The GBT VLBI DAS consists of the major components listed in this section. Refer to Figure 2 for the layout of the VLBI DAS electronics racks located in the VLBI Equipment Room (204). Please see the paragraph on technical references for further details.
This is the overall analog to digital processing section of the system. It performs the function of converting IF signals from the telescope's "front-end" to baseband, sampling the signal and converting it from an analog to digital format, then formatting the data before sending it to a tape recorder directly or via components located in the S2 recorder rack. The rack consists of various components (listed below) designed to carry out specific tasks.
Located in the upper section of the rack are three 5V and two 15V power supplies. These units supply the appropriate voltage required to support the system's various components.
This module provides a frequency reference standard to all modules within the rack requiring it.
This module generates a 32 MHz reference frequency or "clock" signal that is injected into the signal path. This action is required to support functions carried out by the Sampler/Digitizer further into the system. As stated in paragraph 1.0, the precise correlation of VLBI/VLBA data requires synchronized timing of data from the various sites. This component assists with that function.
The IF Distributors amplify the signal before sending it to a set of power splitters which distribute the signal to each Baseband Converter. IF power levels are detected and digitized here as well to allow monitoring by a control computer.
The BBC's perform the bulk of the analog signal processing. They accept the signals from the IF Distributors where, under computer control and direction, a single, optimum signal is selected and processed. Filtering to adjust bandwidth and additional amplification is also carried out here before the signal continues on to the Sampler/Digitizers.
This module, basically an A/D converter, converts the amplified and filtered analog signals from the BBCs to a digital format at a rate of 32 MHz. The data and a 32 MHz clock signal are then transmitted to either the S2-RT VLBI Recording Terminal or Formatter depending on the type of observing run (VLBI or VLBA respectively).
For VLBI observations, the S2-RT Recording Terminal, instead of the Formatter, processes the data signals before they're sent to the VHS type recording units.
For VLBA observations, the Formatter as noted in paragraph 2.1.8 below processes the data signals.
This module, an important part of the Formatter, carries out a number of functions to include sending out a timing pulse used by the formatter and calibration system and synchronizing these pulses with an external timing signal obtained from a GPS satellite. This is extremely important, as the whole concept behind interferometry is the correlation of data from widely separated telescopes.
The formatter accepts the data streams from the Sampler/Digitizer then organizes it before sending it to the tape recording units. This "organizing" routine includes assigning output to specific tape tracks, combining data streams where required, inserting timing and other coding, and adding parity error check bits to name a few. Once the data is formatted, it's sent to the reel to reel tape recorder.
This rack consists of modules and components necessary to accept pre-processed data from the Data Acquisition rack and further process it before recording it to a VHS tape format. The rack consists of various components (listed below) designed to carry out this task.
Two of these modules are mounted vertically in the rack. Each module contains four VHS type tape recorders (known here as tape transports), each capable of recording data for 6 hours and 2 minutes (full data rate, i.e. super long play). The transports are numbered sequentially (0-7) from top to bottom. The VHS tape format is used to support VLBI observing programs.
This unit houses the Data, Signal, and Control Module. This module contains the custom S2 electronics required to manage the system, format and control the data on record, recover the data on playback, and control the transports. In essence, it further processes the data received by the formatter.
This unit acts as an interface between the tape recorders and the data acquisition system. Additional information coming from the data acquisition system (rack) passes through this interface on its way to the recorders. This adapter provides simultaneous outputs for the S2-RT and its recording system as well as other VLBI systems.
This is a battery backup unit used to supply power to the rack during brief power outages.
This is a reel to reel type tape recorder used to support VLBA observing programs. The radio astronomy data, having been processed by the formatter and other system components, is sent here and recorded. This recorder can record data at extremely high density, 3.5 megabytes per square inch, and at very high rates, 128 megabits per second. The recording tapes come in thick (9,000-foot) and thin (18,000-foot) reels. The thin reels can record up to 10.5 hours of observing data. The recorder can also record up to 14 passes on each tape.