Frequency Bands & Performance

by Gustaaf Van Moorsel last modified Jul 08, 2012

The nominal frequency ranges for VLBA receiver systems are shown in Table 3.  Actual frequency ranges are broader; consult the measurements reported by Hronek & Walker (1996) for details.  Updates on frequency-dependent performance across VLBA bands is available at http://www.vlba.nrao.edu/cgi-bin/wbd_dir.pl.  These actual ranges may be especially important for avoiding radio frequency interference (RFI), and for programs involving extragalactic spectral lines, rotation measures (Cotton 1995b; Kemball 1999), and multi-frequency synthesis (Conway & Sault 1995; Sault & Conway 1999).

 

Table 3: Receiver Frequency Ranges & Performance
[1]
[2] [3] [4] [5] [6] [7]
Receiver Nominal Typical Center Typical Baseline Image
Band Frequency Zenith Frequency Peak Sensitivity Sensitivity
Designation Range SEFD for SEFD Gain ΔS2048,2m
ΔIm2048,8h

[GHz] [Jy] [GHz] [K Jy-1] [mJy] [μJy beam-1]
90 cm 0.312 - 0.342 2227 0.326 0.097 (g)  29
(i)  196
50 cm (a) 0.596 - 0.626 2216 0.611 0.088 (g)  81
(j)  553
21 cm (b) 1.35 - 1.75 296 1.438 0.096 1.0 9
18 cm (b) 1.35 - 1.75 303 1.658 0.100 1.0 10
13 cm (c) 2.15 - 2.35 322 2.275 0.093 1.1 10
13 cm (c,d) 2.15 - 2.35 337 2.275 0.090 1.1 11
6 cm (*) 3.9 - 7.9 210 4.990 0.120 0.7 7
4 cm 8.0 - 8.8 307 8.425 0.113 1.0 10
4 cm (d) 8.0 - 8.8 407 8.425 0.106 1.3 13
2 cm 12.0 - 15.4 550 15.369 0.104 1.8 17
1 cm (e) 21.7 - 24.1 502 22.236 0.107 1.7 16
1 cm (e) 21.7 - 24.1 441 23.799 0.107 1.5 14
7 mm 41.0 - 45.0 1436 43.174 0.078 (g)    7
45
3 mm (f) 80.0 - 90.0 4000 86.2 0.025 (h)  26
(k)  224
Notes:
(a) User-selectable filters available to restrict frequencies to 608.2-613.8 MHz.
(b) Different ranges within the same 20 cm receiver.
(c) Filters at NL, LA, and OV restrict frequencies to 2200-2400 MHz.
(d) Using 13/4 cm dichroic.
(e) Different ranges within the same 1 cm receiver. Continuum performance is better at 23.8 GHz, away from the water line.
(f) See Table 4 for individual station details.
(g) Fringe-fit interval 1 minute.     (h) Fringe-fit interval 30 seconds.
(i) Data rate 256 Mbps.                 (j) Data rate 32 Mbps.
(k) 8-station array; 4-hour integration.

(*) These receivers are being upgraded substantially; this table now shows the performance of the new systems.  Additional information on the upgraded receivers, including the current installation schedule, can be found in the New Developments section.

 

Also shown in Table 3 are parameters characterizing the performance of a typical VLBA station for the various frequency bands.  Columns [3] and [5] give typical VLBA system-equivalent-flux-density (SEFD) values at zenith and opacity-corrected peak gains, respectively.  These are means over measurements in both polarization at all ten antennas, at the frequencies in column [4].

The typical zenith SEFD can be combined with the aggregate recorded data rate and appropriate integration times to estimate the root-mean-square (RMS) noise level on a single VLBA baseline, and in a VLBA image.  Characteristic values tabulated in columns [6] and [7] are computed assuming, for most cases, the VLBA's upgraded 2-Gbps recording rate for continuum observations;  a typical fringe-fit interval of 2 minutes; and a total on-source integration time of 8 hours.  Exceptions, indicated in the table notes, apply to the fringe-fit intervals at the lowest and highest frequency bands, where shorter intervals are often required; for the recording rate limits imposed by the available RF bandwidth at the lowest frequency bands; and for most parameters at the extreme 3-mm band.  Performance may be worse than the tabulated estimates on some baselines due to poor primary or subreflector surfaces or poor atmospheric conditions.

 

Table 4: Typical Performance Parameters at 86.2 GHz
Antenna Nominal Typical Typical Typical Baseline

Frequency Zenith Peak Zenith Sensitivity

Range SEFD Gain Tsys
ΔS512,30s

[GHz] [Jy] [K Jy-1] [K] [mJy]
BR 80.0 - 90.0 3500 0.039 135 28.
NL 80.0 - 96.0 4900 0.055 270 33.
FD 80.0 - 96.0 3600 0.034 120 28.
LA 80.0 - 90.0 3100 0.051 160
PT 80.0 - 96.0 4100 0.024 100 28.
KP 80.0 - 96.0 4600 0.025 110 30.
OV 80.0 - 96.0 5800 0.020 100 33.
MK 80.0 - 96.0 4100 0.023 100 28.

 

The 3 mm band extends beyond the design specification for the VLBA antenna, and is challenging for the panel-setting accuracy of the primary reflectors, the figure of the subreflectors, and the pointing of the antennas. In addition, performance in this band is highly dependent on weather conditions. Table 4 gives the approximate current performance at 86 GHz for each antenna, as well as the RMS noise in 30 seconds (at 2 Gbps) on a baseline to LA, which is one of the most sensitive3 mm antennas.