Frequency Bands & Performance

by Jonathan Romney last modified Apr 15, 2019 by Jay Blanchard

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 are 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
as of July 2015
[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 (a) 0.312 - 0.342 2742 0.326 0.077 (h)    39 (j)  266
50 cm (a,b) 0.596 - 0.626 2744 0.611 0.078 (h)  111 (k)  681
21 cm (c) 1.35 - 1.75 289 1.438 0.110 1.0 10
18 cm (c) 1.35 - 1.75 314 1.658 0.112 1.1 11
13 cm 2.2 - 2.4 347 2.269 0.087 1.2 12
13 cm (d) 2.2 - 2.4 359 2.269 0.085 1.3 12
6 cm   (e) 3.9 - 7.9 210 4.993 0.119 0.7 6
7 ghz  (e) 3.9 - 7.9 278 6.660 0.103 1.0 9
4 cm 8.0 - 8.8 327 8.419 0.118 1.2 11
4 cm   (d) 8.0 - 8.8 439 8.419 0.105 1.6 15
2 cm 12.0 - 15.4 543 15.363 0.111 1.9 18
1 cm    (f) 21.7 - 24.1 640 22.236 0.110 2.3 22
24 ghz (f) 21.7 - 24.1 534 23.801 0.118 1.9 18
7 mm 41.0 - 45.0 1181 43.124 0.090 (h)    6 40
3 mm  (g) 80.0 - 90.0 4236 86.2 0.033 (i)   30 (l)  254
Notes:
(*) Receiver band designations are those recognized for SCHED's 'BAND' parameter and in the calibration files.
(a) Both bands processed in a single receiver.  Signal from either or both available in same IFs.
(b) User-selectable filters available to restrict frequencies to 608.2-613.8 MHz.
(c) Different ranges within the 20-cm receiver.
(d) Using the 13/4-cm dichroic system.
(e) Different ranges within the 3.9-7.9 GHz receiver.  Two LOs available, providing 4 IFs in dual polarization.
(f) Different ranges within the 1 cm receiver. Continuum performance better at 23.8 GHz, away from water line.
(g) See Table 4 below for individual station details.
(h) Fringe-fit interval 1 minute.      (i) Fringe-fit interval 30 seconds.
(j) Data rate 256 Mbps.                 (k) Data rate 32 Mbps.
(l) 8-station array; 4-hour integration.

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: Performance Parameters at 86.2 GHz
Antenna Nominal Typical Typical Typical Baseline
Frequency Zenith Peak Zenith Sensitivity
Range SEFD Gain Tsys ΔS2048,30s
[GHz] [Jy] [K Jy-1] [K] [mJy]
BR 80 - 90 3500 0.039 135 28.
NL 80 - 96 4900 0.055 270 33.
FD 80 - 96 3600 0.034 120 28.
LA 80 - 90 3100 0.051 160
PT 80 - 96 4100 0.024 100 28.
KP 80 - 96 4600 0.025 110 30.
OV 80 - 96 5800 0.020 100 33.
MK 80 - 96 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 a snapshot of 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 sensitive 3 mm antennas.