The GBT Spectrometer is in essence a large correlator constructed in quadrants. The system consists of four quadrants, each fed signals from samplers (described later). Each quadrant is capable of providing a varying number of channels depending on the bandwidth chosen. These quadrants can be operated independently i.e., each running at different bandwidths and with different numbers of simultaneous inputs. In reverse, the system can be configured to run all quadrants together forming a single giant correlator with a specified number of channels. As you can see, the Spectrometer offers a lot of flexibility to the observer with the added capability to carry out different observation strategies at the same time. Given this versatility, the Spectrometer should be expected to house a vast number of complex components and it does. The Spectrometer consists of two racks housed within a single cabinet. The left rack contains samplers. The right houses various digital electronics that make up the correlator, a very complex computer. The more important components are described below. The Spectrometer cabinet is located in the electronics equipment room (206).
As noted by its name, this rack consists primarily of two types of samplers, each housed in a module and consisting of complex circuitry, amplifiers, chips, cards, and other parts. A detailed description of all components incorporated in a sampler module is beyond the scope of this document. Please refer to Volume 1 of the GBT Spectrometer Manual for specific details. In general, two types of samplers are used: high speed and low speed.
Eight high-speed samplers are housed in the rack. These samplers are capable of 3-level, Nyquist, and double Nyquist sampling.
32 low-speed samplers are included. These samplers are capable of 3-level, 9-level, Nyquist, and double Nyquist sampling.
The samplers convert their analog input signals into digital data streams by measuring the voltage of the signal a specified number of times per second (100 million for low speed samplers, 1.6 billion for the high speed samplers). These data streams are then sampled at a specified level (discussed below).
These levels refer to the number of different voltage ranges a sampler is capable of measuring. For example, in 3-level sampling a 3-level encoder has two thresholds, a positive one (Vp) and a negative one (Vn). Voltages greater than Vp are digitally encoded as "01". Voltages less than Vn are encoded as "10". Voltages between Vp and Vn are encoded as "00". As you can see, this results in three distinct levels, hence 3-level sampling. Please refer to Table 1 for an illustration of the variety of sampling schemes possible.
This rack houses numerous computer cards and other components known generally as the digital electronics. The more important components are described below.
These cards (four total; one per quadrant) take the output of the samplers and correctly route it to the appropriate memory cards for the observation desired. Each sampler distributor provides data streams to all Spectrometer quadrants. In some modes, all four sampler distributors are needed to operate a single quadrant. These cards are also involved in the "balancing" process, the process that sets the optimal signal level into the samplers. In this role, the sampler distributors measure the number of samples for each voltage range. A final job of these cards is to monitor the output of the samplers by searching for erroneous signal codes. In this capacity, they act as an aid in troubleshooting.
These cards (eight total; two per quadrant; one each type) contain Xilinx field programmable gate arrays (e.g. computer "chips") and serve several functions. Data from the samplers goes through these cards where it is manipulated and temporarily stored. It should also be noted that within these cards, the selection of a specific observing mode could be loaded by selecting a certain set of Xilinx chip "personalities".
These cards (16 total; four per quadrant) accept the signals coming from the Prompt and Delayed Memory cards. They are responsible for generating auto and cross correlation products from an input data signal and can be programmed to support various operating modes. Each card contains sixteen 1024-Lag correlator chips. These chips were designed to enhance the Spectrometer's capabilities to support pulsar observing.
These cards (eight total; one for every two correlator cards) take short-term data integration results from the correlator cards and accumulate them prior to their transfer to the interface card. Since data can enter and pass through these cards at a faster rate than the interface card can accept, the data that's been accumulated and is waiting to be transferred is averaged, further increasing its quality.
This module accepts the integration results from the LTA's and ships them to the VME computer in data "blocks" at a transfer rate of approximately 3 Mbytes/sec per quadrant.
These cards (four total; one per quadrant) serve to program the memory and correlator cards for desired modes of operation prior to an observation run. They're also used to generate control signals for the cards in specific quadrants of the Spectrometer. Programming information passed by these cards comes from the VME control computer.
This card is used to set the mode of the samplers (i.e., simple, Nyquist, 3-level, 9-level, etc.). This card serves an additional role in system troubleshooting. It can be used to send a signal to the samplers thereby programming them to send test signals rather than actual data. This form of testing provides a fairly complete "self-test" of the digital system.
These have no real function at this point. What they do have is the capability to access raw data from the correlator cards at a very high rate of speed. They're included mainly as a slot for future upgrades and for using the Data Tap Card (as of this writing).
This card doesn't reside in the Spectrometer, but can be inserted into a Pulsar Spigot card slot for test purposes. Using this card, testing can be performed on any correlator or long term accumulator card or on the interface card by either injecting a test signal into the data path or tapping into an output signal.
This card is responsible for generating a system wide 1.3 msec cycle for all Spectrometer components requiring one. It also serves to monitor Spectrometer power supplies and rack temperatures. Finally, it's used as a medium for resetting any number of microprocessors via commands from the VME computer.