The DCR and Holography receiver are housed within a single cabinet. Components associated with the DCR are located in the top portion of this rack. The DCR consists of three basic sections: voltage to frequency (V/F) switch, integrating counter, and timing generator. It also incorporates fiber optics technology. The significant components within these sections, as well as other components necessary for operating the DCR, are described below. Please refer to Figure 3 for a layout of these items in the rack. The DCR is located in the electronics equipment room (206).
Components making up the V/F Switch are housed in the vertical slot labeled as such. The switch is a VME computer card designed by the NRAO. As its name implies, it acts as a means of routing (switching) where signals will be directed for further processing. The DCR, via the V/F switch, is capable of supporting 32 input signals split between two banks (defined as A and B). The integrating counters, discussed next, can be connected to either of these banks via computer control. In its design, only one bank can be used at a time. A frequency to voltage converter is also incorporated in this chassis for the purpose of generating two chart recorder outputs. It's doubtful that a mechanical chart recorder will ever be used with the DCR for operational monitoring. The outputs are provided should one be required for test or other purposes. Also incorporated in this chassis is a 5 MHz test clock. The test clock allows for inputting a known reference signal into the system to assist in checking the integrating counters and timing generator.
Components making up the Integrating Counter are housed in the vertical slot labeled INT. CTR. Like the V/F switch, the Integrating Counter is a VME computer card designed by the NRAO. It too is setup in two banks, each consisting of 16 counters (corresponding to the 32 inputs provided via the V/F Switch). In this design, one bank of counters is used to integrate data while a computer reads the data in the other. At a determined point, the counter banks are switched, the computer interrupted, data read, and integrating counters reset. This cycle continues throughout the observing process.
The timing generator is a VME computer card designed by the NRAO as well. It's made up of two identical sections; one used for normal switching of signals, the other for advanced switching. Each section in turn contains a delay generator and a phase period generator. The purpose of these components, and hence the overall purpose of the timing generator within the DCR, is to control various aspects of DCR data acquisition. The delay generator controls the start of a scan. The phase period generator controls the duration and the state of the switching signals.
The DCR uses an VME single-board computer. The DCR's software, which contains interrupt routines, real-time tasks, and various classes, are run by software used on this computer. The main function of this computer is to acquire the integrating counter counts, mark the data, and pass it on to an analysis computer. The computer is also used to control various aspects of the DCR. These include, but are not limited to:
Monitoring data and events occurring within the DCR.
Providing a user interface to control DCR setups.
Controlling hardware and changing the state of the DCR during the varying stages of data processing.
Error detection
The following consists of technical specifications that may not be easily distilled by the reader. The information is included to illustrate the complexity and capabilities of the DCR.
Input Channels: There are 32 input channels. The channels are split into two banks of 16 each. Any combination of channels can be selected within a bank. Only one bank is usable at a time.
Switching Modes: 7 predefined and a single user-defined mode are available. They include:
Total Power with continuous calibration.
Total Power with continuous calibration in asymmetric and symmetric phase configurations.
Total Power without continuous calibration.
Switched Power with continuous calibration.
Switched Power without continuous calibration.
Switched Power with continuous calibration in symmetric phase configuration.
Switched Power without continuous calibration in symmetric phase configuration.
User-defined Mode.
Minimum Integration Time: 100 milliseconds (depends on data transfer to disk time).
Maximum Integration Time per Switch Phase: 250 seconds nominal, 25 seconds maximum.
Chart Recorder Outputs: 2 frequency-to-voltage converters that may be assigned to any V/F input channels under software control.
Input Level Monitor Points: 2 minimum/maximum monitors on V/F outputs that may be assigned to any input channel. Minimum limit trigger is 1 kHz. Maximum limit trigger is 9.5 MHz.
Test Input: For test purposes, an internal 5 MHz signal may be substituted on all channels simultaneously to provide a signal with known characteristics.
External Control Outputs: These include Calibration, Signal/Reference (for load or beam switching), Blanking, and 4 that can be user assigned. In addition, there are two advanced signals that may be assigned to any two of the 6 signals (not blanking) that allow their timing to be advanced by up to 0.5 seconds for driving high-inertia devices such as a subreflector.