This section describes the devices in the signal path at a VLBA station. Devices in Sections 6.1-6.7 are located at the antenna; those in Sections 6.9-6.13 are in the station control building.
An essential component at each VLBA station, although not directly part of the signal path, is the ultra-stable hydrogen maser frequency standard. Its reference signals, at 100 MHz and 5 MHz, and multiplied versions therof, are used throughout the station electronics, both in the antenna and in the station building. A round trip phase measuring scheme monitors the length of the cables that transmit signals to enable corrections for temperature and pointing induced variations. More information on the VLBA signal path is provided by Napier (1995), Thompson (1995), and Rogers (1995).
In collaboration with MIT Haystack Observatory, the CASPER Laboratory at UC Berkeley, and the South African KAT project, NRAO currently is developing equipment that will enable sampling, filtering, and recording of data at a rate of 4 Gbps. We expect the new digital backend and recording systems to be implemented in 2010, at which time Sections 6.10 through 6.12 will become obsolete, and substantial changes will appear in Section 6.13.