The VLA was designed to observe faint cosmic sources for which, under most circumstances, the receiver noise Trx dominates the system temperature Tsys and the contribution of the source Tant is negligible; i.e., Tant << Trx ~Tsys. When observing the Sun, this is no longer true--the Sun dominates the VLA system temperature at all bands, raising it by factors from several hundred to several thousand times the design values: Tsys ~Tant >> Trx. Two fundamental hardware modifications are therefore necessary for solar observing: i) gain reduction; ii) measurement of Tsys.
First, it is necessary to reduce the effective gain of the receiving system of each antenna. This is accomplished by inserting attenuators in the front end of each antenna. The choice of the value for the insertion loss introduced by the switched attenuators is driven by constraints imposed by the ``automatic level control'' or ``ALC" loops, and by the radio-emitting properties of the quiet and active Sun.
From an electrical engineering standpoint, it is much easier to maintain constant signal-power inputs to the correlator than it is to maintain a constant gain for each antenna. An ALC loop uses a variable attenuator to adjust the antenna gain so that the input to the correlator has a constant power level. The ALC loops show a significant phase variation with gain. It is desirable to minimize phase differences introduced by the ALC loop between calibrator and solar scans. Ideally, this is done by ``tuning'' the attenuators to values that most nearly satisfy this requirement.
The choice of insertion loss must, however, be a compromise, because of the radio-emitting properties of the Sun. Radio emission from the Sun may be loosely divided into three categories: (i) an omnipresent background component that varies at the leisurely pace of the eleven-year sunspot cycle; (ii) a slowly varying component associated with active regions on the solar disk, which varies on time scales from hours to days; and (iii) a transient component associated with solar flare activity, which varies on time scales from less than a second to hours.
The gain-reduction scheme implemented at the VLA is a crude compromise between active- and quiet-Sun observing. For all bands, phase-constant 20 dB switchable attenuators are used. Prior to 1990, additional (non-switchable) attenuators were employed at 3.6, 6, and 20 cm. These often introduced problems during calibrator scans and are therefore no longer used.
Since, under the action of the ALC loop, the antenna gain is allowed to vary, the antenna gains must be monitored. During normal observing, a switched noise signal of known amplitude is injected into each receiver input. Expressed in terms of an equivalent temperature, Tcal, the amplitude of the injected signal is typically such that Tcal 00.1 Tsys. Separate noise sources are provided for each band and for each of the two orthogonal senses of polarization. The antenna gains are calibrated by measuring two voltages in the front end of each antenna: the synchronous-detector voltage and the total power voltage. The synchronous-detector voltage VSD measures the difference between the total power levels when the noise source is on and when it is off. The total power voltage VTP measures the power level when the noise source is off. The former is, of course, proportional to Tcal, the injected noise, and the channel gain; the latter is proportional to Tsys and the channel gain. It follows that Tsys is proportional to (VTP/VSD)Tcal. As the gain is inversely proportional to Tsys, the antenna gains are corrected via the so-called ``Tsys correction".
When one observes the Sun, the system temperature is dominated by the Sun itself. The normal Tcal's are of no use because they are orders of magnitude smaller than Tant. Hence, when observing the Sun, special high-temperature noise sources are employed. Initially, because of the expense of retrofitting all twenty-eight VLA antennas with high-temperature noise sources (solar CALs), only four VLA antennas were provided with solar CALs. The situation has changed over the years as receivers systems have been added or upgraded at various bands. As of the year 2000, all antennas are outfitted with solar CALs in the 20 and 3.6 cm bands. Six antennas are outfitted with solar CALs in the 1.3 cm band, four antennas in the 2 and 6 cm bands, three antennas in the 90 cm band, and one antenna in the 7 mm band. More up-to-date information on the status of solar CALs is maintained here.
The solar CALs have been adjusted so that Tcal is 10% of the system temperature that results from quiet-Sun conditions. A stronger solar CAL signal could cause problems with the operation of the correlator under some circumstances. On the other hand, since the minimum CAL signal that may be reliably detected is of order 0.5% of Tsys, the useful dynamic range is restricted to roughly 13 dB, or a factor of twenty over quiet Sun conditions. If one is willing to suffer some degradation in accuracy of the Tsys measurement, then the factor that ultimately limits the dynamic range of the receiving system is saturation of a component--with this criterion, the dynamic range of the receiving system is roughly 20 dB, or a factor of 100 over quiet-Sun conditions.
This document was generated using the LaTeX2HTML translator Version 98.1p7 (June 18th, 1998)
Copyright © 1993, 1994, 1995, 1996,
Nikos Drakos,
Computer Based Learning Unit, University of Leeds.
Copyright © 1997, 1998,
Ross Moore,
Mathematics Department, Macquarie University, Sydney.
The command line arguments were:
latex2html -split 0 -no_navigation hw_notes.tex
The translation was initiated by Tim Bastian on 2000-07-28