telescope) are sampled at 70 MHz with 3-level sampling. First, the signals go through a power levelling loop. Then, two voltage levels are compared with the signal to give a three-level digital output at 70 MHz. Next, digital delay lines introduce delays in each channel of up to 117 microseconds (= 2^13 x (1/70MHz) ) in steps of 0.89 ns (= (1/70MHz)/16 ). Delays can be updated at any rate, but once every 2 seconds is adequate for the 2.4 km baseline.
After delaying, the channels are multiplied in pairs to form 8 cross-correlations. These are still 3-level bit streams clocked at 70 MHz. The digital output is then treated as an analog signal, smoothed by ~0.2 msec, amplified, and sent to the switchable-gain amplifiers (gain/zero system).
In addition to the cross-correlations, there are 8 auto-correlations (one per input IF channel) performed. These are used as a monitor of the constancy of the ALC level and sampling threshholds. Because they remain fairly constant, no switchable-gain amplifiers are necessary, and their analog outputs are sent directly to the A-to-D.
Gain/Zero system The "zero" circuits compensate for drift in the analog signals. After the drift compensation, the signals go through amplifiers that introduce gains of 1, 10, or 100. This allows the gain to be adjusted in accordance with the strength of the radio source to avoid saturating the A/D.
The gain/zero system keeps the signals in the range +10 to -10 volts allowed by the A/D. 16-bit sampling of each of 16 channels is done at a rate that is settable by the computer.
IRIG timing reference
The timing and frequency reference comes from a Sigma Tau Hydrogen Maser, located in the timing center in the GBT equipment room. An IRIG timing generator uses a 5 MHz reference from the maser and is synchronized to time signals from the GPS satellite system. An IRIG signal is transmitted to an IRIG receiver card in the VME chassis. THe MV167 obtains its time information from this card.
MV167 single-board computer
The single-board computer has a Motorola 68040 33 MHz processor, 8 MBytes of on-board memory, and built in ethernet connectivity. This is housed in a VME chassis along with the IRIG board and the A/D and digital interface cards.
Each device control program in the context of the YGOR system is termed a "manager". All managers are structured in a similar way and share certain characteristics. Information is passed to a manager through "parameters". Data collected from hardware devices go into a "sampler". YGOR provides the infrastructure for enabling programs on any computer in the local network to connect to a manager and pass parameters to it, or to receiver information from a sampler.
Certain parameters are defined in the base manager and thus exist in all managers. These common parameters provide a means to define a syncronized scan. The parameters include start time, stop time, scan duration, scan number, source name, project ID. If several devices are supposed to execute a scan simultaneously, they are each given the same scan parameters.
The antenna manager implements numerous maintenance and testing commands as well as normal source tracking. Sampler: the antenna manager samples status data from an antenna about once per second: encoder readouts, brakes and limit status, and cryo compressor data.
Digital Delay Manager
Data Acquisition Manager
CLEO Antenna interface
CLEO Receiver interface
Glish Array Coordinator
Scan Log files
Sampler2log: FITS log files.
Sturgeon: offline data processing.
Web and other remote access to schedules and data.
email@example.com last changed this page 24 May 2000.