Primary Signal Path
This sub-section describes the instrumentation that collects and amplifies radio-frequency (RF) radiation from a source, converts, and transmits it to the station control building. Napier et al. (1994) includes further information on most of the following aspects.
The antenna brings the RF signals to a focus at one of ten feeds. The main reflector is a 25-m diameter shaped figure of revolution with a focal-length-to-diameter ratio of 0.354. A 3.5-m diameter Cassegrain subreflector with a shaped asymmetric figure is used at all frequencies above 1 GHz, while the prime focus is used at lower frequencies. The antenna features a wheel-and-track mount, with an advanced-design reflector support structure. Antenna motions, designed to facilitate rapid source changes, are at 30° per minute in elevation and 90° per minute in azimuth.
The feed couples free-space electromagnetic waves into waveguides for transmission to the receiver system. Feeds at observing bands above 1 GHz are located on a ring at the offset Cassegrain focus, and are selected by rotation of the subreflector with a maximum transition time of about 20 seconds. A dichroic system enables simultaneous 2.3/8.4 GHz observations. The 330 and 610 MHz feeds are crossed dipoles mounted on the subreflector near prime focus; simultaneous 330/610 MHz observations are possible.
The polarizer extracts orthogonal circularly-polarized signals, which are routed separately to dual receiver channels. For receivers above 1 GHz, the polarizer is cooled to cryogenic temperatures.
The receiver amplifies the signal. Most VLBA receivers are HFETs (Heterostructure Field Effect Transistors) at a physical temperature of 15 K, but the 90 cm and 50 cm receivers are GaAsFETs (Gallium Arsenide FETs) at room temperature. Each receiver has 2 channels, in opposite circular polarizations. The 1 cm, 7 mm, and 3 mm receivers also perform an initial frequency down conversion.
The IF converter mixes the receiver output signals with the first LO generated by a front end synthesizer. Two signals between 512 and 1024 MHz are output by each IF converter, one for each sense of circular polarization. The same LO signal is used for mixing with both polarizations in most cases. However, the 4 cm IF converter has a special mode that allows both output signals to be connected to the RCP output of the receiver and to use separate LO signals, thereby allowing the use of spanned bandwidths exceeding 512 MHz. Also, the 90 cm and 50 cm signals are combined and transmitted on the same IFs. The 50 cm signals are not frequency converted, while the 90 cm signals are upconverted to 827 MHz before output.
Four IF cables, designated A, B, C, and D, carry the IF signals from the antenna vertex room to the station control building. Normally only two cables are in use at a time, with the signals from each IF converter transmitted via A and C, or B and D; by convention, RCP is normally carried by IFs A and B, and LCP by C and D. However, switching is available to support other configurations needed for special cases. These include dual-band dual-polarization modes that use all four IFs, and in particular, the capability of the new C-band receiver for dual-polarization observations at two frequencies anywhere within its 4-8 GHz range.
