Frequency Bands & Performance

by Gustaaf Van Moorsel last modified Jun 05, 2013

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 have been upgraded substantially; the performance of the new system is shown.

 

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.