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 observing wavelengths shorter than 30 cm, while the prime focus is used at longer wavelengths. The antenna features a wheel-and-track mount, with an advanced-design reflector support structure. Antenna motions are relatively rapid for an antenna of this size, to facilitate rapid source changes: 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 wavelengths shorter than 30 cm 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 frequency-selective dichroic system enables simultaneous 13/4-cm observations. The 90- and 50-cm feeds are crossed dipoles mounted on the subreflector near prime focus; simultaneous 90/50-cm observations are possible.
The polarizer extracts orthogonal circularly-polarized signals, which are routed separately to dual receivers. For receivers below 30 cm, the polarizer is cooled to cryogenic temperatures.
The receiver amplifies the signal. Most VLBA receivers use HFET (Heterostructure Field Effect Transistor) amplifiers at a physical temperature of 15 K, but the 90- and 50-cm receivers use GaAsFETs (Gallium Arsenide FETs) at room temperature. All receivers produce dual-polarization outputs, in opposite hands of circular polarization.
The IF converter mixes the receiver output signals with the first LO generated by a front end synthesizer. Two IF bands between 512 and 1024 MHz are output by each converter, one in 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, D, carry the IF signals from the antenna vertex room to the station control building. Normally only two IFs 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 several 4-IF configurations needed for special cases. These include dual-polarization observations at two arbitrary frequencies anywhere within the 4-8 GHz range of the new 6-cm receiver, and combinations of dual IF outputs from both the 13- and 4-cm receivers (using the dichroic system described in the paragraph on feeds above), or the 90- and 50-cm receivers.
